oxidation reactions of 1- and 2-naphthols; experimental...

Chapter 3

Oxidation reactions of 1- and 2-naphthols; experimental

and theoretical study

Oxidation reactions of 1- and 2-naphthols; experimental and theoretical study 67

Abstract

Naphthols are naphthalene homologues of phenol. α and β naphthols are isomers which

differ only in the position of the hydroxyl group. Both α and β naphthols are precursors of

variety of insecticides including carbaryl and nodolol. In this chapter the transients formed

during the reactions of oxidizing radicals with 1-naphthol (1) and 2-naphthol (2) in aqueous

medium has been investigated by pulse radiolysis with detection by absorption

spectroscopy and DFT calculations. The transient spectra formed on hydroxyl radical

(•OH) reactions of 1 and 2 exhibited λmax at 340 nm and 350 nm at neutral pH. The rate

constant of the •OH reactions of 1 (2) were determined from build-up kinetics at λmax of the

transients as 9.6 × 109 M-1s-1 (7.3 × 109 M-1s-1). DFT calculations using B3LYP/6-31+G

(d,p) method have been performed to locate favorable reaction sites in both 1 and 2 and

identification of the pertinent transients responsible for experimental results. Calculations

demonstrated that •OH additions can occur mostly at C1 and C4-positions of 1, and at C1

and C8-positions of 2. Among several isomeric •OH adducts possible, the C1 adduct, was

found to be energetically most stable both in 1 and 2. TDDFT calculations in solution phase

have shown that experimental spectrum of 1 was mainly attributed by 1a4 (kinetically

driven •OH-adduct) formed via the addition of •OH at C4 position which was 0.73 kcal/mol

endergonic than 1a1 (thermodynamic •OH-adduct); whereas 2a1 (thermodynamic/kinetic •OH-adduct) was mainly responsible for the experimental spectrum of 2. Naphthoxyl

68 Chapter III

radicals of 1 and 2 have been predicted as the transient formed in the reaction of •OH at

basic pH. In addition, the same transient species resulted from the reactions of oxide radical

ion (O•–) at pH~13 and azide radical (N3•) at pH 7 with 1 and 2. Further, UV photolysis of

aqueous solutions of 1 and 2 containing H2O2 (UV/H2O2) were used for the •OH induced

oxidation product formations up on 60% degradations of 1 and 2; profiling of the oxidation

products were performed by using an ultra-performance liquid chromatography quadrupole

time of flight mass spectrometry (UPLC–Q-TOF) method. According to the UPLC–Q-

TOF analyses, the preliminary oxidation products are limited to di-hydroxy naphthalenes

and naphthoquinones with N2-saturation while some more additional products (mainly

isomeric mono hydroxy naphthoquinones) have been observed in the degradations of 1 and

2 in presence of O2. We postulate that, di-hydroxy naphthalenes are derived explicitly from

the most favorable •OH-adducts speculated (preference is in terms of kinetic/

thermodynamic dominancy of transients) by using theoretical calculations which in turn

substantiate the proposed reaction mechanisms. The observations of •OH-adducts for an

aromatic phenol (herein for both 1 and 2 at pH 7) rather than phenoxyl type radical in the

pulse radiolysis experiments is a distinct and unique illustration. The present study provides

a meaningful basis for the early stages associated with the •OH initiated advanced oxidation

processes of 1- and 2-naphthols.

Publications from this chapter:

i) Sreekanth, R.; Prasanthkumar, K. P..; Sunil Paul, M. M.; Aravind, U. K.;

Aravindakumar, C. T; Pulse radiolysis and theoretical studies of oxidation reactions

of 1- and 2-naphthols. J. Phys. Chem. A 2013, 117, 11261–11270

ii) Sreekanth, R.; Sunil Paul M. M.; Aravind U. K.; Aravindakumar C. T, Hydroxyl

Radical Mediated Oxidation of 1- and 2-Naphthols: A Mass Spectrometric Study,

Proceedings of the International Conference on Frontiers of Mass Spectrometry

(ICMS 2013)(September 6-9, 2013), Kottayam

iii) Sreekanth, R.; Aravindakumar, C. T; Pulse radiolysis studies of 1- and 2-Naphthols,

Proceedings of the Trombay symposium on radiation and Photochemistry (TSRP

2009) Mumbai, 2009


3.1. Introduction

1-Naphthol and 2-naphthol (designated as 1 and 2, Scheme 1) are isomeric

hydroxyarenes, have been the subject of several experimental and associated

theoretical studies. Basically 1 and 2 are released into the environment through

manufacture, handling, use and disposal in the context of their use in dye and

pesticide industries 1-3. Further they entered into the environment owing to the

oxidation of naphthalene (the major constituent of coal tar) or as metabolites of

carbamate pesticides like sevin or carbaryl (1-naphthyl-N-methylcarbamate) by

chemical and biological processes 4-12. The presence of hydroxyl group in 1 and

2 leads to their increased solubility and portability in natural aquifers and is

considered as more toxic than naphthalene and other polycyclic aromatic

hydrocarbons 13. Both 1 and 2 are used as biomarkers for livestock and humans

exposed to polycyclic aromatic hydrocarbons 4, 14, 15. Remarkably, molecules 1

and 2 are regarded as archetypes of ‘photoacids’ and exhibit major difference in

pKa between the ground and excited states due to the dependence of

dissociation constant on electronic state of the molecule 16-23.

Several comprehensive experimental/theoretical studies have been reported for

the •OH induced oxidation reactions of phenolic compounds in view of their so-

called antioxidant or free-radical scavenging properties 24-36. In the case of

phenol (C6H5OH), the •OH reacts by addition to the aromatic ring that lead to

Scheme 3.1: Structures of 1- and 2-naphthols with atomic numbering.

70 Chapter III

the formation of the •OH-adduct as the preferred transient at neutral pH as

demonstrated by ESR and pulse radiolysis studies 25, 26. However, the •OH

addition essentially leads to the formation of isomeric adduct species. Acids and

bases catalyze the dehydration of the primary •OH-adduct(s) to the formation of

phenoxyl radical26-28, 31. Furthermore, the base catalyzed dehydration is reported

to be faster than the acid catalyzed dehydration27. Indeed, it was recognized that

thermodynamically the formation of phenoxyl radical is more favorable than •OH additions 37. Moreover, in substituted phenols the substituent have marked

influence on directing the incoming •OH 30, 32, 38. A number of techniques which

essentially comprise of chemical, biological, catalytic, and electrochemical

procedures were reported for the •OH induced degradation of naphthols in

waste water 39-51. Also, the free electron transfer phenomena of naphthols, their

analogues and derivatives has been extensively studied by fluorescence, laser

flash as well as pulse radiolysis techniques in organic solvents 52-59.

The main interest of the present study is accordingly to understand the

mechanism of primary oxidation reactions of isomeric naphthol molecules with •OH. In order to further understand the oxidations of naphthols, two other

oxidizing inorganic radicals (O•– and N3•) were also selected and their reactions

were compared with •OH reactions. We used pulse radiolysis technique with

optical absorption detection as direct, convenient and reliable method to portrait

the reactions of oxidizing radicals produced in aqueous medium with naphthols.

The advantage of pulse radiolysis method is that, it offers a clean source for the

selective generation of a particular radical like •OH under suitable experimental

conditions to probe its reactions with a substrate. DFT calculations were carried

out to locate the most probable reaction sites and to evaluate the preferred

kinetic/thermodynamic transient(s) that would be formed in the pulse radiolysis

experiments. Additionally, analysis of the oxidation products resulting from the


reactions of •OH (generated via UV/H2O2 method) with 1 and 2 has been

carried out using UPLC-Q-TOF-MS technique to elucidate the primary radical

intermediates in the pulse radiolysis studies.

3.2. Results and Discussion

3.2.1. Pulse radiolysis studies

(A) Reactions with ••••OH with 1- and 2-naphthols

The transient absorption spectra formed on the reactions of •OH with 1 and 2

at pH 7 and 10.5 are presented in Figure 3.1. The ordinate in all these plots

represents the product of G-value 72 and absorption coefficient (ε) of the

transient at a particular wavelength. It should be noted that, the ground state

pKa values of 1 and 2 are respectively 9.3 and 9.6 60, therefore their anionic

forms are reacting at higher pHs. The spectrum measured after 5 µs shows

λmax 340 nm for 1 at pH 7 (Figure 3.1A), whereas at pH 10.5 the transient

absorption shows λmax 410 nm and a broad but minor absorption centered

around 540 nm (Figure 3.1B). The transient spectrum produced in the

reaction of •OH with 2 (Figure 3.1C) showed λmax 350 nm at pH 7, whereas

the same reaction at pH 10.5 is characterized by three absorptions with λmax

330 nm, 370 nm and 480 nm (Figure 3.1D). In all these cases we have

observed no change in the spectral maxima with time except the decrease in

signal intensities. Subsequently, from time resolved spectral studies we

postulate the formations of relatively long-lived transient(s) in the reactions

of •OH with 1 and 2 in neutral and alkaline solutions. Further, it has been

found that, the transient with absorptions at either 340 or 350 nm in the

reactions •OH with 1 or 2 at pH 7 decayed via second order kinetics. The

second order rate constants for the reactions of •OH with 1 and 2 were

determined from the slope of the plot of observed rate constant (kobs) as a

function of concentrations of naphthols measured at their transient λmax at

72 Chapter III

340 nm and 350 nm respectively. The slopes of these plots (Figure 2) gave

rate constants of 9.6 × 109 and 7.3 × 109 M-1s-1 for 1 and 2 respectively;

these values indicate that the reactions of •OH with 1 and 2 are nearly

diffusion controlled. Also, these values are exceptionally not far-off to the

reported rate constant of 1.4 × 1010 M-1s-1 for the •OH additions to phenol 24;

hence the observed rate coefficients are consistent with the well-known

electrophilic addition nature of •OH to aromatic rings. The

hydroxycyclohexadienyl-type radical formed via the addition of •OH to the

benzenoid unit of phenol and a number of benzene derivatives 24, 28, 61 were

reported to have λmax around 330 nm. Obviously, a number of competent •OH addition sites are present in benzenoid units of both naphthol molecules.

However, the prosperity of •OH addition to a particular ring atom is highly

dependent upon the electron density on that carbon atom.


As the spectral features (in terms of both λmax and G × ε values) are apparently

different at neutral and basic pH for •OH reactions of both naphthol

molecules, one can presume that different transient species are observed at

neutral and basic pH. One probable mechanism would be the formation of an •OH adduct radical at neutral pH; naphthoxyl radical formation via •OH attack

to the naphthoxide ion followed by rapid elimination of OH– from the

so-formed adduct at basic pH. The due reason for the supposition of •OH

addition followed by OH– elimination at basic pH can be well admired on the

basis of similar type of reactions studied by Roder and co-workers on phenol 27.

Their studies have revealed that phenoxyl radical formation through the OH–

elimination of •OH adduct of phenol (pKa = 9.9) at pH 11 is almost 6-8 times

Figure 3.1: Transient spectra for •OH reactions with 1- and 2-naphthols (0.1 mM) recorded at 5 µs after irradiation. (A) 1-naphthol at pH 7, (B) 1-naphthol at pH 10.5 (C) 2-naphthol at pH 7 and (D) 2-naphthol at pH 10.5. Dose = 15.4 Gy/pulse

74 Chapter III

faster than the H2O-elimination of the •OH adduct in highly acidic medium.

Therefore in the present case (at basic pH), the initial reaction is expected to be

an addition of •OH to naphthoxide ion followed by a rapid elimination of OH–

to give naphthoxyl radical. Demonstration of naphthoxyl radical formation will

be more obvious when we look at the O•– and N3• reactions of naphthols.

It can be noted that, under the pulse radiolysis time scale, the naphthols at

neutral pH can react with •OH according to three possible pathways as depicted

in Scheme 3.2. In one mechanistic pathway, the electrophilic additions of •OH

can occur to either benzenoid units of naphthol molecules which lead to the

formations of so-called •OH-adducts. There could be two types of •OH addition

to hydroxyl group bearing (i.e. phenolic) benzenoid unit, one is addition to ipso-

position and the other being addition at un-substituted carbon atoms. The

quantification of isomeric •OH-adducts formed in the case of phenol has been

reported on the basis of product analysis and pulse radiolysis data26. The ipso- as

well as meta- additions accounts 8% each, whereas 48% and 36% accounts for

Figure 3.2: Plots of kobs as a function of concentrations of 1- and 2-naphthols at the λmax of the transients formed on their reactions with •OH at pH 7.


the •OH additions at ortho- and para- positions of phenol 26. The relative yields

for addition of •OH to ipso-, meta-, ortho-, and para- positions of phenol reported

recently by Albarran and Schuler are 8%, 25%, 4% and 34% respectively 34. The

direct outer-sphere electron transfer reactions can lead to the formations of radical

cations of 1 and 2. The third possibility is the H-abstractions from the phenolic

OH units leading to naphthoxyl radicals. The differentiation of these three

competing mechanistic pathways (viz. •OH-adducts formations, direct electron

transfer and H-abstraction) is rather very difficult; however, the reactions of

specific one-electron oxidants such as O•– and N3• would rather eliminate the

possibility of one or two pathways.

Scheme 3.2: Possible reaction channels for •OH interaction with 1- and 2-naphthols.

76 Chapter III

(B) Reactions with O••••– and N3••••

The transient spectra (Figure 3.3) obtained for O•– reactions with anionic forms of

1 and 2 are identical to that generated from their •OH reactions at pH 10.5.

Moreover, it was found that spectra shown in Figure 3.3 are in good agreement

with the spectra reported for the naphthoxyl radicals in the reactions of 1 and 2

with N3• in aqueous solutions 53, 60. Apparently, we could also reproduce identical

spectra (not shown herein) for N3• reactions with 1 and 2 in aqueous solutions at

pH 7 and 10.5. As N3• reacts with aromatic compounds mainly via direct electron

transfer pathway, the results presented above clearly establish identical transient

(i.e. naphthoxyl radical) formations in the reactions of •OH at pH 10.5 and of O•–

at pH~13. Concomitantly, the possibility of electron transfer and thereby the

formations of radical cations of 1 and 2 can also be ruled out. Therefore, the

spectral data presented herein infer the operation of two different mechanisms

under the experimental conditions used (neutral and basic pH) for •OH reactions

of 1 and 2. If one considers an addition of •OH at neutral pH (for both 1 and 2) as

in the case of phenol, then the chances of H-abstraction from phenolic OH

(thereby naphthoxide radical) of naphthols can easily be precluded.

Figure 3.3: Transient spectra for O•– reactions with 1- and 2-naphthols (0.1 mM) recorded at 5 µs after irradiation at pH~13. (A) 1-naphthol and (B) 2-naphthol. Dose = 15.4 Gy/pulse.


The reactions of O•– and N3• with 1 and 2 underlined our previous hypothesis of

naphthoxyl radical formations in the reaction of •OH at basic pH. Therefore, the

observed experimental data in combination with the results of O•– and N3•

reactions unequivocally establish the formation of •OH-adduct(s) at pH 7.

However, there are many possible •OH addition sites in naphthol skeleton and

hence the experimental observations alone are unable to resolve the question of

the most probable adduct(s) and is the rationale for DFT studies. DFT

calculations have unfailingly supported us in previous studies by resolving the

otherwise complicated pulse radiolysis experimental results and serves well in

assigning the exact transient(s) formed therein 63-69.

3.2.2. Theoretical studies

We have carried out theoretical modeling mainly to find the reactive sites in

naphthol molecules for •OH additions and as an alternate tool to predict the

most probable experimental transient(s) by exploring the formation energies and

predictions of λmax of •OH-adducts. The gas phase optimized geometries of the

isomeric naphthols are depicted in Figure 3.4 along with selected bond lengths.

Essentially several orientations of phenolic hydrogens are possible for both

naphthols and we have presented the most stable conformations. It was found

that both benzenoid rings of 1 and 2 are in the same plane and also phenolic

hydroxyl units are positioned in the same molecular plane. Calculations showed

that, the interaction takes place between the LUMO of •OH, located at -5.05 eV,

and the HOMOs of 1 located at -5.77 eV and of 2 located at −5.92 eV. HOMOs

of both naphthol molecules color mapped onto van der Waal's surfaces are

presented in Figure 3.4.

78 Chapter III

The contribution of each atom towards the HOMO may be considered as an

indicator for the electrophilic addition of •OH towards that site. It can be seen

from the MO plot (Figure 3.4) that, C4 and C1 positions formulate larger

contribution towards the HOMO for compound 1; whereas C1 and C8 are the

major contributors for the HOMO of compound 2. Other key contributors for

the HOMO of 1 are phenolic O, C2, C5, and C8, similarly for the HOMO of 2

are phenolic O, C2, C4, C5, and C6. The interaction of •OH with the phenolic

oxygen in both naphthols would primarily result in direct one-electron oxidation

as well as H-abstraction reactions and these possibilities are ruled out while

discussing the experimental results at neutral pH. Therefore, we have

Figure 3.4: Optimized geometries of (a) 1-naphthol and (b) 2-naphthol with selected bond lengths in Å unit. HOMO of (c) 1-naphthol and (d) 2-naphthol are color-mapped onto van der Waal's surfaces and the MO coefficients of major donor atoms are also shown.


theoretically modeled the •OH additions at vulnerable sites C1, C2, C4, C5 and

C8 of 1 and the corresponding adducts are represented as 1a1, 1a2, 1a4, 1a5, and

1a8. Similarly, the •OH additions at C1, C2, C4, C5, C6 and C8 of 2 lead to

adduct molecules represented as 2a1, 2a2, 2a4, 2a5, 2a6, and 2a8. Attempts to

find pre-complexes for the •OH-adduct formations lead product-like (i.e.

adduct) structures for both 1 and 2. Therefore, it seems that •OH-additions are

very fast and occurs via energy free or they are barrier-less processes.

The optimized geometries of adduct molecules 1a1, 1a2, 1a4, 1a5, and 1a8 in

solution phase are illustrated in Figure 3.5 with selected bond lengths. The

BSSE corrected Eint values of the adduct systems in gas phase are also depicted

in Figure 3.5. The Eint values illustrate that the adduct systems are much more

stable than the separate entities viz., 1 and •OH. Also, the 1a8 is more stable

than other available adduct molecules. However, the solution phase studies

point at the influence of solvation on the stabilities of the radicals. The relative

electronic energies (∆E0), enthalpies (∆H), and free energies (∆G) of •OH

adduct molecules of 1 in solution phase are listed in Table 3.1. It can be found

that, formations of all adducts systems are likely due to negative values of

enthalpy and free energy of formations. The difference in free energy between

the most stable 1a1 and the least stable 1a2 accounts for 3.92 kcal/mol.

However, the formations of 1a1 and 1a4 (via additions at C1 and C4) are

thermodynamically more feasible than other adduct molecules. Interestingly,

the high electron density reserves at C1 and C4 positions are also in favor of the

formations of 1a1 and 1a4 as obvious from the HOMO picture (Figure 3.4).

Therefore, adducts 1a1 and 1a4 arises by the kinetic/thermodynamic harmony of •OH reaction with 1. It can also be noted from Table 1 that, except in 1a2 the

unpaired electron spin is confined to the same benzenoid ring to which •OH gets

added. The fewer stability associated with 1a2 can be attributed as a result of

80 Chapter III

unpaired electron spin delocalization onto both benzenoid units which renders

the reduction of inherent aromaticity of both the •OH added and spectator

benzenoid ring.

The solution phase optimized geometries of adduct molecules 2a1, 2a2, 2a4, 2a5,

2a6, and 2a8 are presented in Figure 3.6 with selected bond lengths. The

calculated Eint values (with BSSE correction) showed that 2a1 is the most stable

adduct molecule in gas phase. The Eint values of 2a2, 2a4, 2a5, and 2a8 are found

to be lower than that of 1a2, 1a4, 1a5, and 1a8. The solution phase ∆E0, ∆H, and

∆G value of •OH adducts of 2 are presented in Table 3.1 and the

thermodynamic parameters (in a. u.) of •OH, 1-naphthol, 2-naphthol and

various •OH adducts of 1-naphthol & 2-naphthol are given in table 3.2. The

addition of •OH at C1-position of 2 results in the formation of most stable

radical 2a1 followed by the addition at C8-position leading to 2a8. The

stabilities of the •OH adducts follows the order 2a1 > 2a8 > 2a5 > 2a4 > 2a2 >

2a6. The difference in free energy between the most stable 2a1 and the least

stable 2a6 accounts for 5.37 kcal/mol. Interestingly, the ipso-addition of •OH

leads to the thermodynamically most stable adduct (i.e. 1a1) in 1 in contrast to 2

where the ipso-addition causes the formation of one of the least stable adduct

(i.e. 2a2). Also it can be noted from Table 3.1 that, for all •OH-adducts of 2 the

odd electron spin density is mainly dispersed onto the carbon atom adjacent to

the •OH added carbon. As noted in the case of 1a2, there is marked odd electron

spin delocalization into both benzenoid units of 2a2 and 2a6 and which accounts

for the fewer stabilities associated with these two species. Also it seems that the

stabilities of the •OH adducts of 2 in solution are lower than that of 1. Albeit,

the stabilities of 2a1 and 2a8 are consistent with the intuitive reactivity of C1

and C8 positions and therefore we can conclude that formations of 2a1 and 2a8

occur via the kinetic/thermodynamic control of •OH reaction with 2.


Figure 3.5: Solution phase optimized geometries of adducts formed via •OH additions at C1, C2, C4, C5, and C8 positions of 1-naphthol. Bond lengths are in Å unit. BSSE corrected Eint values (in kcal/mol) in gas phase are shown in parenthesis.

82 Chapter III

Figure 3.6: Solution phase optimized geometries of adducts formed via •OH additions at C1, C2, C4, C5, C6 and C8 positions of 2-naphthol. Bond lengths are in Å unit. BSSE corrected Eint values (in kcal/mol) in gas phase are shown in parenthesis.


Table 3.1: Calculated relative electronic energies (∆E0), enthalpies (∆H), free energies (∆G), Mulliken spin densities, λmax and corresponding oscillator strengths (ƒ) of the •OH adducts of 1- and 2-naphthols in solution phase at B3LYP/6-31+G(d,p) level of theory.

Transient ∆E0 (kcal/mol)

∆H (kcal/mol)

∆G (kcal/mol)

Spin density (a.u.) λmax

(nm) ƒ

1a1 -20.48 -21.44 -11.65 C2(0.56), C4(0.62) 328 0.130

1a2 -16.48 -17.43 -7.73 C1(0.58),C6(0.29),

C8(0.30) 300 0.106

1a4 -19.60 -20.52 -10.91 C1(0.48), C3(0.59) 336 0.145

1a5 -18.58 -19.52 -9.95 C6(0.57), C8(0.65) 347 0.197

1a8 -17.64 -18.86 -8.40 C5(0.66), C7(0.61) 343 0.146

2a1 -19.79 -20.70 -11.26 C2(0.46), C4(0.62) 344 0.174

2a2 -16.00 -16.94 -7.13 C1(0.66), C6(0.29) 280 0.070

2a4 -17.69 -18.53 -9.15 C1(0.66), C3(0.58) 305 0.182

2a5 -18.24 -19.17 -9.56 C6(0.59), C8(0.64) 321 0.133

2a6 -14.68 -15.62 -5.89 C5(0.73), C2(0.26) 313 0.056

2a8 -18.70 -19.61 -10.08 C5(0.64), C7(0.57) 347 0.238

84 Chapter III

Table 3.2: Thermodynamic parameters (in a. u.) of •OH, 1-naphthol, 2-naphthol and various •OH adducts of 1-naphthol & 2-naphthol calculated in solution phase at B3LYP/6-31+G(d,p) level of theory.

System Gas phase Solution phase

SCF Energy

BSSE correction

SCF Energy

Sum of electronic and zero-

point Energies

Sum of electronic

and thermal Enthalpies

Sum of electronic and thermal Free

Energies

•OH -75.73901 -- -75.75146 -75.74412 -75.74081 -75.76110

1-Naphthol (1) -461.14524 -- -461.16322 -461.01380 -461.00483 -461.04640

1a1 -536.92631 0.00295 -536.95223 -536.79055 -536.77981 -536.82606

1a2 -536.92431 0.00290 -536.94682 -536.78419 -536.77342 -536.81981

1a4 -536.92511 0.00257 -536.95171 -536.78916 -536.77835 -536.82489

1a5 -536.92380 0.00256 -536.94982 -536.78754 -536.77675 -536.82335

1a8 -536.93095 0.00282 -536.94944 -536.78603 -536.77571 -536.82088

2-Naphthol (2) -461.14549 -- -461.16421 -461.01507 -461.00608 -461.04772

2a1 -536.93105 0.00302 -536.95314 -536.79073 -536.77989 -536.82678

2a2 -536.91933 0.00274 -536.94599 -536.78469 -536.77389 -536.82019

2a4 -536.92227 0.00257 -536.94916 -536.78739 -536.77642 -536.82340

2a5 -536.92345 0.00256 -536.95024 -536.78827 -536.77744 -536.82406

2a6 -536.91705 0.00241 -536.94439 -536.78259 -536.77179 -536.81820

2a8 -536.92465 0.00261 -536.95096 -536.78900 -536.77815 -536.82488

Calculated λmax and corresponding oscillator strengths of •OH adducts of 1 and

2 by the application of TDDFT method on solution phase optimized geometries

are also presented in Table 3.1. The λmax 336 nm calculated for the adduct 1a4


agrees well with the experimentally observed transient λmax 340 nm in the

reaction of •OH with 1 at neutral pH. Obviously, in terms of free energy, the

stability of this kinetic adduct is only 0.74 kcal/mol less than the

thermodynamic adduct 1a1. The thermodynamic adduct 1a1 can also contribute

towards the experimental spectrum even though the calculated λmax 328 nm

differs by 12 nm (blue shift) with respect to the experimental λmax; a difference

(12 nm) which is in the acceptable limit by considering the uncertainty in the

experimental transient absorption spectroscopy and the TDDFT theoretical

calculations. Although, the calculated λmax values of 1a5 and 1a8 are coinciding

with the experimental λmax, the feasibility of the formations of these adducts are

less in accordance with their enthalpy and free energy formations as compared

to 1a1 or 1a4. Therefore, with the aid of the prevailing theoretical results, the

experimental spectrum in the reaction of •OH with 1 at neutral pH is assigned as

a result of adducts 1a1 and 1a4.

The optical absorptions calculated for the most stable thermodynamic adduct

2a1 with λmax 343 nm and the next stable thermodynamic adduct 2a8 (λmax 347

nm) agrees well with the experimental λmax of 350 nm observed in the reaction

of •OH with 2 at neutral pH. Moreover, 2a1 and 2a8 are produced as a result of •OH additions to the leading contributors of the HOMO of 2 (Figure 3.4). Thus,

the λmax calculations are also in favor of the formations of

kinetic/thermodynamic driven products (viz., 2a1 and 2a8) of •OH reaction with

2. Accordingly, on the basis of theoretical calculations it can be perceived that

the experimental spectrum for •OH reactions of 2 at neutral pH is attributed due

to 2a1 and 2a8 formations.

86 Chapter III

3.2.3. Oxidation product analyses

Further insights to •OH reaction mechanisms of 1 and 2 were obtained by the

evaluation of preliminary oxidation products derived from UV/H2O2 method.

Table 3.3 summarizes the results (the retention time and m/z values of the major

products) of the analyses by using UPLC–Q-TOF-MS technique performed on

degraded samples solutions of 1 and 2 via the UV/H2O2 method. Three major

products were identified in the •OH reactions of 1 with N2-saturation; two of

them have an [M-H]– value of 159.04 with retention time (RT) at 2.5 and 2.9

min while the third one possess an [M+H]+ value of 159.04 with RT at 2.1 min.

First stage ESI-MS spectra of the oxidation products of 1-naphthol & 2-

naphthol (with and without N2 -saturation) analyzed by using UPLC-Q-TOF-

MS technique is given in figure 3.7 to 3.10.

Figure 3.7: First stage ESI-MS spectra of oxidation products of 1-naphthol (with N2-

saturation)


Figure 3.8: Additional peaks observed for 1-naphthol (without N2-saturation)

Based on the analyses of MS/MS fragmentation patterns and also taking

account of the feasibilities of initial •OH attack to the ring of 1 (based on our

theoretical studies as shown above), we have deduced the structures of the

degradation products as the di-hydroxy naphthalenes 1p1 and 1p4 and the

naphthoquinone 1q4 (Scheme 3.3). The product 1p1 results from the ipso-

addition of •OH (most stable thermodynamic transient) while 1p4 results from

the initial •OH attack at C4 (next stable thermodynamic but the most feasible

kinetic transient). Obviously, Scheme 3.3 represents one of the possible

mechanistic pathways for product formations. However, in the presence of

oxygen (O2) additional peaks have been observed in the total ion chromatogram

88 Chapter III

(TIC) besides 1p1, 1p4 and 1q1. The TIC in the presence of oxygen in the

negative ionization mode have species with identical [M-H] – mass of 159.04

observed at RT 1.9, 2.5, and 2.9 min; these peaks were assigned as due to the

formations of isomeric di-hydroxy naphthalenes 1p5, 1p4 and 1p1. The TIC in

the positive ionization mode has a peak with an [M+H]+ value of 159.04 at RT

2.1 min and the product is identified as 1q4. Products with identical [M-H]–

mass of 173.02 were observed at RT 1.0, 1.5 and 2.0 min; which is consistent

with hydroxylation of 1q4 and the isomeric products were assigned as 1q41, 1q42

and 1q43 (Scheme 3.4).

Figure 3.9: First stage ESI-MS spectra of oxidation products of 2-naphthol (with N2-saturation)


Figure 3.10: Additional peaks observed for 2-naphthol (without N2-saturation)

90 Chapter III

Table 3.3: The retention time and m/z values obtained from UPLC-Q-TOF-MS analyses of the UV/H2O2 treated 1- and 2-naphthols and the identified products based on Scheme 3.3 to 3.6.

Parent Molecule

Retention Time (min)

Experimental (m/z) Assigned Product

[M+H] + [M-H] – Designation Mass

1-Naphthol

A. With N2-saturation

2.1 159.04 1q4 158.04

2.5 159.04 1p4 160.05

2.9 159.04 1p1 160.05

B. Without N2-saturation

1.0 173.02 1q41 174.03

1.5 173.02 1q42 174.03

1.9 159.04 1p5 160.05

2.0 173.02 1q43 174.03

2.1 159.04 1q4 158.04

2.5 159.04 1p4 160.05

2.9 159.04 1p1 160.05

2-Naphthol

A. With N2-saturation

2.1 159.04 2q1 160.05

2.6 159.04 2p1 160.05

3.0 159.04 2p8 158.04

B. Without N2-saturation

1.1 173.02 2q11 174.03

1.4 173.02 2q12 174.03

1.6 173.02 2q13 174.03

2.1 159.04 2q1 158.04

2.2 173.02 2q14 174.03

2.6 159.04 2p1 160.05

3.0 159.04 2p8 160.05

3.1 159.04 2p5 160.05


Scheme 3.3: A possible route of product formations in the oxidation of 1-naphthol by UV/H2O2 method with N2–saturation.

92 Chapter III

As in the case of 1, three major products were obtained for the degradation of

2 by the UV/H2O2 method in the presence of N2-saturation. As can be seen

from Table 3.3 that, two of them have an [M-H]– value of 159.04 with

retention time (RT) at 2.6 and 3.0 min while the third one possess an [M+H]+

value of 159.04 with RT at 2.1 min. One possible mechanism that accounts the

formations of these oxidation products is shown in Scheme 3.5. The product

2p1 and 2p8 arises from the initial •OH attack at C1 (the most feasible kinetic

site for •OH addition that leads to the most stable thermodynamic adduct) and

at C8 (the next feasible kinetic site for •OH addition that leads to the second

stable thermodynamic adduct). Further oxidation of 2p1 results in the

Scheme 3.4: Proposed channels for oxidation product formations from 1-naphthol by UV/H2O2 method without N2-stauration.


formation of 2q1. The degradation of 2 in the presence of O2 renders more

products compared to N2-saturation; at the same time the number is higher than

that of 1 under similar experimental conditions. In addition to the peaks of 2q1,

2p1 and 2p8 (RT respectively 2.1, 2.6 and 3.0 min) the TIC comprise of

additional peaks at RT 1.1, 1.4, 1.6 and 2.2min (all having identical mass

173.02) and another one at 3.1 min (mass 159.04). The peaks at 1.1, 1.4, 1.6 and

2.2 min are considered as the mono hydroxy naphthoquinones via the

hydroxylation of 2q1 and these products are designated as 2q11, 2q12, 2q13, and

2q14, while the 3.1 min peak with mass 159.04 is regarded as 2p5 (Scheme 3.6).

Scheme 3.5: A possible route of product formations in the oxidation of 2-naphthol by UV/H2O2 method with N2–saturation.

94 Chapter III

Therefore, the evaluation of the preliminary degradation products of 1 and 2 by

the UV/H2O2 method in the presence of N2-saturation provide complementary

data for the pulse radiolysis experiments.

3.3. Conclusions

The present study using pulse radiolysis technique, allows the qualitative

understanding of the preliminary oxidation products (the transient species)

formed during the reaction of •OH with 1 and 2. Absorption spectra of the

transients produced for •OH reactions at pH 7 were exhibited very close λmax for

these isomeric naphthol molecules. Rate constant determinations demonstrated

Scheme 3.6: Proposed channels for oxidation product formations from 2-naphthol by UV/H2O2 method without N2-stauration.


the diffusion controlled nature of •OH reactions of 1 and 2. Results of DFT

calculations for •OH reactions of both naphthols provide a conceptual

framework of the most applicable reaction mechanism. The preferential attack

of •OH at C4 site of 1 is mainly responsible for the experimental spectrum,

whereas addition at C1 is dictated in the case of 2; agreement between

experimental and theoretical λmax max of the kinetic transients in both 1 and 2

validate this possibility again. However, naphthoxyl radical was proposed as the

transient observed at pH 10.5 via the •OH induced oxidation of 1(2). Moreover,

the reactions of 1 and 2 with specific one-electron oxidants O•– (at pH~13) and

N3• (at pH 7) also point at naphthoxyl radical formations. We suggest that, in

addition to •OH, the other radicals viz. O•– and N3• used in the present study are

also effective for the degradation of naphthols, but comprehensive experimental

studies are required to determine their effectiveness in advanced oxidation

processes. The formations of isomeric dihydroxy naphthalenes as products in

the oxidation via UV/H2O2 treatment clearly demonstrate the selective addition

of •OH at C1, C4 & C5 of 1 and at C1, C5 & C8 of 2. Our present experimental

and theoretical results along with UPLC-Q-TOF data should serve as useful

guides to the understanding of •OH induced oxidation of naphthols.

96 Chapter III

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