chapter: 2 modification of guar gum -...

Chapter: 2

Page 86

CHAPTER: 2 MODIFICATION OF GUAR GUM

Chapter based on various methods of preparation of carboxymethyl

guar gum (CMGG) is divided into three parts

1. Carboxymethylation in heterogeneous and homogeneous solvent

system

2. Carboxymethylation using Friedel Craft Acylation method

3. Carboxymethylation in solid phase (solvent free method)

1. Part-1 describes preparation of sodium salt of partially carboxy-

methylated guar gum (Na-PCMGG) of different degree of

substitution (DS). The reaction parameters for this reaction were

optimized.

2. Part-2 covers the preparation of CMGG by Friedel Craft Acylation

method. Acetyl chloride was used as acetylating agent. The effect of

catalyst viz. FeCl3 and AlCl3 and phase transfer catalyst were

studied. The DS of the product was measured.

3. Part-3 covers the solvent free method to synthesize acetylated guar

gum. In this method no solvent was used. Acetic anhydride and

iodine was used as acetylating agent and catalyst respectively.

Acetic anhydride provide liquid base for better mixing of raw

materials. The DS of the product was measured.

The resulting CMGG was tested for DS, swelling index and

viscosity. The synthesized CMGG was characterized by Fourier Transform

Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM),

Thermogravimetry Analysis (TGA).

2.0 INTRODUCTION

Natural polysaccharides are often incorporated in the design of

controlled drug delivery such as those target delivery of the drug to a

specific site in the gastro intestinal tract (GIT), this can be achieved by

various mechanisms including coating granules, pellets, tablets with

polysaccharides having pH dependent solubility, or incorporating non-

digestible polysaccharides that are degraded by bacterial enzymes present

Chapter: 2

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in the colon, this property made these polysaccharides potentially useful

in the formulation of colon-targeted drug delivery systems [1, 2].

Natural polysaccharides have gained importance due to their

properties and their products of tailor made properties. The development

in this field leads the increased consumption of natural polysaccharide.

The guar galactomannan has the unique property of imbibing large

quantities of water, resulting in dispersions of extremely high viscosity.

High viscosity coupled with the branched character of the polymer is

responsible for adhesion of guar gum to hydrophilic surfaces. Guar gum

products showed a pronounced temperature thinning effect when their

solutions are heated. This is caused by loss of water of hydration around

the polymer molecule which made the guar gum most applicable natural

polymer [3]. Because of these properties, guar gum is used for a large

number of industries viz. textile, petroleum, paper, explosive,

pharmaceutical and food applications [4, 5].

Guar gum was used to deliver drug to colon due to its drug release

retarding property and susceptibility to microbial degradation in the large

intestine. The gelling property retards release of the drug from the dosage

form as well as it is susceptible to degradation in the colonic environment

[6, 7].

Guar gum and its derivatives were used as a binder and

disintegrate in tablets to add cohesiveness to drug powder. Guar gum was

also used as a controlled release agent for the drug due to high hydration

rate (swelling in aqueous media) [8].

Guar galactomannan was a block copolymer containing a galactose-

rich region and an unbranched mannan region. The guar galactomannan

had the unique property of imbibing large quantities of water, resulting in

dispersions of extremely high viscosity. High viscosity coupled with

branched character of the polymer is responsible for adhesion of guar gum

to hydrophilic surfaces. Because of these properties, guar gum is used for a

large number of industrial and food applications. However, in many cases,

the native gum has been found wanting in certain end-use properties.

As discussed earlier due to uncontrolled rate of viscosity,

uncontrollable rate of hydration, guar gum finds limited use in virgin

forms. There so, it has been chemically modified into various properties to

expand its industrial applications such as in food, paint and pigments, oil

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field, mining, paper, water treatment, personal care, pharmaceutical and

new types of superabsorbent. Natural polysaccharides like starch,

cellulose, chitosan etc. were modified to carboxymethyl derivatives [9].

The most specific property of the guar gum and their derivatives

was that they has hydroxyl groups, which made them suitable for making

changes in their structure formula and functionalization. A lot of research

has been done on guar gum for the changing their physical and chemical

properties by grafting, blending and compositing with synthetic and

natural polymers [10, 11].

Possible processing of guar gum depends on chemical modifications.

Various treatments are instrumental in developing functional

characteristics which made guar gum versatile and useful in a variety of

industrial applications. The simplest change is by varying the degree of

polymerization by controlled hydrolysis which is the means of controlling

viscosity. Furthermore, the abundance of hydroxyl groups in the

galatomannan molecule lends itself-like in cellulose-to a variety of

chemical reactions. They can be easily esterified, resulting in a variety of

interesting compounds.

Alkoxylation with ethylene or propylene oxides is also easily carried

out producing the corresponding ethers. Carboxyalkyl and cyanoalkyl

ethers are another example of functional modifications, e.g.

O-carboxymethyl derivative-prepared by reacting galactomannan with

chloroacetic acid-forms viscous aqueous solutions that are stable to

strongly alkaline reagents [12].

There were lost-of chemical processes involving galactomannans

some of them patented-designed to endow the natural gums with a variety

of desired properties-including anionic and cationic. Complexing reactions

are worth mentioning as they lead to cross-linking of the molecules

resulting in a three dimensional network which manifests itself in gel

formation. These reactions are not peculiar to galactomannans, being

characteristic of linear molecules having an abundance of adjacent

hydroxyl groups in cis positions. The complexing reaction of polyvinyl

alcohol with borax is an example.

Among others, copper salts from complexes with galactomannans.

Fehling’s solution, for instance, does not reduce those polysaccharides

even on prolonged boiling. An insoluble, gel-like complex is formed

instead. Salts of Ca, Al and Cr have the same gel forming capacity at

Chapter: 2

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certain pH levels. Perhaps the most characteristic, and important, is the

reaction involving borate ions. Like in the case of PVA borate ion co-

ordinates with 4-hydroxyl groups of two chain molecules, resulting in a di-

diol complex.

GG derivatives like O-(2-hydroxyethyl), O-(2-hydroxypropyl) were

reported but this fails to more or less extent in getting used in

pharmaceutical industries due to introduction of substituents groups to

the galactomannan polymer which increased branching and

entanglements and therefore higher viscosity [4]. Modified natural

polysaccharides like carboxymethyl cellulose, carboxymethyl starch finds

application in pharmaceutical industries [13, 14].

In the present work guar gum was carboxymethylated by

heterogeneous and homogeneous solvent method. Reaction parameters

were optimized to get best result.

Part 1: Carboxymethylation in heterogeneous and

homogeneous solvent system

Part 1 deals with the preparation of carboxymethyl guar gum by

conventional method. In this method guar gum was treated with NaOH

and chloroacetic acid in the presence of suitable solvent. The degree of

substitution (DS) of the product was determined by titrimetric method.

Carboxymethylation of guar gum employed the Williamson ether

synthesis procedure, which is a consecutive two-step reaction, proceeding

with a strong base-such as sodium hydroxide-that deprotonates the free

hydroxyl groups (particularly, the hydroxyl group of (-CH2OH) in guar

gum) to form alkoxides, thereby increasing their nucleophilicity.

Carboxymethyl groups are then formed in a reaction between guar

alkoxides and chloroacetic acid. A side reaction simultaneously takes

place, resulting in the formation of sodium glycolate from sodium

chloroacetate and sodium hydroxide.

The four selected process parameters that influence the efficiency of

the carboxymethylation process were the following

1. Volume of sodium hydroxide

2. Amount of chloroacetic acid

3. Reaction time

4. Reaction temperature

Chapter: 2

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The reaction conditions were optimized to get optimum value of

Degree of substitution.

2.1 Materials and Methods

2.2 MATERIALS

Guar gum, sodium hydroxide, monochloro acetic acid were

purchased from Sigma- Aldrich. Methanol and iso propyl alcohol were

purchased from National chemicals, Baroda. Solvents and other laboratory

chemicals were used after routine purification and they were of analytical

reagent (AR) grade. Double distilled water was used.

2.3 METHODS

2.3.1 Carboxymethylation of Guar Gum

Guar gum was carboxymethylated by two methods

1. Heterogeneous solution method

2. Homogenous solution method

In heterogeneous method iso propyl alcohol is used as a solvent and

in homogeneous method water is used as a solvent.

2.3.1.1 Purification of guar gum

2gm of the guar gum was boiled with 8ml of 70% (v/v) ethanol for

1hr under reflux. The sample was filtered, washing with 95% (v/v) ethanol

and dried at 600C for 3hrs in vacuum oven.

2.3.1.2 Heterogeneous Method

Purified guar gum was dispersed in 150ml of iso propyl alcohol, in

250ml of three neck flask equipped with a mechanical stirrer and contact

thermometer for control of temperature. After the gum was well dispersed,

the appropriate volume of NaOH solution (30%) was added at a rate of

1ml within 15min.

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Figure: 1 Assembly for CMGG

Then required amount of monochloro acetic acid was added to the

reaction mixture with continuous stirring over a period of 10min. The

reaction mixture was heated and maintained at a specific temperature

with continuous stirring for constant time to drive the reaction process to

completion. After completion of reaction carboxymethyl guar gum was

precipitated with the help of methanol and the precipitated product was

purified.

2.3.1.3 Homogenous Solution Method

In homogenous solution method iso propanol was replaced by water

by keeping other parameters as same as heterogeneous method.

2.1.3.4 Purification of crude CMGG

Crude carboxymethyl guar gum purified with the help of water.

Product was dialyzed against distilled water for 48hrs and then

carboxymethyl guar gum was precipitated with the help of methanol.

Finally product was dried in an oven at temperature 40-450C.

Chapter: 2

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2.3.2 Degree of Substitution in Modified Guar Gum

Figure: 2 Reaction scheme of carboxymethyl guar gum

The degree of substitution (DS) is the average number of sodium

carboxymethyl groups bound per anhydroglucose unit. This method is

used to determine the number of substituent groups added to the guar

gum backbone. Based on the DS determination study one can find how

many hydroxyl groups were converted into carboxymethyl group. Degree

of substitution markedly affects the properties of the compound.

1gm of Na- CMGG was dissolved in known amount of water. Then

this solution was passed through regenerated Amberlite IRA 96 anion

exchange resin no. of times till it become acidic. Then solution was divided

into two equal parts labeled as solution 1 and solution 2. The exhausted

resin was regenerated by passing 1 N HCl solution (3-4 times) followed by

washing with distilled water to remove any excess acid.

Solution 1 was taken into previously weighed beaker. The solution

was heated until dryness on hot plate and then cooled and weighed Na-

CMGG.

Solution 2 was titrated against a standard solution of NaOH. Note

down the burette reading and find out the degree of substitution by

following equation (1) & (2).

Chapter: 2

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2.3.2 Optimization study of parameters

Four parameters were selected to study their influence on the

efficiency of the carboxymethylation process, and they are

1. Volume of NaOH

2. Amount of mono chloroacetic acid

3. Reaction time

4. Reaction temperature

In this thesis the dependent variables includes the degree of

substitution (DS).

2.3.2.1 Effect of NaOH

The effect of NaOH on DS was carried out. Reaction was carried out

by varying the amount of NaOH from 5-15ml.

2.3.2.2 Effect of amount of chloro accetic acid

The effect of amount of chloroaceticacid on DS was carried out.

Reaction was carried out by varying the amount of chloro acetic acid from

5gm-20gm.

2.3.2.3 Effect of time

The effect of time on DS was carried out. Reaction was carried out

by varying reaction time from 4hrs-7hrs.

2.3.2.4 Effect of Temperature

The effect of temperature on DS was carried out. Reaction was

carried out by varying temperature from 40-700C.

2.3.3 Swelling and gel fraction studies

Swelling and gel fraction studies were carried out. Briefly, samples

weighing 0.01gm purified or modified guar gum were placed in small

dishes that were carefully inserted into glass flasks. A total volume of

60ml distilled water was slowly poured into each glass flask. The samples

were allowed to soak for 2hrs at room temperature, after which the excess

Chapter: 2

4

solution was carefully removed, and the gelled samples remaining in the

glass bottle were weighed. The gelled samples were lyophilized for three

days and then weighed again. The swelling ratio and percentage of gel

fraction were calculated using following equation.

Where,

Wwater is the weight of the sample after 2hrs soaking

Wgel is the weight of the sample after lyophilization

Wsolid is the initial weight of the sample.

2.3.4 FTIR spectroscopy

Fourier transform infrared spectroscopy (FTIR) is a technique

which is used to obtain an infrared spectrum of absorption, emission,

photoconductivity or Raman scattering of a solid, liquid or gas. An FTIR

spectrometer simultaneously collects high spectral resolution data over a

wide spectral range. This confers a significant advantage over a dispersive

spectrometer which measures intensity over a narrow range of

wavelengths at a time. In infrared spectroscopy, IR radiation is passed

through a sample. Some of the infrared radiation is absorbed by the

sample and some of it is passed through (transmitted). The resulting

spectrum represents the molecular absorption and transmission, creating

a molecular fingerprint of the sample. Like a fingerprint no two unique

molecular structures produce the same infrared spectrum. This makes

infrared spectroscopy useful over several types of analysis.

The resulting products were characterized by FTIR spectroscopy

using Perkin Elmer spectrum GX instrument, by the KBr pallet method.

2.3.5 Thermo gravimetric analysis

Thermogravimetric analysis or thermal gravimetric analysis (TGA)

is a method of thermal analysis in which changes in physical and chemical

properties of materials are measured as a function of increasing

temperature (with constant heating rate), or as a function of time (with

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constant temperature and/or constant mass loss). TGA can provide

information about physical phenomena, such as second-order phase

transitions, including vapourization, sublimation, absorption, adsorption,

and desorption. Likewise, TGA can provide information about chemical

phenomena including chemisorptions, desolvation (especially

dehydration), decomposition, and solid-gas reactions (e.g., oxidation or

reduction).

The TGA instrument continuously weighs a sample as it is heated

to temperatures of up to 20000C for coupling with FTIR and Mass

spectrometry gas analysis (GCMS). As the temperature increases, various

components of the sample are decomposed and the weight percentage of

each resulting mass change can be measured. Results are plotted with

temperature on the X-axis and mass loss on the Y-axis. The data can be

adjusted using curve smoothing and first derivatives are often also plotted

to determine points of inflection for more in-depth interpretations. TGA

instruments can be temperature calibrated with melting point standards

or Curie point of ferromagnetic materials such as Fe or Ni. A

ferromagnetic material is placed in the sample pan which is placed in a

magnetic field. The standard is heated and at the Curie point the material

becomes paramagnetic which nullifies the apparent weight change effect

of the magnetic field.

TGA of prepared CMGG and Borax cross-linked CMGG was carried

out by heating sample from temperature of 500C to 7000C at a heating rate

of 100C/min using Perkin Elmer Pyris 1 TGA instruments.

2.3.6 Scanning electron microscopy

A scanning electron microscope (SEM) is a type of electron

microscope that produced images of a sample by scanning it with a focused

beam of electrons. The electrons interact with atoms in the sample,

producing various signals that can be detected and that contain

information about the sample's surface topography and composition. The

electron beam is generally scanned in a raster scanpattern, and the

beam's position is combined with the detected signal to produce an image.

SEM can achieve resolution better than 1 nanometer. Specimens can be

observed in high vacuum, low vacuum, dry conditions (environmental

SEM), and at a wide range of cryogenic or elevated temperatures.

The most common mode of detection is by secondary electrons

emitted by atoms excited by the electron beam. On a flat surface, the

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plume of secondary electrons is mostly contained by the sample, but on a

tilted surface, the plume is partially exposed and more electrons are

emitted. By scanning the sample and detecting the secondary electrons, an

image displaying the topography of the surface is created. Since the

detector is not a camera, there is no diffraction limit for resolution as in

optical microscopes and telescopes.

The resulting carboxymethylatedd guar gum and other products

were characterized by SEM using Philips made ESEM EDAX XL-30 model

instrument.

2.4 Result and Discussion


Carboxymethylation of guar gum is a consecutive two-step reaction

proceeding with a strong base such as sodium hydroxide that deprotonates

the free hydroxyl groups (particularly, the hydroxyl group of (-CH2OH) in

guar gum) to form alkoxides, thereby increasing their nucleophilicity.

Carboxymethyl groups are then formed in a reaction between guar

alkoxides and chloroacetic acid.

2.4.1.1 Effect of NaOH

Table: 1 Effect of NaOH

The results were tabulated in table: 1. Carboxymethylation reaction

was carried out by varying the amount of NaOH from 5-15ml is shown in

table 1. The DS is increase by increasing the amount of NaOH up to 10ml,

further increase in amount of NaOH results in to the decrease in DS.

Increase in amount of NaOH leads to the alkali degradation of polymer.

The lower amount of NaOH leads to lower number of free hydroxyl group

deprotonated to form alkoxide which was resulted into lower value of DS.

NaOH

(in ml)

Temperature

( in 0C)

Chloroacetic

acid

( in gm)

Time

(in

hours)

DS

5 (30%)

600C

10

5

0.212

10 (30%) 1.144

15 (30%) 1.012

Chapter: 2

Page 97

2.4.1.2 Effect of amount of chloroaccetic acid

Table: 2 Effect of chloroacetic acid


was carried out in the range of 5-20gm keeping other parameters constant.

It can be concluded from the table that DS increased with increase in

amount of chloroacetic acid from 5-10gm. Mono chloroacetic acid reacts

with alkoxides groups formed by reaction between guar gum and NaOH

and converted them into caroxymethyl group. If the amount of

monochloroacetic acid used is less than the less amount of carboxymethyl

groups formed which leads to low value of DS. Further increase in the

amount of monochloro acetic acid leads to neutralization of alkali and

formation of salts which leads to formation of CMGG having lower DS

value.

2.4.1.3 Effect of time

Table: 3 Effect of time


was carried out in the range of 4-7hrs keeping other parameters constant.

It can be concluded from the table that DS increased with increase in time

from 4-6hrs but DS decrease with further increase in temperature.

Chloroacetic

acid

( in gm)

NaOH

(in ml)

Temperature

( in 0C)

Time

(in hours)

DS

5

10

(30%)

60

5

0.368

10 1.144

15 0.836

20 0.768

Time

(in

hours)

NaOH

(in ml)

Temperature

( in 0C)

Chloroacetic

acid

( in gm)

DS

4

10 (30%)

60

10

0.401

5 0.598

6 1.144

7 1.146

Chapter: 2

Page 98

Further, carrying reaction for longer time may result into alkali

degradation of polymer. It can be concluded that optimum DS is obtained

by carrying reaction for 6hrs further increase in reaction time has no effect

on the DS. Carriying out the reaction for longer time leads to increase in

the production cost.

2.4.1.4 Effect of Temperature

Table: 4 Effect of the temperature


was carried out in the range of 40-700C keeping other parameters

constant. It can be concluded from the table that DS increased with

increase in temperature from 40-600C but at 700C DS obtained is almost

same as obtained at 600C. So we select 600C as an optimum reaction

temperature against 700C because to maintain higher temperature, more

energy consumption which leads to increase in production cost.

2.4.1.5 Optimum Parameters

The following are the optimum parameters for the reaction

1. Volume of NaOH 10ml(30%)

2. Amount of chloro acetic acid (10gm)

3. Time (5hrs)

4. Temperature (600C)


The results were tabulated in table: 5. Carboxymethyl guar gum

displayed a higher swelling ratio relatively to unmodified guar gum.

Temperature

( in 0C)

NaOH

(in ml)

Chloroacetic

acid

( in gm)

Time

(in hours)

DS

40

10

(30%)

10

5

0.768

50 0.834

60 1.144

70 1.148

Chapter: 2

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DS Swelling ratio % Gel fraction

0.401 2.01 69.70

0.598 2.60 63.90

0.839 3.78 55.73

1.144 4.98 51.80

0.768 3.35 58.60

Table: 5 Swelling ratio and % Gel fraction of CMGG

The carboxymethylation of carbohydrates relies on maximizing the

degree of substitution. However, the water solubility of carboxymethylated

carbohydrates increases with increasing substitution, which, in turn,

reduces the gelling properties, which is the main reason for studying the

gel fraction percentage.


The FTIR spectrum of guar gum and carboxymethyl guar gum were

shown in figure: 3 and figure: 4 respectively. The IR spectrum of

carboxymethyl guar gum shown a reduced intensity of the absorption

band located at 3432.56cm-1, as compared to guar gum IR spectrum due to

-OH is stretching, indicating that some -OH group were

carboxymethylated. The sharp absorption band located at 2925.43cm-1

may be attributed to CH group stretching.

The C-O symmetrical and asymetrical vibrations at a

frequency of 1025.65cm-1 and 1149.93cm-1 confirms the incorporation of

the carboxymethyl group on to the guar gum molecule, which is absent in

the guar gum spectra.

Chapter: 2

Page 100

Figure: 3 FTIR Spectra of GG

Figure: 4 FTIR spectra of Na-PCMGG

Chapter: 2

Page 101

2.4.5 Thermo gravimetric analysis

Differential thermo gravimetric curves of the guar gum and CMGG

were shown in figure: 5 and figure: 6 respectively.

Figure: 5 TGA of GG

Figure: 6 TGA of CMGG

Thermo gravimetric analysis of guar gum essentially reveals two

distinct zones of weight loss. The initial weight loss occurred in the 50-

1000C range, due to the moisture traces present in the sample. The second

step represents the degradation of the polymer backbone, having started

at 2000C and lasting until 3000C. In addition to these zones of weight loss,

the thermal degradation of carboxymethyl guar gum shows a third zone in

the 400-5000C range, due to the degradation of the carboxymethyl groups

Chapter: 2

Page 102

incorporated in the polymer moiety. CMGG shown 65.83% weight loss as

compared to 68.25% of guar gum at 4000C indicates improved thermal

stability of CMGG.

2.4.6 Scanning electron microscopy

The figure: 7 and 8 represents SEM micrograph of Guar gum and

CMGG.

Figure: 7 SEM of GG

Figure: 8 SEM of GMGG

SEM micrograph of CMGG indicates the reduction in porosity of

guar gum by carboxymethylation. The most of the porosities were

eliminated by carboxymethylation process. It also confirm in the % gel

study which is decreased by carrying modification of guar gum.

2.5 Conclusion

The carboxymethylation of guar gum carried out successfully.

Carboxymethylation improves the properties of guar gum especially, rate

of hydration, viscosity. The optimization study for CMGG was carried out.

The optimum DS of 1.144 was obtained by carrying reaction at 600C for

5hrs by adding 10 ml(30%) of NaOH and 10gm of chloro acetic acid. The

reaction was carried out in homogeneous and heterogeneous phase. The

heterogeneous method was found superior to homogeneous method

because in heterogeneous method granules of the CMGG was obtained

while in the homogeneous method lumps of the CMGG was obtained.

Chapter: 2

Page 103

Granules of CMGG were easy to handle compared to lumps. It is

easy to control temperature in the heterogeneous method.

Part 2: Carboxymethylation using Friedel Craft

Acylation method

This part deals with the preparation of carboxymethyl guar gum by

Friedel craft acylation method. Natural polysaccharides were

carboxymethylated by conventional method using NaOH and Na-salt of

chloroacetic acid [15, 16, 17]. In organic chemistry number of compound

were synthesized by well-known Friedel Craft reaction, using AlCl3 and

FeCl3 as catalyst. But there was no work reported on carboxymethylation

of natural polysaccharide via Friedel Craft acylation reaction. There so, we

synthesized carboxymethyl guar gum via Friedel Craft acylation reaction

using AlCl3 and FeCl3 as catalyst.

In the present work guar gum was carboxymethylated by Friedel

craft acylation methods by using AlCl3 and FeCl3 as a Friedel craft

catalyst, while tetra ethyl ammonium bromide used as phase transfer

catalyst.

Effect of AlCl3 and FeCl3 on reaction was studied. The effect of time,

temperature, solvent were also studied and compared with other

conventional method of synthesized CMGG.

2.0 Material and Methods

2.1 MATERIALS

Guar gum, acetyl chloride, ferric chloride, aluminium chloride and

tetra ethyl ammonium bromide were purchased from Sigma- Aldrich.

Ethanol and iso propyl alcohol were purchased from National chemicals,

Baroda. Solvents and other laboratory chemicals were used after routine

purification and they were of analytical reagent (AR) grade. Double

distilled water was used.

2.2 METHODS

Guar gum was carboxymethylated by Friedel craft acylation method

by using two different catalyst viz. FeCl3, AlCl3, phase transfer catalyst

like tetra ethyl ammonium bromide and iso propyl alcohol as a solvent.

Chapter: 2

Page 104

2.2.1 Purification of guar gum

Guar gum was purified by method described in Part: 1.

2.2.2 Friedel Craft Acylation

Purified guar gum was dispersed in 150ml of iso propyl alcohol, in

250ml round bottom flask equipped with a magnetic stirrer. After the gum

was well dispersed, catalyst AlCl3 and the phase transfer catalyst

tetraethyl ammonium bromide were added. After that acetylating agent

(acetyl chloride, 5ml) was added and the reaction was continued at room

temperature with constant stirring for 5hrs. After completion of reaction

carboxymethyl guar gum was precipitated with the help of methanol and

the precipitated product was purified.

2.2.2.1 Purification of modified guar gum

The insoluble CMGG in its acidic form was dialyzed against

distilled water for 48hrs. The suspension was precipitated with ethanol,

followed by washing with solvent exchange (ethanol, acetone, ether) and

dried under reduced pressure.

2.2.3 Degree of substitution in modified guar gum

1gm of CMGG was dissolved in known amount of water. Then this

solution was passed through regenerated Amberlite IRA 96 anion

exchange resin no. of times till it become acidic. Then the solution was

divided into two equal parts labeled as solution 1 and solution 2. The

exhausted resin was regenerated by passing 1 N HCl solution (3-4 times)

followed by washing with distilled water to remove any excess acid.

Solution 1 was taken into previously weighed beaker. Evaporate

water by heating on a hotplate and cool it into desiccator and weigh it

again. Find the weight of residue left in the beaker. Find out concentration

by evaporation.



following equation

Chapter: 2

Page 105


Two parameters were selected to study their influence on the

efficiency of the Friedel craft Acylation method, and they are

1. Catalyst

2. Reaction time

In this thesis the dependent variables includes the degree of

substitution (DS).

2.2.4.1 Effect of Catalyst

The effect various catalyst on efficiency of freidel craft acylation

reaction was carried out. In the present study two catalyst viz. AlCl3 and

FeCl3 were used to study their effect.

2.2.4.2 Effect of Time

The effect of time on reaction efficiency was studied. The reaction

was carried out by varying time from 4-6hrs.


Swelling and gel fraction studies were carried on the basis of a

previously reported protocol. Briefly, samples weighing 0.01gm purified or

modified guar gum were placed in small dishes that were carefully

inserted into glass flasks. A total volume of 60ml distilled water was

slowly poured into each glass flask. The samples were allowed to soak for

2hrs at room temperature, after which the excess solution was carefully

removed, and the gelled samples remaining in the glass bottle were

weighed. The gelled samples were lyophilized for three days and then

weighed again. The swelling ratio and percentage of gel fraction were

calculated using following equation

Chapter: 2

Page 106

Where,

Wwater is the weight of the sample after 2hrs soaking

Wgel is the weight of the sample after lyophilization

Wsolid is the initial weight of the sample





2.3.1 Effect of Catalyst

Catalyst Degree Of

Substitution (DS)

AlCl3 0.7

FeCl3 0.5

Table: 6 Effect of catalyst

The results were tabulated in table: 6. Carboxymethylation of guar

gum is a consecutive two-step reaction proceeding with a generation of an

acylium ion. The acylium ion is formed by the removal of halide by the

Lewis acid catalyst AlCl3. This acylium ion reacts with free hydroxyl group

of guar gum to generate carboxymethyl guar gum. It can be concluded

from the table that high DS was obtained by using AlCl3 compared to

FeCl3 as catalyst. AlCl3 gives high DS value because it will coordinate to

halogens, and facilitate the breaking of these bonds. In doing so, it

increased the electrophilicity of its binding partner, making it much more

reactive so high value of DS was obtained.

Chapter: 2

Page 107

2.3.2 Effect of Time

Time (in hrs.) Catalyst DS

4

AlCl3 0.31

FeCl3 0.20

5

AlCl3 0.7

FeCl3 0.58

6

AlCl3 0.65

FeCl3 0.44

Table: 7 Effect of Time

The results were tabulated in table: 7. Reaction was carried out for

4, 5 and 6hrs. By carrying reaction for longer time will not improve value

of DS. The optimum DS was obtained by carrying out reaction for 5hrs

using AlCl3 as catalyst.


Catalyst DS Swelling ratio % Gel fraction

FeCl3

0.20 1.40 72.29

0.44 1.96 66.34

0.58 2.10 62.88

AlCl3

0.31 1.70 70.19

0.65 2.50 60.67

0.70 3.03 53.54

Table: 8 Swelling ratio and % Gel fraction of CMGG

The results were tabulated in table: 8. It can be concluded from

result that % gel fraction of product obtained by AlCl3 is low compared to

product obtained by FeCl3 catalyst. Swelling ratio of product is higher for

AlCl3 compared to FeCl3 catalyst.


The IR spectrum of guar gum and carboxymethyl guar gum were

shown in figure: 9 and figure: 10 respectively.

Chapter: 2

Page 108

Figure: 9 FTIR spectra of GG

.

Figure: 10 FTIR spectra of CMGG

The IR spectrum of carboxymethyl guar gum shown a reduced

intensity of the absorption band located at 3439.35cm-1, as compared to

guar gum IR spectrum due to -OH is stretching, indicating that some -OH

group were carboxymethylated. The C-O symmetrical and asymetrical

vibrations at a frequency of 1090.67cm-1 and 1156.06cm-1 confirms the

incorporation of the carboxymethyl group on to the guar gum molecule,

which is absent in the guar gum spectra.

Chapter: 2

Page 109

2.4 Conclusion

The carboxymethylation of guar gum carried out successfully via

Friedel craft acylation method. Carboxymethylation improves the

properties of guar gum especially, rate of hydration, viscosity. The effect of

catalyst on DS was studied. Optimum DS of 0.7 was obtained carrying

reaction for 5hrs using AlCl3 catalyst. The reaction was carried out in

heterogeneous phase. In heterogeneous method granules of the CMGG

was obtained.

Part 3 Carboxymethylation in solid phase (solvent free

method)

Part 3 deals with the solvent free method to synthesize

carboxymethyl guar gum. In this method no solvent was used.

Chloroacetic acid and iodine was used as carboxymethylating agent and

catalyst respectively. The DS of the product was measured.

Acetylation of polysaccharides has been known for many decades.

For example, acetylation of cellulose and starch in the presence of acetic

acid and acetic anhydride is well known [18]. In the case of cellulose

acetate, it is customary to produce cellulose triacetate (CTA) first, and

then hydrolyze it to produce cellulose acetate (CA) with the desired degree

of substitution [19]. Conventional acetylation processes typically involve

solvents such as methylene chloride or high temperatures with sulfuric or

perchloric acid as a catalyst. There were been some recent studies in

polysaccharide esterification, particularly the use of ionic liquids to

dissolve cellulose and prepare cellulose acetates [20]. Other paths for

esterification include dialkylcarbodiimide, N,N-carbonyldiimidazole,

iminium chlorides, trans-esterification, and ring-opening esterification. An

alternative approach reported is to use iodine as a catalyst for the

esterification of cellulose and starch in the presence of acetic anhydride

[21]. The reactions are generally conducted at 1000C without the use of

additional solvents. Starch acetates are produced using both conventional

or microwave heating. In this article, we aim to review this development,

summarize the major results, and evaluate its usefulness [22].

Falling in to about trends in the present work we synthesized

CMGG by solvent free method using iodine as catalyst.

Chapter: 2

Page 110

2.0 Materials and Methods

2.1 MATERIALS

Guar gum, chloroacetic acid, sodium thiosulfate and iodine were

purchased from Sigma- Aldrich. Acetone was of A.R. grade and use

without any purification. All other reagents and solvents were of LR

grade.

2.2 METHODS

2.2.1 Carboxymethylation of guar gum

Iodine (0.50gm) was added into pre-dried round bottom flask. Acetic

anhydride (5ml) was added and the reaction mixture was stirred for

15min. After that guar gum (1.0gm) was added to the mixture. The

reaction mixture was refluxed for 4hrs. As the reaction was completed,

excess iodine was removed by adding a saturated solution of sodium

thiosulfate. Thus white precipitates of CMGG was formed which was

filtered off and washed thrice with cold water and then re-precipitated

from acetone. The product was dried under vacuum at 500C for 12hrs.

2.2.2 Degree of substitution in modified guar gum

The D.S. of the product find out by method described earlier. 1gm of

CMGG was dissolved in known amount of water. Then this solution was

passed through regenerated Amberlite IRA 96 anion exchange resin no. of

times till it become acidic. Then the solution was divided into two equal

parts labeled as solution 1 and solution 2. The exhausted resin was

regenerated by passing 1 N HCl solution (3-4 times) followed by washing

with distilled water to remove any excess acid.

Solution 1 was taken into previously weighed beaker. Evaporate

water by heating on a hotplate and cool it into desiccator and weigh it

again. Find the weight of residue left in the beaker. Find out concentration

by evaporation.



following equation.

Chapter: 2

Page 111

2.2.3 Reaction mechanism

Chloroacetic acid and guar gum were reacted in the presence of

iodine as catalyst. The general mechanism was shown in figure: 9. In the

first step chloroacetic acid activated by iodine in the presence of -OH

groups. The oxygen of R-OH attacks the carbonyl carbon resulting in sp3

hybridization. The acetic ester is formed by the reaction of the iodine ion

with the iodine atom that was attached to the oxygen of the carbonyl

group, resulting in free iodine. Acetic acid is formed as a by-product. At

the end of the reaction, addition of a saturated solution of sodium

thiosulfate removes all the free iodine.

Figure: 11 Reaction mechanism


Swelling and gel fraction studies were carried on the basis of a

previously reported protocol. Briefly, samples weighing 0.01gm purified or

Chapter: 2

Page 112

modified guar gum were placed in small dishes that were carefully

inserted into glass flasks. A total volume of 60ml distilled water was

slowly poured into each glass flask. The samples were allowed to soak for

2hrs at room temperature, after which the excess solution was carefully

removed, and the gelled samples remaining in the glass bottle were

weighed. The gelled samples were lyophilized for three days and then

weighed again. The swelling ratio and percentage of gel fraction were

calculated using following equation.

Where,

Wwater is the weight of the sample after 2hrs soaking,

Wgel is the weight of the sample after lyophilization, and

Wsolid is the initial weight of the sample.

2.2.5 Effect of amount of iodine

The reaction was carried out by varying the amount of iodine. Effect

of iodine on DS was studied.






The results were tabulated in table: 9. It can be concluded form

table that as the DS of the product increased % gel fraction is decreased.

Chapter: 2

Page 113

DS Swelling ratio % Gel fraction

0.268 1.56 71.30

0.620 2.60 57.90

0.432 1.80 63.71

Table: 9 Swelling ratio and % Gel fraction

2.3.2 Effect of amount of iodine

The reaction was carried out by varying the amount of iodine.

Amount of

Iodine (gm)

DS

0.4 0.268

0.5 0.620

0.6 0.432

Table: 10 Effect of amount of iodine

The results were tabulated in table: 10. The DS CMGG increased as

amount of iodine increased. Iodine promotes reactive dissolution of guar

gum in chloroacetic acid. By carrying out reaction with less amount of

iodine low value of DS was obtained because at low concentration of iodine

reaction barely proceeded. By using high amount of iodine DS was

decreased because high amount of iodine may cause acid hydrolysis.

2.3. 3 FTIR spectroscopy

The IR spectrum of guar gum and carboxymethyl guar gum were

shown in figure: 12 and figure: 13 respectively. The IR spectrum of

carboxymethyl guar gum shown a reduced intensity of the absorption

band located at 3438.46cm-1, as compared to guar gum IR spectrum due to

-OH is stretching, indicating that some -OH groups were

carboxymethylated. The sharp absorption band located at 2925.67cm-1

may be attributed to CH group stretching.

Chapter: 2

Page 114

Figure: 12 FTIR of GG

Figure: 13 FTIR of CMGG

The C-O symmetrical and asymetrical vibrations at a frequency of

1027.33cm-1 and 1151.97cm-1 confirms the incorporation of the

carboxymethyl group on to the guar gum molecule, which is absent in the

guar gum spectra.

Chapter: 2

Page 115

2.4 Conclusion

In the present study, we focused on carboxymethylation of guar

gum, using iodine as catalyst. Hence the conventional reagent chloroacetic

acid was explored using iodine as catalyst. This esterification method has

two advantages over other methods. First is the carboxymethylating

reagent is used is not expensive than p-toluene sulfonyl chloride, 1,1’-

carbonyldiimidazole, etc. Second is the reaction carried out in

homogeneous phase and no expensive solvent is used for the reaction.

Carboxymethylation of natural polysaccharides and its derivatives

can be carried out using different carboxymethylating agents as well as

catalyst. Traditionally used catalysts such as aluminium chloride, sodium

hydroxide required the use of costly solvent to carry out the reaction in

homogeneous phase. But the use of iodine as catalyst can overcome the

use of costly solvent also benefited by prevention of glycosidic linkage of

polysaccharides ring which can be broken by traditional catalyst.

Carboxymethylation of guar gum under solvent-free conditions

using iodine as catalyst was carried out successfully. The DS of the

product obtained was 0.6.

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