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School of Sciences, Gujarat University, Ahmedabad, INDIA. Page 260

Chapter 7A

Photosensitized Reaction of

4 & 5 – Methylthiazole

and Benzophenone

Abstract: The photosensitized reaction of the 4 – Methylthiazole (4-MT) & 5 – Methylthiazole (5-MT) with benzophenone (BP) in alkaline medium using visible light has been studied. 4-MT & 5-MT show the λmax at 245 nm and at 240 nm in the pH range of 2 -12 respectively. The triplet – triplet energy transfer from the triplet excited state of the aromatic ketone benzophenone to the substrate molecule takes place during the photo sensitized reaction. The triplet excited 4-MT/5-MT breaks down on further exposure and photo product formation takes place. The reaction shows the participation of singlet oxygene during the photo reaction. The sulfate has been observed as photo product. The rate of the reaction has been calculated and the effect of pH, concentration of the sensitizer, the light intensity on the rate of the reaction has been studied. The quantum efficiency of the photo chemical reaction is calculated using potassium ferri oxalate actinometer and the effect of the concentration of the substrate on the quantum efficiency is calculated. The reaction mechanism and the excited states involved have been suggested.

7A.1 Introduction

4 – Methylthiazole is a Colorless to straw pale yellow to dark yellow clear liquid and occurs in

asparagus, cooked pork, coffee, roasted peanut. It has been used as pharmaceutical intermediate,

pesticide intermediate and as a flavor ingredient in the food flavors, nut flavors, vegetable and

vegetative flavors, baked goods, dairy products, beverages.

The thiazoles trimethylthiazole, 2-methylthiazole and 2-ethylthiazole have been used as gelation

inhibitors in the polymer industry [1, 2]. Trimethylthiazole has also been used as a promoter for the

carbonylation of butadiene to 3-pentenoic acid [3]. 2-methylthiazole and 4-methylthiazole have

been used for the prepration of pyridine free Karl-Fisher reagent [4]. 4-Methyl-5-vinylthiazole has

been used as a lubricant [5] and as a high water content hydrogel [6]. 2-acetylthiazole has been used

as oxytocin antagonist [7] , 2-methylthiazole has been used in the development of an inhibitor for

the gastric acid secretion [8], 4, 5-dimethylthiazole as a raw material in the development of

neuroprotective drugs [9].

School of Sciences, Gujarat University, Ahmedabad, INDIA. 61

Due to it's multipurpose use, thiazole derivatives are frequently detected in both waste water

treatment plant and surface water. In the last decade photochemical study of such compounds has

been a topic of keen interest and an extensive work has been reported on the photochemical study of

substituted thiazole.

Maurizio D' Auris [10] has reported the ab inition study on the isomerization of 2- phenylthiazole

and 2-acetylthiazole. The photochemical isomerization of thiazole derivative involves the formation

of the Dewar isomer. The irradiation of thiazole do not give product [11] but 2- methyl, 4-methyl

and 5-methyl thiazole gave the corresponding isothiazole in trifluoroacetic acid [12]. Pavlik et al

[13] have reported that the methylisothiazoles undergo photo transposition in neutral solution to

methylthiazoles by a single permutation process. The isothiazole to thiazole transposition was

observed to occur upon photolysis in a variety of solvent but the reverse transposition of thiazole to

isothiazole is not observed.

Owing to the interest in the environment protection, the photo-degradation may play an important

role in their elimination from aquatic environment. The photodegradation study of substited

methylthiazole using aromatic ketone as a sensitizer in the presence of the visible light is not

reported elsewhere.

The present study reports the photochemical reaction of methyl thiazole in the aqueous alkaline

medium on irradiation with visible light. The aromatic ketone, benzophenone has been used as

photo sensitizer. There are very few report in the literature for the sensitized study of methyl

thiazole.

Aromatic ketone has been used as a photo sensitizer [14-16] in a number of photochemical

reraction. It shows energy transfer process by photo – oxygenation mechanism and by proton

abstraction mechanism.

Benzophenone has been used as photo sensitizer by a number of workers with different type of

compound. Canonica et. al. [17] have reported the aqueous oxidation of phenylurea herbicides by

triplet excited state of aromatic ketones via singlet O2. A laser flash photolysis study for the

reactivity of aromatic amines with triplet 1, 8 dihydroxy anthraquinone has been reported by Y.Pan

et. al. [18]. Morsi et. al. [19] have reported the photo-oxidation of cis-polyisoprene by singlet

oxygen in the presence of sensitizer benzophenone by uv radiation. Backstrom [20] has presented

the mechanism for the benzophenone-photosensitized oxygenation of alcohols and aldehydes. Iqbal

et. al. [21] have reported the photo oxygenation of tinosponone with singlet oxygen using different


combinations of sensitizers, solvents and singlet oxygen scavengers. Santi Nonell et al. [22] have

reported that aromatic ketones as standards for singlet molecular oxygen photosensitization.

Benzophenone shows proton abstraction [23] and electron transfer reactions [24] in the excited

triplet state. Ichimura et. al. [25] has reported kinetic study by the quenching the rate of DPA by

triplet benzophenone photo sensitizer and have also reported that the excitation energy effect on the

reaction with 2- bromo methyl naphthalene. Jockusch et al [26] have reported that Photo-induced

energy and electron transfer processes between ketone triplet state and organic dyes (methylene

blue, thiopyrinine, safranine and phenosafranine). Koji Yamada et. al. [27] have reported that

Nitrile-forming radical elimination reactions of 1-naphthaldehyde O-(4-substituted benzoyl) oximes

activated by triplet benzophenone. R.G.Brinson et. al. [28] have reported the proton abstraction and

electron transfer photo reaction by anthraquinone. R.E.Galian et. al. [29] have reported that the

intramolecular electron transfer between tyrosin and trytophan photo sensitized by aromatic ketone.

Cai et. al. [30] have suggested mechanism of Sensitized reaction by benzophenone in the triplet

excited state.

The present study reports the benzophenone photosensitized reaction of the 4-MT and 5-MT in the

aqueous alkaline medium in the presence of the visible light. The reaction is monitored by

measuring the spectrum change of 4-MT and 5-MT. The kinetics of the photo degradation reaction

has been studied. The effects of the pH, the concentration of the sensitizer, the concentration of 4-

MT and 5-MT and the light intensity have been evaluated on the rate of the reaction. The reaction

has been used to calculate the quantum efficiency and wether the reaction is monophotonic or

biphotonic. The effect of the oxygen and free radical scavenger has been studied on the reaction.

The mechanism of the photo sensitized reaction has been suggested.

7A.2 Results

7A.2.1 Spectral characteristics

The UV spectrum of the pure methyl substituted thiazoles [4-MT and 5-MT (2 X 10-4 M)] were

recorded in the range of 200 nm – 400 nm at different pH to determine the different species present.

The pH of the solution was maintained using suitable concentration of the HCl or the NaOH in the

solution.

The initial spectrum of methyl substituted thiazoles [4-MT / 5-MT (2 X 10-4 M)] in the aqueous

solution in the pH range 2 - 12 is represented by the dash / continuous line in Fig 7A.1 which


exhibits one well-defined maximum for π → π* transition at 245 nm and at 240 nm for 4-MT and 5-

MT respectively.

Substitution of proton by a methyl group in the azole ring alters the absorption spectrum of thiazole.

It induces bathochromic shift in the order 4- > 5-, with significant increase in the absorption

intensity. The spectra shows (Fig 7A.1) one maximum at 245 nm (ε = 3,310L mol-1 cm-1) and at

240 nm (ε = 3,000 L mol-1cm-1) for 4-MT and 5-MT respectively in the whole pH range of 2 - 12.

4-MT and 5-MT have no tautomeric form. The molecules do not show protonation or

deprotonation.

Goyal et al [31] have reported that the UV spectral behaviour at pH < pKa and pH > pKa is

essentially the same in the case of substituted thiazole compound. The λmax and molar absorptivities

of 4-MT and 5-MT are reported in Table 7A.1.

The exposure of the solutions containing only methyl substituted thiazoles [4-MT / 5-MT (2 X 10-4

M)] in the pH range 2 – 12 to the visible radiation does not result in the change of the original

spectrum of the solution. The direct photolysis of 4-MT and 5-MT on the irradiation by the visible

light does not occur in the acidic or in the alkaline medium and 4-MT and 5-MT are photo stable.

The reaction mixtures of methyl substituted thiazoles [4-MT / 5-MT (2 X 10-4 M)] with the suitable

concentration of BP were prepared and the pH of the solutions were maintained from 2 to 12. The

solutions were kept in the dark and their spectrums were recorded against a reagent blank which

matched with the original spectrum of the 4-MT and 5-MT. The methyl substituted thiazoles [4-MT

/ 5-MT] and BP do not interact in the ground state.


Spectrum of the 4-MT in the acidic and alkaline medium.

210 220 230 240 250 260 270 280

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

4MT5MT

Wavelength (nm)

abso

rban

ce

Fig 7A.1 Spectrum of the 4-MT in the acidic and alkaline medium.

Substrate: 4-MT & 5-MT (2 × 10 -4

M),

In Acidic Medium: pH 2,

In Alkaline Medium: pH 11.5


Absorption spectra of the photosensitized reaction of 4-MT with BP

210 220 230 240 250 260 270 280

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

1.000

T0

T30

T60

T90

T150

T180

T210

Wavelength (nm)

Absorb

ance

Fig 7A.2a Absorption spectra of the photosensitized reaction of 4-MT with BP on exposure at 11 pH.

4-MT (2 × 10 -4

M)

BP (1 × 10-4

M)

Time interval: 30 mins

Source: 100 W tungsten lamp


Absorption spectra of the photosensitized reaction of 5-MT with BP

210 220 230 240 250 260 270 280

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

T0

T25

T50

T75

T125

T150

T175

Wavelength (nm)

ab

so

rba

nce

Fig7A. 2b Absorption spectra of the photosensitized reaction of 5-MT with BP on exposure at 11 pH.

5-MT (2 × 10 -4

M)

BP (1 × 10-4

M)




The solutions containing methyl substituted thiazoles [4-MT / 5-MT (2 X 10-4 M)] and BP (1 X 10-4

M) maintained at different pH were exposed to the visible radiation and the spectrum of the

solutions were recorded against the reagent blank. The solutions maintained at pH 2 to 6 do not

show change in the spectrum but solutions having pH 8 to 12 show change in the spectrum. The

methyl substituted thiazoles [4-MT / 5-MT] are photo stable in the acidic medium and do not

undergo photoreaction with BP when exposed to the visible radiation but it shows photoreaction in

the presence of BP used as a sensitizer in the alkaline medium.

The change in the spectrum of methyl substituted thiazoles [4-MT / 5-MT (2 X 10-4 M)] is observed

when the reaction mixture containing a suitable concentration of BP in the aqueous alkaline

medium is exposed to the visible radiation. The spectrum of the photo sensitized reaction was

recorded against the reagent blank. (Fig 7A.2a/7A.2b) The absorbance bands at the λmax 245 nm

and λmax 240 nm of 4-MT and 5-MT respectively decreased. The decrease of the absorbance with

the time was monitored at the λmax 245 nm and 240 nm for 4-MT and 5-MT respectively.

Table 7A.1 λmax and molar absorptivity of 4-MT and 5-MT in the aqueous solution of the

pH range 2 - 12

λmax Molar Absorptivity

4-MT 245 nm 3,311 L mol-1 cm-1

5-MT 240 nm 3,000 L mol-1 cm-1

Table 7A.1 λmax and molar absorptivity of 4-MT and 5-MT in the aqueous solution of the

pH range 2-12, Substrate: 4-MT and 5-MT: (2 × 10 -4

M), Acid Medium: pH 3,

Alkaline Medium: pH 11

7A.2.2 Determination of the rate constant

The progress of the photosensitized reaction was monitored by recording UV spectra of the methyl

substituted thiazoles [4-MT / 5-MT (2 X 10-4 M)] with BP (1 X 10-4 M) in the range 200 - 400 nm at

fixed time interval. The spectrum of the 5ml aliquot of the exposed solutions, withdrawn after 10

min time intervals in the range of 200 nm – 400 nm against the reagent blank were recorded. The

absorbance of the solutions for 4-MT and 5-MT were also measured at 245 nm and at 240 nm

respectively.


The absorbance at 245 nm and at 240 nm decreases and becomes constant after 180 mins exposure.

The results of a typical run for the change in the absorbance of the methyl substituted thiazoles [4-

MT / 5-MT] with time have been presented in Fig 7A.3.

The change in the absorbance with time has been used for the calculation of the rate of the reaction.

The reactions follow the first order reaction kinetics as the plot of 2 + log OD (optical density) vs.

time is a straight line with a positive slope. (Fig 7A.4) The rate constants for type I reactions of

benzophenone triplets with several alkenes have been reported by Encinas et. al. [32]. The rate

constant has been determined by the expression:

Rate constant = 2.303 x slope

7A.2.2.1 Effect of the pH on the rate of the reaction

The spectrum of the pure methyl substituted thiazoles [4-MT / 5-MT (2 X 10-4 M)] were recorded at

different pH in the range of 200 nm – 400 nm and the effect of the pH on the rate of the

photosensitized reaction was carried out by changing the pH of the solution between pH 2 – 12 and

the calculations for the rate of the reaction were carried out at different pH. (Table 7A.2)

The spectrum of the methyl substituted thiazoles [4-MT / 5-MT (2 X 10-4 M)] at different pH shows

the same λmax at 245 nm and at 240 nm for 4-MT and 5-MT respectively. The photosensitized

reaction is slow between the pH 2 – 6 and increases as the pH of the solution increases till the pH is

10 and then becomes constant (Fig 7A.5). The subsequent studies were carried out at pH 11.

The spectrum of the exposed reaction mixture between the pH 2 to 6 remains the same as the

control solution. The photosensitized reaction of methyl substituted thiazoles [4-MT / 5-MT(2 X 10-

4 M)] with BP occurs only in alkaline medium and is very slow in the acidic medium.

The photo effect of benzophenone is sensitive to OH¯ ion concentration of the solution therefore

the increase in OH¯ ion concentration increases the sensitivity of the sensitizer, which shows higher

proton abstraction capability of benzophenone [33]. Similar effect of OH¯ ion concentration has

been observed in the present study.


Absorption Vs time plot of the photosensitized reaction of 4-MT & 5-MT with BP

Fig 7A.3 Absorption Vs time plot of the photosensitized reaction of 4-MT & 5-MT with BP on

exposure.

4-MT & 5-MT (2 × 10 -4

M)

BP (1 × 10-4

M)



0 20 40 60 80 100 120 140 160 180 200

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

5MT

4MT

Time (min)

Ab

so

rba

nc

e


2 + log OD Vs Time plot of the photosensitized reaction of 4-MT & 5-MT with BP

0 20 40 60 80 100 120 140 160

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

25MT

Linear Regression for

5MT

4MT

Linear Regression for

4MT

Time (min)

2 +

lo

g O

D

Fig 7A.4 2 + log OD Vs Time plot of the photosensitized reaction of 4-MT & 5-MT with

BP on exposure.

4-MT & 5-MT (2 × 10-4

M)

BP (1 × 10-4

M)




Table 7A.2 Effect of pH on the rate of the reaction

pH of the solution

4-MT

Rate of the Reaction (K x 10-3

)

mole / L min [Avg K + 0.4]

5-MT

Rate of the Reaction (K x 10-3

)


2 2.78 2.303

4 2.78 2.303

6 3.67 3.224

8 5.37 4.990

10 7.36 6.740

10.5 7.68 6.909

11 7.68 6.909

11.50 7.68 6.909

Table 7A.2 Effect of the pH on the rate of the reactionSubstrate: 4-MT and 5-MT: (2 × 10-4

M), Sensitizer: BP (1 × 10-4

M), Source: 100 W tungsten lamp

7A.2.2.2 Effect of the concentration of the sensitizer on the rate of the reaction

The effect of the concentration of the BP on the rate of the photosensitized reaction has been

studied. The study was carried out by varying the concentration of the BP in the range of 0.6 x 10-4

M to 1.4 x 10-4 M.

The rate of the reaction remains constant initially with the increase of the BP concentration in the

reaction mixture, but decreased as the BP content reached a higher level due to the self deactivation

of the sensitizer. The results indicate that 1 x 10-4 M BP had an optimal concentration and gave the

best performance under the experimental condition. (Fig 7A.6)


Effect of the pH on the rate of the reaction

0 2 4 6 8 10 12 14

0

1

2

3

4

5

6

7

8

9

5MT

4MT

pH of the solution

Rate

of th

e r

ea

ction

(k x

10-3

) m

ole

/ L

min

Fig 7A.5 Effect of the pH on the rate of the reaction measured at 245 nm.

4-MT & 5-MT (2 × 10 -4

M)

BP (1 × 10-4

M)




Effect of the sensitizer concentration on the rate of the reaction

5 6 7 8 9 10 11 12 13 14 15

5

6

7

8

9

10

5MT

4MT

Con of the sensitizer (C x 10-5)

Ra

te o

f th

e r

ea

ctio

n (

k x

10

-3)

mo

le /

L m

in

Fig 7A.6 Effect of the sensitizer concentration on the rate of the reaction at 11 pH

4-MT & 5-MT (2 × 10-4

M)

BP (0.6 × 10-4

M to 1.6 × 10-4

M)




7A.2.2. 3 Effect of the concentration of the substrate on the rate of the reaction

The effect of the initial concentration of the methyl substituted thiazoles [4-MT / 5-MT] on the rate

of the reaction was studied. The rate of the reaction has been calculated with the different initial

concentration of the substrate in the range of (1.6 – 2.4) x 10-4 M. The rate of the reaction remains

constant in the concentration range shows that the reaction is independent of the initial

concentration of the substrate. (Fig 7A.7) The half life time of the reaction has also been calculated

at the different concentration of the substrate and t1/2 value is constant over the above range of the

substrate concentration. The observation suggests that the photochemical reaction is of the first

order. (Table 7A.3)

Table 7A.3 Effect of the concentration of substrate on the rate of the reaction

Concentration

of Substrate

C x 10-4

4-MT Rate of

the Reaction

(k x 10-3

) mole /

L min

[Avg k + 0.4]

4-MT

t1/2

L min / mole

5-MT Rate of the

Reaction

(k x 10-3

) mole / L

min

[Avg k + 0.4]

5-MT

t1/2

L min / mole

1.6 7.31 94.93 6.98 99.28

1.7 7.54 91.91 7.12 97.33

1.8 7.79 88.96 7.21 96.12

2.0 7.79 88.96 7.21 96.12

2.2 7.79 88.96 7.21 96.12

2.4 7.75 89.42 7.18 96.52

Table 7A.3 Effect of the substrate concentration on the rate of the reaction at 11 pH,

Substrate: 4-MT and 5-MT: (2 × 10-4 M), Sensitizer: BP (1 × 10-4 M), Time

interval: 30 mins, Source: 100 W tungsten lamp


Effect of the substrate concentration on the rate of the reaction

1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5

6.4

6.6

6.8

7

7.2

7.4

7.6

7.8

8

5MT

4MT

Concentration of substrate (c x 10-4)

Ra

te o

f th

e r

ea

ctio

n (

k x

10

-3)

mo

le /

L m

in

Fig 7A.7 Effect of the substrate concentration on the rate of the reaction at 11 pH.

4-MT & 5-MT (1.6 × 10-4 M to 2.4 × 10-4 M)

BP (1 × 10-4 M)




7A.2.2.4 Effect of the light intensity

The solutions of the the different concentration of the substrate [4-MT / 5-MT] and the sensitizer

BP were prepared in the aqueous alkaline solution and irradiated with the visible light. The rate of

the reaction was calculated.

The increase of the light intensity [Einstein / second (E/s)] shows positive effect and the rate of the

reaction increases as the light intensity increases. The higher number of the photons increases the

number of the excited sensitizer molecule and the rate of the reaction also increases. A linear

relationship has been observed between the light intensity and the rate of the reaction. (Table 7A.4)

Table 7A.4 Effect of the Light Intensity on the rate of the reaction and on the quantum

efficiency

Light Intensity

( I × 108

) E/s

4-MT (K × 10-3

)

mole / L min

K + 0.5

5-MT (K × 10-3

)

mole / L min

K + 0.5

4-MT

Quantum

Efficiency

5-MT

Quantum

Efficiency

5 2.54 2.21 0.3435 0.5463

10 4.76 4.56 0.3431 0.5462

15 7.68 6.95 0.3436 0.5465

20 9.75 9.27 0.3433 0.5464

25 12.57 11.86 0.3432 0.5463

Table 7A.4 Effect of the light intensity and quantum efficiency on the rate of the reaction at

11 pH, Substrate: 4-MT and 5-MT: (2 × 10-4


M),

Time interval: 30 mins, Source: 100 W tungsten lamp


7A.2.2.5 Effect of the anaerobic condition

The reaction was studied in the anaerobic condition to observe the effect of the oxygen on the rate

of the reaction.


BP were prepared in the aqueous alkaline medium. The reaction mixtures were de-aerated by

purging with purified nitrogen for 30 min via a needle through the cap. The maximum

deoxygenated reaction mixtures of the 4-MT / 5-MT and the BP were exposed to the visible light. It

is also reported that thiol system does not absorb oxygen in the dark but show high reactivity with

oxygen when exposed to radiation. [34]

The rate of the reaction was calculated. The rate of the reaction shows marked decrease in the

anaerobic condition as compared to the aerobic condition. The participation of the singlet oxygen

during the photo reaction is an important step. (Table 7A.5)

The φ value of the reaction was calculated for the anaerobic condition of the reaction which

decreases in the anaerobic condition. The observation suggests that the oxidation reaction takes

place during the photo sensitized reaction.

Table 7A.5 Effect of the anaerobin condition on the rate of the reaction

Rate of the reaction in aerobic aqueous

alkaline medium K × 10-3

mole / L min

Rate of the reaction in anaerobic aqueous

alkaline medium K × 10-3

mole / L min

4-MT at

245 nm 7.86 2.303

5-MT at

240 nm 6.909 1.989

Table 7A.5 Effect of the anaerobin condition on the rate of the reaction at 11 pH, Substrate:

4-MT and 5-MT: (2 × 10-4


M), Time interval: 30

mins, Source: 100 W tungsten lamp


7A.2.2.6 Effect of the free radical scavenger

The effect of the solvent used as free radical scavenger on the photo sensitized reaction of the

methyl substituted thiazoles [4-MT / 5-MT] was studied by changing the medium from aqueous

alkaline to the methanolic alkaline.


BP were prepared in the alkaline methanolic solution and irradiated with the visible light. 4-MT and

5-MT show λmax at 245 nm and at 240 nm respectively in the alkaline methanolic solution. The rate

of the reaction was calculated.

The photochemical reaction shows decrease in the alkaline methanolic solvent suggests that there is

a free radical formation during the reaction. Similar results were obtained using NaN3 as a free

radical scavenger. (Table 7A.6)

Table 7A.6 Effect of the free radical scavenger on the rate of the reaction

Rate of the reaction in aqueous

alkaline medium K x 10-3

mole / L min

Rate of the reaction in alkaline

methanolic medium K x 10-3

mole / L min

4-MT

at245 nm

7.67 2.21

5-MT

at240 nm

6.78 1.87

Table 7A.6 Effect of the free radical scavenger on the rate of the reaction at 11 pH,

Substrate: 4-MT and 5-MT: (2 x 10-4M), Sensitizer: BP (1 x 10

-4M), Time



7A.3 Quantum efficiency

The quantum efficiency of the photo reactions of the methyl substituted thiazoles [4-MT / 5-MT]

and the BP have been calculated using potassium ferrioxalate actinometer and was determined with

a number of initial concentration of the methyl substituted thiazoles [4-MT / 5-MT].

The graph of the different initial concentrations of the methyl substituted thiazoles [4-MT / 5-MT]

and the quantum efficiency is a straight line with a positive slope which suggests that the quantum

yield of the reaction is dependent on the substrate concentration and the triplet – triplet energy

transfer from the triplet excited state of the benzophenone to the substrate molecule takes place

during the photo sensitized reaction. (Fig 7A.8)

The plot of the inverse of the quantum efficiency versus inverse of the concentration of the

substrate is a straight line with a positive slope suggests that the product formation takes place from

the triplet excited state of the substrate molecule and does not involve the exciplex formation. [35]

(Fig 7A.9)

Substrate

Concentartion

( C × 10-4

M )

4MT

Quantum

Efficiency

(Φ)

5MT

Quantum

Efficiency

(Φ)

Inverse of

substrate

concentration

(C × 103)

4MT

Inverse of

quantum

efficiency

5MT

Inverse of

quantum

efficiency

1.6 0.036 0.128 6.250 27.8 7.813

1.8 0.096 0.217 5.556 9.00 4.61

2.0 0.145 0.306 5.0 6.90 3.27

2.2 0.206 0.386 4.545 4.85 2.59

2.4 0.263 0.465 4.167 3.80 2.151

2.6 0.343 0.546 3.846 2.90 1.83

Table 7A.7 Different initial substrate concentration and quantum at 11 pH, Substrate: 4-

MT and 5-MT: (2 x 10-4M), Sensitizer: BP (1 x 10

-4M), Time interval: 10 mins,



Plot of quantum efficiency Vs substrate concentration

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

0.000

0.100

0.200

0.300

0.400

0.500

0.600

4MTLinear Regression for 4MT5MTLinear Regression for 5MT

Concentartion of the Substrate c Χ 10-4

Qua

ntum

Effi

cien

cy

Fig 7A.8 Plot of quantum efficiency Vs substrate concentration at 11 pH.

4-MT & 5-MT (1.6 × 10-4

M to 2.6 × 10-4

M)

BP (1 × 10-4

M)




Plot of inverse of the quantum efficiency Vs inverse of the substrate concentration

3.6 3.8 4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6

0.000

1.000

2.000

3.000

4.000

5.000

6.000

7.000

8.000

9.000

10.000

4MTLinear Re-gression for 4MT5MTLinear Re-gression for 5MT

Inverse of the substrate concentration

Inve

rse

of th

e qu

antu

m e

ffici

ency

Fig 7A. 9 Plot of inverse of the quantum efficiency Vs inverse of the substrate concentration at 11 pH

4-MT & 5-MT (0.8 × 10-4

M to 2.8 × 10-4

M)

BP (1 × 10-4

M)




7A.3.1 Effect of the light intensity on the quantum efficiency

The quantum efficiency of the reaction of 4-MT and 5-MT with BP as a sensitizer was determined

using light intensity in the range 5 – 25 E/s.

The solutions of the the different concentration of the substrate and the sensitizer BP were prepared

in the aqueous alkaline solution and irradiated with the visible light of different intensity for the

fixed time intervals. The quantum efficiency of the photoreaction was calculated.

The φ value remains constant in this range of the light intensity. The graph of φ value vs light

intensity is horizontal line with zero slope suggests a monophotonic reaction. (Fig 7A.10) (Table

7A.4)


Plot of light intensity Vs quantum efficiency

Fig 7A.10 Plot of light intensity Vs quantum efficiency at 11 pH.

4-MT & 5-MT (0.8 × 10-4

M to 2.8× 10-4

M)

BP (1 × 10-4

M)



0 5 10 15 20 25 30

0.0000

0.1000

0.2000

0.3000

0.4000

0.5000

0.6000

4MT

5MT

Light Intensity (I x 108) E/s

Qu

an

tum

Eff

icie

nc

y


7A.4 Photo product identification

The photo product of the reaction of the methyl substituted thiazoles [4-MT / 5-MT] and the BP in

aqueous alkaline solution has been evaluated under experimental condition.

The analysis of the reaction mixture for photo product does not show the presence of organic

compound and the test for S-2 and SCN- was found negative with lead acetate and FeCl3 solution.

Test for the SO4-2

ion: The reaction mixture was further tested for SO4-2 ion with BaCl2 and lead

acetate solution which gave white precipitation. The triplet excited state molecule of 4-MT and 5-

MT undergoes decomposition to give sulfate as a photo product.

7A.5 Discussion

The photo-reaction product of the methyl substituted thiazoles [4-MT / 5-MT] and BP were isolated

and analysed for the product identification.

The methyl substituted thiazoles [4-MT / 5-MT] show the same λmax and molar absorptivity in the

acidic solution as well as in the alkaline solution indicates that same molecular species is present in

the acidic and in the alkaline medium. The visible light is not absorbed by the methyl substituted

thiazoles [4-MT / 5-MT] as their λmax is below 300 nm and is photo stable.

The photosensitized reaction of methyl substituted thiazoles in the presence of BP is slow in the

acidic medium and the maximum reaction takes place at pH 10 and above.

The reaction mixtures of the methyl substituted thiazoles and the sensitizer BP show λmax at 245 nm

and at 240 nm for 4-MT and 5-MT respectively do not react in the ground state but when exposed

to the visible light the absorbance decreases with time at 245 nm and at 240 nm and remains

constant after 180 mins exposure indicating the completion of the reaction.

The benzophenone molecule absorbs near UV radiation present in the visible radiation and is

excited to the singlet state. The singlet excited BP undergoes ISC to give triplet excited state. The

triplet excited state of BP acts as proton abstractor [23] and also producer of singlet state O2 by

transferring energy to triplet state O2 molecule.

The rate of the photoreaction is independent of the concentration of the substrate and the

concentration of the sensitizer but is dependent on the light intensity.


The free radical acavenger effect of methyl alcohol and NaN3 show free radical formation. It

appears that triplet excited state of BP abstracts a proton from 4-MT / 5-MT forming an excited free

radical.

The singlet oxygen plays an important role in the reaction and oxygen consuming process proceeds

mainly via photochemical reaction. The rate of the reaction and the quantum efficiency have shown

decrease when the reaction is carried out in the anaerobic condition. The oxidation reaction takes

place by singlet state oxygen.

Tanielian et. al. [36] have reported that in the absence of O2 quenching, the reaction of triplet

benzophenone takes place both through hydrogen atom-abstraction to form radicals and through

quenching by the double bond.

The methyl substituted thiazole's free radical reacts with singlet oxygen and gives sulfate as a photo

product on ring cleavage.

Wang et. al. [34] have reported the hydrogen transfer reaction between thiol and free radical and

oxidation of thiol itself takes place easily.

The plot of the quantum efficiency vs the concentration of the substrate is linear with a positive

slope indicating that the energy transfer from the excited sensitizer molecule to the substrate

molecule involves a triplet state. The plot of the inverse of the quantum efficiency vs inverse of the

concentration of the substrate is a straight line with a positive slope suggests that the product

formation is taking place by excited triplet state and does not involve exciplex formation.

The singlet excited state of the BP molecule formed after absorption of the visible light undergoes

ISC and forms triplet excited state which transfers it's energy to the ground state of the methyl

substituted thiazoles and the methyl substituted thiazoles go to triplet excited state.

----------- (1)

----------- (2)

Where and

Eq 2 represents the dependance of inverse of the quantum efficiency upon the inverse of the

concentration of the substrate. As the concentration of the methyl substituted thiazoles increases,


the plot of the inverse of the quantum efficiency vs inverse of the concentration of the substrate is

linear with positive slope.

The horizontal graph of the quantum efficiency vs the light intensity with zero slope suggest a

monophotonic process during the product formation.

The benzophenone acts as a sensitizer for 4 - & 5 - methyl thiazoles in the triplet excited state. The

experimental observation suggests that triplet excited state of the benzophenone acts a proton

abstactor as well as a singlet state oxygen producer.

The excited benzophenone molucule abstracts a proton from 4 - and 5 - methyl thiazole to produce

a free radical. The free radical undergoes oxidative decomposition to give SO4-2

as the final product.

School of Sciences, Gujarat University, Ahmedabad, INDIA. 7

7A.6 Mechanism

Benzophenone (BP) absorbs visible radiation and forms singlet excited molecule. This singlet

excited state of BP gives triplet excited state after ISC.

The methyl substituted thaizoles have absorbance below 400 nm and do not absorb visible light and

are photo stable. The photosensited reactions of methyl substituted thiazoles with BP involve both

the proton abstration and photo – oxygenation reaction. The reaction follows the Type I photo-

oxygenations reaction mechanism in which the sensitizer triplet state (3S*) interacts with a substrate

(XH), which gives rise to a free radical, by electron-transfer or hydrogen-transfer mechanisms. The

free radicals produced react with singlet oxygen to regenerate the sensitizer to ground state and to

form the photo product.

The triplet excited state of BP acts as proton abstractor and abstract proton from the CH3 of the

methyl thiazole and forms the methylene thiazole free radical in the triplet excited state and BP

molecule comes back to the ground state. The excited methylene thaizole free radical undergoes

photo – oxygenation by singlet oxygen and gives sulfate and alkylcynide as a photo product on ring

cleavage.


substrate suggests that photo-product is formed from the triplet excited state of methyl substituted

thaizoles and no exciplex formation takes place during the photo reaction.


Mechanism of 4-MT (5-MT) Degradation


7A.7 Conclusion

The photosensitized reaction of the 4 – Methylthiazole (4-MT) & 5 – Methylthiazole (5-MT) with

benzophenone (BP) in alkaline medium using visible light has been studied. The triplet – triplet

energy transfer from the triplet excited state of the aromatic ketone benzophenone to the substrate

molecule takes place during the photo sensitized reaction. The triplet excited 4-MT/5-MT breaks

down on further exposure and photo product formation takes place. The reaction shows the

participation of singlet oxygene during the photo reaction. The triplet excited state of BP acts as

proton abstractor and abstract proton from the CH3 of the methyl thiazole and forms the methylene

thiazole free radical in the triplet excited state and BP molecule comes back to the ground state. The

excited methylene substituted thaizole’s fee radical undergo photo – oxygenation by singlet

molecule oxygen and gives sulfate and alkylcynide as a photo product on ring cleavage.


7A.8 Referances

[1] U.S. Pat. 4 161 571; 17/7/79.

[2] U.S. Pat. 4 080 493; 21/3/78.

[3] U.S. Pat. 5 962 732; 5/10/99.

[4] U.S. Pat. 4 740 471; 26/4/88.

[5] U.S. Pat. 6 127 481; 3/10/00.

[6] U.S. Pat. 5 439 950; 8/8/95.

[7] U.S. Pat. 5 726 172; 10/3/98.

[8] U.S. Pat. 3 950 522; 13/4/76.

[9] U.S. Pat. 5 843 971; 1/12/98.

[10] Maurizio D' Auris, Tetrahedron, 58(40), (2002), 8037-8042

[11] J. P. Catteaum, Labalche – Combier and A. Pollet, J. Chem. Soc. Chem. Commun, (1969),

1018

[12] J. K. Pavlik, C. R. Pandit, C. J. Samuel and A. C. Day, J. Org. Chem, 58, (1992), 3407

[13] James W. Pavlik, Chennagiri R. Pandit, Christopher J. Samuel and A. Colin Day, J. Org.

Chem., 58, (1993), 3407–3410

[14] J.M.Pena, N.S. Allen, M.Edge, C.M Liauw, I. Roberts and B. Valange, Polymer

degradation and stability,70, (2000) 437-454.

[15] C.H. Tung, L.P.Zhang and Y. Li., Macromolecules, 31, (1998), 8794-8801.

[16] Y. Kawamura, R.TaKayama, M. Nishiuchi and M.Tsukayama., Tetrahedron letters,

41, (2000), 8101-8106.


[17] S.Canonica, B.Hellrung, P.Muller and J.Wirz., Environ.Sci.Tech., 40(21), (2006),

6636-6641.

[18]Y. Pan, Y.Fu, S.Liu, H.Yu, Y.Gao , Q.Guo and S.Yu, J.Phys.Chem.A, 110,(2006),

7316-7322.

[19] S.E. Morsia, A.B. Zakia, S.H. Etaiwa, W.M. Khalifaa and Al-Sayed Salem, Polymer, 22(7),

(1981), 942-945

[20] Backstrom L. I., Z. Physik. Chem. Abt. B, 25, (1934), 99-138.

[21] Jawaid Iqbal, Adil Husain and Anamika Gupta, Acta Chim. Slov. 2005, 52, 455–459

[22] C.Marti, O.Jurgens, O.Cuenca, M.Casals and S. Nonell, J.Photochem. Photobiol. A:

Chemistry 97, (1996), 11-18.

[23] K.Yamada, M.Sato, K. Tanaka, A. Wakabayashi, T. Igarashi and T. Sakurai, Journal

of Photochemistry and Photobiology A: Chemistry, 183(1-2), (2006), 205-211

[24] R. G. Brinson and P. B. Jones, J. Photochem. PhotoBio. C, 175(2-3), (2005), 118-124.

[25] Teijiro Ichimura, T. Suzuki, M. Nagano and S. Watanabe, J. Photochem. Photobiol. A:

Chemistry, 136, (2000), 7 - 13

[26] S. Jockusch, H. J. Timpe, W. schnabel and N. J. Turro, J. Phys. Chem., A. 101(4),

(1997), 440.

[27] K.Yamada, M.Sato, K. Tanaka, A. Wakabayashi, T. Igarashi and T. Sakurai, Journal

of Photochemistry and Photobiology A: Chemistry, 183(1-2), (2006), 205-211

[28] R. G. Brinson and P. B. Jones, J. Photochem. PhotoBio. C, 175 (2-3) ,(2005), 118-124.

[29] R. E. Galian, Pastor perez L., M.A. Miranda and J.Perez-pgieto, J.Am.Chem.Soc. 127,

(2005), 255-269.


[30] X.Cai, M. Hara, K. Kawai, S. Tojo and T. Majima, Chem. Phy. Lett., 371, (2003), 68-

73.

[31] R.N.Goyal, Rajeshwari and N.C.Mathur, J.Indian Chem.Soc., LVX, (1988), 490-494

[32] M. V. Encinas and J. C. Scaiano, Reaction of benzophenone triplets with allylic hydrogens. A

laser flash photolysis study, J. Am. Chem.Soc., 1981, 103, 6393–6397.

[33] K. K. Rohatgi-Mukherjee, “Fundamentals of photochemistry”, 3rd Eds., New age

international (P) limited, New Delhi, India, (1997)

[34] Wang Erjian, Li Miaozhen, chang zhiying and Feng Xinde, Chinese Journal of Polymer

science, No 3, 1987, 214 – 219.

[35] P. K. Freeman, Jung-Suk Jang and N. Ramnath, J. Org. Chem. 56, (1991), 6072

[36] Charles Tanielian, Claude Schweitzer,‡ Rachid Seghrouchni, Marc Esch and Robert Mechin,

Photochem. Photobiol. Sci., 2003, 2, 297–305


Chapter 7B

Photosensitized Proton Abstraction

Reaction of 2–Methyl 2-Thiazoline

with Benzophenone

Abstract

The photosensitized reaction of the 2 – Methyl 2 - thiazoline (2-MTh) with benzophenone (BP) in alkaline medium using visible light has been studied. The spectrum of the 2-MTh shows λmax at 260 nm in the acidic medium while in the alkaline medium it absorbs at 230 nm and 245 nm nearly in equal concentration. The two tautomeric forms of 2-MTh are in equilibrium in the pH range 8 – 12. The triplet – triplet energy transfer from the triplet excited state of the aromatic ketone benzophenone to the substrate molecule takes place during the photo sensitized reaction. The triplet excited 2-MTh breaks down on further exposure and photo product formation take place. The thiocyanate has been observed as photo product. The rate of the reaction has been calculated and the effect of pH, concentration of the sensitizer, the light intensity on the rate of the reaction has been studied. The quantum efficiency of the photo chemical reaction is calculated using potassium ferri oxalate actinometer and the effect of the concentration of the substrate on the quantum efficiency is calculated. The reaction mechanism and the excited states involved have been suggested.

7B.1 Introduction

2 Methyl 2- thiazoline (2-MTh) is pale yellow liquid to solid having sulfurous musty meaty nutty

odor. It has m.p. at 62.0°C and boiling Point at 143.0 to 145.0°C @ 760.00 mm Hg. It is in

flammable and is irritating to skin and eyes.

2-MTh is an industrially important compound used in manufacturing of flavoring agents and in the

products like dairy products, fats, oils, and fat emulsions, processed fruit, processed vegetables,

confectionery, cereals and cereal products, for particular nutritional uses, non-alcoholic ("soft")

beverages, dairy products, alcoholic beverages, incl. alcohol-free and low-alcoholic counterparts,

ready-to-eat and composite foods (e.g. casseroles, meat pies, mincemeat).

Lang et. al. [1, 2] have investigated the opening of the ring of 2-methyl- thiazoline to give the

sulfhydryl compound N- acetyl- P-mercaptoethylamine and its S-acetyl isomer. Calvin [3] have


reported that the thiazoline ring formation occurs in strongly acid solutions of glutathione and this

has been supported by Garfinkel [4] and Martin [5]. The rate of hydrolysis of 2-methyl 2-thiazoline

has been studied by Martins et. al. [6] as a function of pH from concetitrated HC1 to buffer

solutions at pH 9.

Matsuura et. al. [7] have reported that the photoaromatization of 2-methyl 2- thiazoline gave N –

vinylthioacetamide as a photoproduct in acetone solution.

Sharma et. al. [8] have reported the photochemical reaction of indol-2, 3-dione derivatives with 2-

thiazoline-2-thiol. Pardasan et. al. [9] reported the reaction of indol-2.3-dione derivatives with 2-

thiazoline-2-thiol under thermal as well as photochemical conditions.

Smith et. al. [10] have reported the hydrolysis of 2-methyl-2-thiazoline-4-carboxylic acid. Sheehan

et. at. [11] have reported the reaction of 2 – methyl 2 – thiazoline with phthaloglycyl acid. Durstan

et. al. have reported the acylation of 2 – methyl 2 – thiazoline [12].

Azam et. al. have reported the metallation of 2-methyl-2-thiazoline at a triosmium cluster at

ambient temperature [13].

The photo-degradation may play an important role in the elimination of 2-methyl-2-thiazoline from

aquatic environment. The photodegradation study of substited 2-methyl 2-thiazoline using aromatic

ketone as a sensitizer in the presence of visible light is not reported elsewhere.

The present study reports the photochemical reaction of methyl thiazoline in the aqueous alkaline

medium on irradiation with visible light. The aromatic ketone, benzophenone has been used as a

photo sensitizer [14-16] in a number of photochemical reraction. It shows energy transfer process

by photo – oxygenation mechanism and by proton abstraction mechanism. There are very few

report in the literature for the sensitized study of methyl thiazoline.

Benzophenone has been used as photo sensitizer by a number of worker with different type of

compound. Canonica et. al. [17] have reported the aqueous oxidation of phenylurea herbicides by

triplet excited state of aromatic ketones via singlet O2. A laser flash photolysis study for the

reactivity of aromatic amines with triplet 1, 8 dihydroxy anthraquinone has been reported by Y.Pan

et. al. [18]. Morsi et. al. [19] have reported the photo-oxidation of cis-polyisoprene by singlet

oxygen in the presence of the sensitizer benzophenone by UV radiation. Backstrom [20] has

presented the mechanism for the benzophenone-photosensitized oxygenation of alcohols and

aldehydes. Iqbal et. al. [21] have reported the photo oxygenation of tinosponone with singlet


oxygen using different combinations of sensitizers, solvents and singlet oxygen scavengers. Nonell

et al. [22] have reported that aromatic ketones as standards for singlet molecular oxygen

photosensitization.

Benzophenone shows proton abstraction [23] and electron transfer reactions [24] in the excited

triplet state. Teijiro Ichimura et al [25] has reported kinetic study by the quenching the rate of DPA

by triplet benzophenone photo sensitizer and have also reported that the excitation energy effect on

the reaction with 2- bromo methyl naphthalene. S. Jockusch et al [26] have reported that Photo-

induced energy and electron transfer processes between ketone triplet state and organic dyes

(methylene blue, thiopyrinine, safranine and phenosafranine). Yamada et. al. [27] have reported that

Nitrile-forming radical elimination reactions of 1-naphthaldehyde O-(4-substituted benzoyl) oximes

activated by triplet benzophenone. Brinson et. al. [28] have reported the proton abstraction and

electron transfer photo reaction by anthraquinone. Galian et. al. [29] have reported that the

intramolecular electron transfer between tyrosin and tryptophan photo sensitized by aromatic

ketone. Cai et. al. [30] have suggested mechanism of sensitized reaction by benzophenone in the

triplet excited state.

The present study reports the photosensitized reaction of the 2-MTh with the benzophenone (BP) as

a sensitizer in the aqueous alkaline medium in the presence of the visible light. The reaction is

monitored by measuring the spectrum change of 2-MTh. The kinetics of the photo reaction and the

mechanism of the photo sensitized reaction have been studied.

7B.2 Results

7B.2.1 Spectral characteristics

The UV spectrum of the pure 2-MTh (1.5 X 10-4 M) was recorded at different pH, to determine the

different species present at the different pH of the aqueous solution. The pH of the solution was

maintained using suitable concentration of the HCl or the NaOH in the solution.

The initial spectrum of 2-MTh (1.5 X 10-4 M) in the aqueous acidic solution in the pH range 2 - 6 is

represented by the continuous line in Fig 7B.1 which exhibits one well-defined maximum at 260

nm (ε = 5,300 L mol-1 cm-1).

2-MTh shows (Fig 7B.1) maximum at 230 nm (ε = 2,700 L mol-1 cm-1) and at 245 nm (ε = 2,653 L

mol-1 cm-1) in the aqueous alkaline solution in the pH range 8 – 12 (dash line).


Spectrum of 2-MTh

200 220 240 260 280 300 320

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

Acidic Medium

Alkaline Medium

Wavelength (nm)

Ab

so

rba

nc

e

Fig 7B.1 Spectrum of 2-MTh (1.5 × 10-4

M)

(a) In acidic medium at pH 2 (

)

(b) In alkaline medium at pH 11.5 (----)


The spectrum of the 2-MTh is stabilised in the acidic medium and shows only one λmax at 260 nm.

The two tautomeric forms are observed in the alkaline medium which absorb at 230 nm and 245 nm

nearly in equal concentration. The two tautomeric forms of 2-MTh are in equilibrium in the pH

range 8 – 12. The marked changes have been observed in the spectrum of 2-MTh in the aqueous

acidic and in the aqueous alkaline medium. Martin et. al. [6] have reported that the spectrum of the

free base in water is very close to that of the pure liquid, whereas marked differences occur on the

addition of a proton to the nitrogen of the thiazoline ring.

Goyal et al [31] have reported that the UV spectral behaviour at pH < pKa and pH > pKa is

essentially the same in the case of substituted thiazole compound. The λmax and molar

absorptivities of the tautomeric forms of 2-MTh in the acidic and in the alkaline aqueous medium

are quoted in Table 7B.1.

The exposure of the solution containing only 2-MTh (2 X 10-4 M) in the pH range 2 – 12 to the

visible radiation does not result in the change of the original spectrum of the solution. The direct

photolysis of 2-MTh on irradiation by the visible light does not occur in the acidic or in the alkaline

medium and 2-MTh is photo stable.

The reaction mixture of 2-MTh with the suitable concentration of BP was prepared and the pH of

the solution was maintained from 2 to 12. The solutions were kept in the dark and their spectrums

were recorded against a reagent blank which matched with the original spectrum of the 2-MTh for

the solution having pH value 2 – 6. 2-MTh and BP do not interact in the ground state in the aqueous

acidic solution.

The spectrum of 2-MTh with the suitable concentration of BP shows difference from the original

spectrum of 2-MTh only in the aqueous alkaline medium in the pH range 8 - 12. 2-MTh and BP do

not interact in the ground state in the aqueous alkaline medium but it shows only one broad band

due to the merging of two separate bands in the aqueous alkaline medium instead of two bands at

230 nm and 245 nm.

The solutions containing 2-MTh (2 X 10-4 M) and BP (1 X 10-4 M) maintained at different pH

between 2 to 12 were exposed to the visible radiation and the spectrum of the solutions were

recorded against the reagent blank. The solutions having pH 2 to 6 show very small change in the

spectrum but solutions having pH 8 to 12 show much faster change in the spectrum of 2-MTh

undergoes very slow reaction in the acidic medium with BP when exposed to the visible radiation

but it shows much faster photoreaction in the presence of BP used as a sensitizer in the alkaline

medium.


Absorption spectra of the photosensitized reaction of 2-MTh with BP

200 210 220 230 240 250 260 270 280 290 300

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

T0

T30

T60

T90

T120

T150

T180

T210

T240

Wavelength (nm)

Absorb

ance

Fig 7B.2 Absorption spectra of the photosensitized reaction of 2-MTh with BP on exposure at 11 pH.

2-MTh (2 × 10 -4

M)

BP (1 × 10-4

M)




The change in the spectrum of 2-MTh is observed when the reaction mixture containing a suitable

concentration of BP in the aqueous alkaline medium is exposed to the visible radiation.The

spectrum of the photo sensitized reaction was recorded against the reagent blank. (Fig 7B.2) The

absorbance of the broad band decreased and an intence band at 220 nm was observed. The product

of the photo-reaction absorbs at a shorter wavelength in comparison to the starting material. The

decrease of the absorbance with the time was monitored and an isobestic point was not observed.

Table 7B.1 λmax and molar absorptivity of 2-MTh in the acidic and alkaline aqueous

medium

Medium λmax Molar Absorptivity

Acidic (pH 3) 260 nm 5,300 L mol-1 cm-1

Alkaline (pH 11) 230 nm

245 nm

2,700 L mol-1 cm-1

2,653 L mol-1 cm-1

Table 7B.1 λmax and molar absorptivity of 2-MTh in the aqueous solution of the pH range

2-12, Substrate: 2-MTh: (1.5 × 10 -4

M), Acid Medium: pH 3, Alkaline Medium:

pH 11

7B.2.2 Determination of the rate constant

The progress of the photosensitized reaction was monitored by recording UV spectra of 2-MTh (2

X 10-4 M) with BP (1 X 10-4 M) in the range 200 - 400 nm at different time interval. The spectrum

of the 5ml aliquot of the exposed solution, withdrawn after 10 min time intervals in the range of 200

nm – 400 nm against the reagent blank were recorded. The absorbance of the solution was also

measured at 230 nm.

The absorbance decreases at the broad band of 2-MTh and a band at 220 nm is observed on

irradiation of the reaction mixture and becomes constant after 210 mins exposure. The absorbance

shows different rate of decrease of 2-MTh broad band having λmax at 230 nm and 245 nm. It is

observed that the absorbance at 245 nm decreases at a faster rate than at 235 nm. The decrease of

the absorbance with time for 2-MTh at the λmax 245 nm have been used to calculate the rate of the

reaction. The results of a typical run for the change in the absorbance of 2-MTh with time have

been presented in Fig 7B.3.


Absorption Vs time plot.

0 50 100 150 200 250 300

0.000

0.100

0.200

0.300

0.400

0.500

0.600

Time (min)

Absorb

ance

Fig 7B.3 Absorption Vs time plot on exposure at 11 pH.

2-MTh (2 × 10-4

M)

BP (1 × 10-4

M)




2 + log OD Vs Time plot

0 50 100 150 200 250 300

0.8

1

1.2

1.4

1.6

1.8

2

Time (min)

2 +

log O

D

Fig 7B.4 2 + log OD Vs Time plot of the photosensitized reaction of 2-MTh with BP.

2-MTh (2 × 10-4

M)

BP (1 × 10-4

M)




The change in the absorbance with time has been used for the calculation of the rate of the reaction.

The reaction follows the first order reaction kinetics as the plot of 2 + log OD (optical density) vs.

time is a straight line with a positive slope. (Fig 7B.4) The rate constant has been determined by the

expression:

Rate constant = 2.303 x slope

7B.2.2.1 Effect of the pH on the rate of the reaction

The spectrum of the pure 2-MTh solution (2 X 10-4 M) was recorded at different pH in the range of

200 nm – 400 nm and the effect of the pH on the rate of the photosensitized reaction was carried out

by changing the pH of the solution between pH 2 – 12 and the calculations for the rate of the

reaction were carried out at different pH. (Table 7B.2)

The spectrum of 2-MTh shows the λmax at 260 nm in the acidic medium and a broad band having

the λmax at 230 nm and 245 nm in alkaline medium. The photosensitized reaction takes place slowly

between the pH 2 – 6 but increases till the pH of the solution is 10 and then becomes constant. (Fig

7B.5) The subsequent studies were carried out at pH 11. The photosensitized reaction of 2-MTh

with BP occurs faster in the alkaline medium.

The photo effect of Benzophenone is sensitive to OH- ion concentration of the solution therefore the

increase in OH- ion concentration increases the sensitivity of the sensitizer, which shows higher

proton abstraction capacity of benzophenone [32]. Similar effect of OH- ion concentration has been

observed in the present study.


Table 7B.2 Effect of pH on the rate of the reaction

pH of the solution 2-MTh Rate of the Reaction (k x 10

-3)


2 0.384

4 0.384

6 0.768

8 5.37

10 6.14

11 6.14

11.50 6.14

Table 7B.2 Effect of the pH on the rate of the reaction, Substrate: 2-MTh: (2 × 10 -4

M),

Sensitizer: BP (1 × 10-4

M), Source: 100 W tungsten lamp


Effect of the pH on the rate of the reaction

0 2 4 6 8 10 12 14

0.000

1.000

2.000

3.000

4.000

5.000

6.000

7.000

pH of the solution

Rat

e of

the

reac

tion

( K x

10-

3 ) m

ole

/ L m

in

Fig 7B.5 Effect of the pH on the rate of the reaction

2-MTh (2 × 10 -4

M)

BP (1 × 10-4

M)




7B.2.2.2 Effect of the concentration of the sensitizer on the rate of the reaction

The effect of the concentration of the BP on the rate of the photosensitized reaction has been

studied. The study was carried out by varying the concentration of the BP in the range of 0.6 x 10-4

M to 1.6 x 10-4 M.

The rate of the reaction remains constant with the increase of the BP concentration in the reaction

mixture. The results indicate that 1 x 10-4 M BP had an optimal concentration and gave the best

performance under the experimental condition. (Fig 7B.6)


7B.2.2.3 Effect of the concentration of the substrate on the rate of the reaction

The effect of the initial concentration of 2-MTh on the rate of the reaction was studied. The rate of

the reaction has been calculated with the different initial concentration of the substrate in the range

of (1.6 – 2.4) x 10-4 M. The rate of the reaction remains constant initially and than slightly decreases

at the higher concentration of the substrate. It shows that the reaction is independent of the initial

concentration of the substrate. (Fig 7B.7) The half life time of the reaction has also been calculated

at the different concentration of the substrate and t1/2 value is constant over the above range of the

substrate concentration. The observation suggests that the photochemical reaction is of the first

order. (Table 7B.3)

Table 7B.3 Effect of the concentration of substrate on the rate of the reaction

Concentration of

Substrate C x 10-4

2-MTh Rate of the Reaction

(K x 10-3

) mole / L min

[Avg K + 0.4]

t ½

L min / mole

1.6 6.14 112.87

1.8 6.14 112.87

2.0 6.14 112.87

2.2 6.14 112.87

2.4 5.73 120.94

Table 7B.3 Effect of the substrate concentration on the rate of the reaction at 11 pH,

Substrate: 2-MTh: (2 × 10-4 M), Sensitizer: BP (1 × 10-4 M), Time interval: 30



Effect of the sensitizer concentration on the rate of the reaction

4 6 8 10 12 14 16 18

0.000

2.000

4.000

6.000

8.000

10.000

Concentration of Sensitizer (c x 10-5) M

Rate

of

the r

eactio

n (

k x

10-3

) m

ole

/ L

min

Fig 7B.6 Effect of the sensitizer concentration on the rate of the reaction at 11 pH.

2-MTh (2 × 10-4

M)

BP (0.6 × 10-4

M to 1.6 × 10-4

M)




Effect of the substrate concentration on the rate of the reaction

1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5

0.000

2.000

4.000

6.000

8.000

10.000

Concentration of Substrate ( c x 10-4) M

Rat

e of

the

Rea

ctio

n (k

x 1

0-3)

mol

e / L

min

Fig 7B.7 Effect of the substrate concentration on the rate of the reaction at 11 pH.

2-MTh (1.6 × 10-4

M to 2.4 × 10-4

M)

BP (1 × 10-4

M)




7B.2.2.4 Effect of the light intensity

The solutions of the the different concentration of the substrate [2-MTh] and the sensitizer BP were

prepared in the aqueous alkaline solution and irradiated with the visible light for the fixed time

intervals. The rate of the reaction was calculated.

The increase of the light intensity [Einstein / second (E/s)] shows positive effect and the rate of the

reaction increases as the light intensity increases. The higher number of the photons increases the

number of the excited sensitizer molecule and the rate of the reaction also increases. A linear

relationship has been observed between the light intensity and the rate of the reaction. (Table 7B.4)

Table 7B.4 Effect of the Light Intensity on the rate of the reaction and on the quantum

efficiency

Light Intensity

( I × 108

) E/s

2-MTh

(k × 10-3

) mole / L min [K + 0.5]

2-MTh

Quantum Efficiency

5 2.24 0.3305

10 4.86 0.3311

15 6.68 0.3306

20 9.05 0.3307

25 11.97 0.3302

Table 7B.4 Effect of the light intensity and quantum efficiency on the rate of the reaction at

11 pH, Substrate: 2-MTh: (2 × 10 -4


M), Time



7B.2.2.5 Effect of the anaerobic condition

The reaction was studied in the anaerobic condition to observe the effect of the oxygen on the rate

of the reaction.


prepared in the aqueous alkaline medium.The solutions of the reaction mixture were de-aerated by

purging with purified nitrogen for 30 min via a needle through the cap. The maximum

deoxygenated reaction mixture of the 2-MTh and the BP was exposed to the visible light. The rate

of the reaction was calculated. The rate of the reaction does not show change and remains the same

as in the aerobic condition. (Table 7B.5)

The φ value of the reaction was also calculated in the anaerobic condition of the reaction which

remains constant and same as in the aerobic condition. This suggests that the singlet oxygen does

not participate in the photosensitized reaction.

Table 7B.5 Effect of the anaerobin condition on the rate of the reaction

Rate of the reaction in aerobic aqueous


mole / L min

Rate of the reaction in anaerobic aqueous


mole / L min

2-MTh 6.58 6.23

Table 7B.5 Effect of the anaerobin condition on the rate of the reaction at 11 pH, Substrate:

2-MTh: (2 × 10 -4


M), Time interval: 30 mins,



7B.2.2.6 Effect of the free radical scavenger

The effect of the solvent used as free radical scavenger on the photo sensitized reaction of 2-MTh

was studied by changing the medium from aqueous alkaline to alkaline methanol.


prepared in the alkaline methanol and irradiated with the visible light. 2-MTh shows the broad band

at the λmax at 230 nm (ε = 2,600 L mol-1 cm-1) with a sholder at 245 nm (ε = 2,250 L mol-1 cm-1) and

a small peak at 266 nm (ε = 1,000 L mol-1 cm-1) in the alkaline methanolic solution. The rate of the

reaction was calculated.

The photochemical reaction shows decrease in the alkaline methanolic solution suggests that there

is free radical formation during the reaction. The similar results were obtained using NaN3 as a free

radical scavenger. (Table 7B.6)

Table 7B.6 Effect of the free radical scavenger on the rate of the reaction

Rate of the reaction in aqueous


mole / L min

Rate of the reaction in alkaline

methanolic medium K x 10-3

mole / L min

2-MTh 6.67 2.321

Table 7B.6 Effect of the free radical scavenger on the rate of the reaction at 11 pH,

Substrate: 2-MTh: (2 × 10 -4


M), Time interval: 10



7B.3 Quantum efficiency

The quantum efficiency of the photo reaction of the 2-MTh and the BP has been calculated using

potassium ferrioxalate actinometer and was determined with a number of initial concentration of the

2-MTh.

The graph of the different initial concentrations of the 2-MTh and ø-values is a straight line with a

positive slope which suggests that the quantum yield of the reaction is dependent on the substrate

concentration. (Fig 7B.8)


substrate is a straight line with a positive slope which suggests that the triplet excited substrate

decomposes to form a radical of the substrate in the triplet excited state and the product formation is

via triplet excited state of the substrate [33] (Fig 7B.9).

Substrate

Concentartion

( C × 10-4

M )

Quantum Efficiency

(Φ)

Inverse of substrate

concentration

(C × 103)

Inverse of quantum

efficiency

1.6 0.026 6.250 38.46

1.8 0.086 5.556 10.31

2.0 0.135 5.0 7.407

2.2 0.196 4.545 5.102

2.4 0.257 4.167 3.891

2.6 0.330 3.846 3.03

Table 7B.7 Different initial substrate concentration and quantum at 11 pH, Substrate: 2-

MTh: (2 × 10 -4


M), Time interval: 10 mins, Source:

100 W tungsten lamp


The plot of quantum efficiency Vs substrate concentration

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

Concentartion of Substrate ( c x 10-4) M

Qua

ntum

Eff

icie

ncy

Fig 7B.8 Plot of quantum efficiency Vs substrate concentration at 11 pH.

2-MTh (1.6 × 10-4

M to 2.6 × 10-4

M)

BP (1 × 10-4

M)




Plot of inverse of the quantum efficiency Vs inverse of the substrate concentration

3.500 4.000 4.500 5.000 5.500 6.000

0.000

2.000

4.000

6.000

8.000

10.000

12.000

1/ concentartion of substrate

1/

qu

an

tum

effi

cie

nc

y

Fig 7B.9 Plot of inverse of the quantum efficiency Vs inverse of the substrate concentration at 11 pH

2-MTh (1.6 × 10-4

M to 2.8 × 10-4

M)

BP (1× 10-4

M)




7B.3.1 Effect of the light intensity on the quantum efficiency

The quantum efficiency of the reaction of 2-MTh with BP as a sensitizer was determined using light

intensity in the range 5 – 25 E/s.

The solutions of the the different concentration of the substrate and the sensitizer BP were prepared

in the aqueous alkaline solution and irradiated with the visible light of different intensity for the

fixed time intervals and the quantum efficiency was calculated.

The φ value remains constant in this range of the light intensity. The graph of φ value vs light

intensity is horizontal line with zero slope suggests a monophotonic reaction. (Fig 7B.10) (Table

7B.4)


Plot of light intensity Vs quantum efficiency

0.000 5.000 10.000 15.000 20.000 25.000 30.000

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0.600

Light Intensity (I x 108) E/s

Quantu

m E

ffic

iency

Fig 7B.10 Plot of light intensity Vs quantum efficiency

2-MTh (1.6 × 10-4

M to 2.8 × 10-4

M)

BP (1× 10-4

M)




7B.4 Photo product identification

The λmax and the molar absorptivity of the photo product of the reaction of the 2-MTh and the BP

in aqueous alkaline solution have been evaluated under experimental condition. The molar

absorptivity of the substrate and product has been calculated by measuring the absorbance of a

number of known concentration solutions.

A sharp absorption band having λmax at 220 nm is observed which remains constant. The analysis of

the reaction mixture for photo product shows the presence of organic compound and the test for S-2

and SCN- was found negative with lead acetate and FeCl3 solution. The photo product shows the

absorbance maxima in UV spectrum at 220 nm.

7B.4.1 Test for the Methyl thiocyanate: The reaction mixture gave the specific onion

odor. The photoproduct was isolated by the procedure described in the chapter 2 and analysed by

GC-MS. A single peak was observed in the gas chromatogram suggesting that there is only one

photoproduct. The RT of the reaction product matches with the RT of the standard solution of

Methylthiocyanate. (Fig 7B.11 & 7B.12) The mass spectrum of the isolated product shows two

main peaks. The peak at: m/z = 73 (M+1, 100 %) which is the base peak also corresponds to [m+1]

protonated molecular ion. The second is observed at m/z = 72.

It appears the photoproduct of the photo reaction of 2-MTh with BP is methylthiocyanate. The

triplet excited state molecule of 2-MTh undergoes decomposition to give methylthiocyanate as

photo product.


GC spectra of the photoproduct of the photosensitized reaction of 2-MTh and BP

7B.11 Mass spectra of the photoproduct of the photosensitized reaction of 2-MTh and BP


7B.12 GCMS spectra of the Methyl thiocyanate from library


7B.5 Discussion

The photo-reaction product of 2-MTh and BP were isolated and analysed for product identification.

The λmax and molar absorptivity of the photo product has been calculated and compared with

irradiated reaction mixture of the the standard methylthiocyanate with BP in the same reaction

condition and result suggest that methylthiocyanate is the photoproduct.

The 2-MTh shows different λmax and molar absorptivity in the acidic solution and in the alkaline

solution. 2-MTh shows one absorption band at 260 nm in the aqueous acidic medium and two

absorption bands at 230 nm and 245 nm for the two tautomeric forms in the aqueous alkaline

medium. Both the tautomeric forms have nearly equal concentartion and are in equilibrium. The

equilibrium is shifted to the deprotonated form as the deprotonated form of 2-MTh undergoes

degradation in the alkaline medium.

The visible light is not absorbed by the 2-MTh as it's λmax is below 300 nm and is photo stable.

Benzophenone (BP) absorbs visible radiation and is excited to the singlet state and undergoes ISC

to give triplet state which shows photosensitized reaction.

The reaction mixture of the 2-MTh and the sensitizer BP also shows λmax at 260 nm and do not react

in the ground state in the acidic solution but in the alkaline medium reaction mixture shows a broad

peak having λmax at 230 nm & 245 nm. The reaction mixture when exposed to the visible light

shows decrease in the absorbance with time. The absorbance at 245 nm decreases at a faster rate

than at 235 nm and a new λmax appears at 220 nm which remains constant after 210 mins exposure.

The influence of pH on the photo sensitized reaction has been investigated, the pH of the medium

affects the photo chemical reaction. The lone pair of electrons present on nitrogen atom co-ordinate

with the proton in the acidic medium. The protonated form of the benzophenone does not form an

exciplex with the alkyl group of the substrates.

The pH effect study on the rate of the reaction suggests that only deprotonated species of the

benzophenone undergo photo sensitized reaction in the pH range 8 to 12. The rate constant of the

photosensitized reaction has been calculated. It has been observed that the maximum reaction takes

place at pH 10 and above. The rate of the photoreaction is independent of the concentration of the

substrate and the concentration of the sensitizer but is dependent on the light intensity.

The rate of the reaction and the quantum efficiency remain same when the reaction was carried out

in the anaerobic condition as in the aerobic condition. The reaction does not show participation of

O2 and oxidation process is not involved in the product formation.


The free radical scavenging effect of the methanol is positive. The photo reaction proceeds via a

free radical mechanism.

Hydrogen abstraction and intramolecular single electron transfer both reactions have also been

observed by Jones et. al [34]. Mohan et al. [35, 36] have reported electron and proton transfer

process in the reaction involving benzophenone.

The plot of the quantum efficiency vs the concentration of the substrate is linear with a positive

slope indicating that the quantum yield of the reaction is dependent on the substrate concentration.

The plot of the inverse of the quantum efficiency vs inverse of the concentration of the substrate is

is a straight line with a positive slope which suggests that the triplet excited substrate decomposes

to form a free radical of the substrate in the triplet excited state and the product formation is via

triplet excited state of the substrate.

The singlet excited state of the BP molecule formed after absorption of the visible light undergoes

ISC and forms triplet excited state which transfers it's energy to 2-MTh and then comes back to the

ground state.

----------- (1)

----------- (2)

Where and

Eq 2 represents the dependance of the inverse of the quantum efficiency upon the inverse of the

concentration of the substrate. The plot of the inverse of the quantum efficiency vs inverse of the

concentration of the substrate is linear with positive slope indicating that the energy transfer from

the triplet excited sensitizer molecule to substrate molecule involves a triplet state of the 2-MTh

without exciplex formation during the reaction.

The horizontal graph of the quantum efficiency vs the light intensity suggest a monophotonic

process during the product formation.

The free radical formation occurs by the fission of the triplet excited substrate to give the triplet

excited 2-MTh free radical. The triplet excited 2-MTh free radical gives methylthiocyanate photo

product on ring cleavage.


7B.6 Mechanism

The 2-MTh exists as two tautomeric forms in equilibrium in the aqueous alkaline medium and have

λmax at 230 nm and 245 nm which do not absorb visible light and are photo stable.

Benzophenone (BP) absorbs visible radiation and forms singlet excited molecule. The singlet

excited state of BP undergoes ISC to give triplet excited state which is more basic than the ground

state and known as a proton abstractor.

Both the tautomeric forms show decrease in the absorbance with time suggesting both the forms

undergo the reaction. The deprotonated form of 2-MTh undergoes degradation reaction in the

presence of the sensitizer BP in the alkaline medium and the concentration of the deprotonated form

of 2-MTh decreases as the reaction proceeds. The equilibrium shifts to right as reaction proceeds

and the protonated form is converted to deprotonated form.


substrate suggests the product formation from triplet excited state of the substrate.

The free radical scavenging effect of the methanol is positive. The photo reaction proceeds via a

free radical mechanism.

There is an interaction between the N-alkyl group of the substrates and the negatively charged

triplet excited benzophenone and proton of the alkyl group of the substrate molecule is transferred

to the triplet excited benzophenone. The proton abstraction takes place forming a •CH2 radical in the

molecule. H. Mohan et al. [37, 38] have reported electron and proton transfer process observed in

benzophenone. The radical excited state undergoes ring cleavage to form methylthiocyanate on

reorganization.


Mechanism of 2-MTh Degradation


7B.7 Conclusion

The photosensitized reaction of the 2 – Methyl 2 - thiazoline (2-MTh) with benzophenone (BP) in

alkaline medium using visible light has been studied. The spectrum of the 2-MTh shows only one

λmax at 260 nm in the acidic medium while in the alkaline medium it absorbs at 230 nm and 245 nm

nearly in equal concentration. The two tautomeric forms of 2-MTh are in equilibrium in the pH

range 8 – 12. Both the tautomeric forms show decrease in the absorbance with time suggesting both

the forms undergo the reaction. The deprotonated form of 2-MTh undergoes degradation reaction in

the presence of the sensitizer BP in the alkaline medium and the concentration of the deprotonated

form of 2-MTh decreases as the reaction proceeds. The equilibrium shifts to right as reaction

proceeds and the protonated forms is converted to deprotonated forms. The triplet – triplet energy

transfer from the triplet excited state of the aromatic ketone benzophenone to the substrate molecule

takes place during the photo sensitized reaction. The triplet excited 2-MTh breaks down on further

exposure and photo product formation take place. The thiocyanate has been observed as photo

product.


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