Brij Raj Sharma,
Vineet Kumar,
Purshottam L. Soni
Center of Advance Studies
in Chemistry of Forest
Products,
Forest Research Institute,
Dehra Dun, India
Cyanoethylation of Cassia tora Gum
Cyanoethylation of Cassia tora gum was carried out with acryionitrile in presence of sodium
hydroxide under different reaction conditions. Variables studied were concentrations of sodium
hydroxide and acryionitrile, reaction temperature and time. The degree of substitution, reaction
efficiency and total extent of etherification were determined. The optimum conditions for preparing
cyanoethyl Cassia tora gum (DS 0.44) were 0.608 mol acryionitrile and 0.625 mol sodium
hydroxide at 30 °C for 4 h. Rheo-logical properties of cyanoethyl Cassia tora gum solutions
showed non-Newtonian pseudopiastic behavior regardless of the degree of substitution (DS). At a
constant rate of shear the apparent viscosity of cyanoethyl Cassia tora gum solutions increased with
the increase in %N of the product.
Keywords: Cassia tora gum; Cyanoethylation; Extent of reaction
1 Introduction
Plant gums derived from seeds, especially guar (Cyamopsis tetragonolobus) and panwar (Cassia
tora), have attracted considerable attention in recent years due to their phenomenal rheological
behavior. During the last two decades guar gum acquired great importance as a source of industrial
polysaccharides. Recently hydrocol-loid industries have started investigating other sources of seed
gums. In this regard Cassia tora gum (CTG) has been found to be equally effective as guar gum in
some of its properties [1], for example good gelling and fairly stable viscosity of gum solution from
pH 5 to 9.
Cassia tora gum is an abundantly available polymeric material, which can be modified to be more
suitable for applications in industry. Very few reports are available on the chemical modification of
CTG to enhance the quality and acceptability of CTG products [2-4]. Cyanoethylation of
biopolymers such as inulin [5], guaran [6], starch [7-9], bagasse [10], cotton [11,12] and cellulose
[13-15] have been reported while no report is available on cyanoethylation of CTG. The present
communication deals with the cyanoethylation of CTG and the effect of this modification on the
rheological properties.
2 Materials and Methods 2.1 Materials
Cassia tora gum was isolated from seeds as per the method described by Son/and Pa/[1]. Sodium
hydroxide, ethanol and acetic acid are laboratory grade chemicals.
Correspondence: Purshottam L Soni, Center of Advance Studies in Chemistry of Forest Products,
Forest Research Institute, P. O. New Forest, Dehra Dun-248006, India, e-mail: [email protected].
Acryionitrile (Aldrich Chemical Company, Inc., St. Louis, MO) was freshly distilled before use.
2.2 Cyanoethylation
The cyanoethylation reaction was performed as follows: in a 500 mL beaker containing 250 ml_
aqueous solution of sodium hydroxide (0.625 to 1.25 mol), 0.246 mol of CTG was gradually added
with continuous stirring and heating (70 °C). After 1 h, the reaction mixture was cooled to 30 °C
and acryionitrile (AN) (0.243 to 0.851 mol) was added dropwise with continuous stirring. The
reaction mixture was stirred at 30 °C or 70 °C for a certain period of time (1 to 4 h). Subsequently,
the reaction mixture was cooled and neutralized with dilute acetic acid and the product precipitated
by pouring the reaction contents in ethanol with vigorous stirring. The precipitated product was
centrifuged, filtered, washed with acetone and dried over calcium chloride under vacuum.
2.3 Analysis and measurements
- The nitrogen content of the CE-CTG was determined by the Kjeldahl's method.
- The carboxyl group content of the CE-CTG was determined according to a reported method [16].
- The degree of substitution (DS) and reaction efficiency (RE) were calculated as follows [7]:
-
The rheological properties of the pastes prepared from the cyanoethylated samples were
measured at 25 °C
- using a Brookefield Digital Viscometer 'RVTD' Soughton, USA.
- The apparent viscosity was calculated using the following equation:
3 Results and Discussion
Cyanoethyiation of CTG was carried out by reacting it with AN in presence of sodium hydroxide
under a variety of conditions. The variables studied were concentration of AN and sodium
hydroxide, temperature and duration of reaction. The changes in the chemical structure of CTG
brought about by cyanoethyiation were assessed by the change in its physical properties such as
solubility and rheology of cyanoethyl-CTG (CE-CTG). The reaction between CTG and AN may be
represented as follows:
Beside the cyanoethyiation reaction (Eq. 1), other reactions are expected to occur as shown later
and the magnitude of these reactions will be governed by the reaction conditions.
3.1 Effect of sodium hydroxide concentration
The effect of sodium hydroxide concentration on the %N and DS of CE-CTG and RE of
cyanoethyiation reaction is shown in Tab. 1. As can be seen, the extent of the cyanoethyiation
reaction (expressed as %N) decreases significantly by increasing the sodium hydroxide concentration from 0.625 to 1.25 mol. Most probably, the cyanoethyl groups of CE-CTG undergoes partial
alkaline hydrolysis under the influence of higher sodium hydroxide concentration to yield
carboxyethyl groups via amide groups (Eqs. 2 and 3). This is also in agreement with the results
reported in the literature [7, 11].
3.2 Effect of acrylonitrile concentration
Tab. 2 shows the effect of AN concentration on %N and DS of CE-CTG samples and RE of
cyanoethyiation reaction taking optimum concentration (0.625 mol) of sodium hydroxide. Results
show the distinct pattern of increase in DS on increasing the concentration of AN (0.243 to 0.851
mol) which becomes optimized at 0.608 mol. No signi-icant increase in DS occurs on further
increase in the concentration of AN beyond 0.608 mol by using the optimum dose of sodium
hydroxide concentration. This shows that there was not sufficient CTG-alkoxide available for reaction with excess AN and AN may have been subjected to side reactions as shown in Eqs. 4-6.
Similar observations have also been reported in the literature [7].
Thus, the optimum concentration of AN for cyanoethyiation of CTG was found to be 0.608 mol.
Tab. 1. Effect of sodium hydroxide concentration on cyanoethyiation of CTG.
3.3 Effect of temperature and duration of reaction
The effect of temperature and duration of cyanoethylation reaction on %N and the carboxyl groups
derived thereof is shown in Tab. 3. The dependence of temperature and duration can be studied in
terms of nitrogen content, carboxyl content and total extent of etherifi-cation.
3.3.1 Nitrogen content
Tab. 3 shows the variation of %N of the CE-CTG samples with reaction time and temperature. As is
evident, the %N is temperature and time dependent. For instance, at 30 °C the %N increases from
1.21 to 2.73% by increasing the duration of reaction within the range studied (1 to 4 h), indicating
that reactions suggested by Eqs. 1 and 2 prevail over the other side reactions as shown previously.
However, the extent of reaction (%N) is adversely affected at 70 °C which may be due to
conversion of some of the -CN and/or -CONH2 groups to -COOH groups via alkaline hydrolysis
(Eq. 3). This is in accordance with the results reported elsewhere [7-9].
3.3.2 Carboxyl content
Tab. 3 shows the carboxyl content of the CE-CTG samples prepared at 30 °C and 70 °C for
different periods of time (1 to 4 h). It is clear that the carboxyl content increases by increasing the
duration of reaction. This is rather a proof that the cyanoethyl groups are converted to amide groups
which in turn are converted to carboxyl groups (Eqs. 2 and 3). Such conversions seem to occur from
the beginning of the reaction and concurrently with cyanoethylation (Eq. 1). Reaction temperature
favors these conversions since the carboxyl content is higher at higher temperature with the
concomitant decrease in %N at higher temperature (Tabs. 3 and 4). This change in carboxyl content
is in accordance with the results reported elsewhere [7, 9].
3.3.3 Total extent of etherification
Tab. 4 shows the total extent of the reaction occurring between CTG and AN, expressed as the sum
of nitrogen content and carboxyl content (in mmol/100 g sample). Calculation of nitrogen content
and carboxyl content are based on the data of Tab. 3. It is seen that at 30 °C, the total extent of
etherification reaction performance vis-a-vis at 70 °C is significant on increasing the reaction time
from
Tab. 4. Dependence of nitrogen content of CE-CTG samples and carboxyl groups derived thereof
on time and temperature of cyanoethylation.
1 to 4 h, e. g., when the reaction was carried out for 4 h, a relative increase in N content (65 mol)
and decrease in carboxyl content (8.7 mmol) is observed at 30 °C with respect to 70 °C, thereby
showing the extent of etherifica-tion as well as cyanoethylation is favored at 30 °C.
3.3.4 Degree of substitution and reaction efficiency
Tab. 5 shows the dependence of the DS of CE-CTG and RE of cyanoethylation on time and reaction
temperature. Calculation of the DS was determined assuming that nitrogen content represents only
cyanoethylation (Eq. 1). It is seen that the DS and RE increase by increasing the reaction time at
both 30 °C and 70 °C. However, DS decreases on increasing the temperature which could be due to
the prevailing of the side reactions (Eqs. 2-6) and decrease in RE may be due to the degradation of
the CTG backbone. Thus, within the range studied the optimum time for cyanoethylation of CTG is
4 h at 30 °C. This is in agreement with the results reported elsewhere [7].
4 Rheological Properties
Hydrocolloids and their derivatives are generally used as viscosifier and thickener in the form of
solutions; there-Tab. 5. Dependence of the DS of CE-CTG samples and RE of cyanoethylation on
time and temperature of reaction.
fore, it is of interest to study the rheological properties of their solutions. The solutions of CE-CTG
were prepared in 3% concentration and their rheological properties were studied (Tabs. 6 and 7). It
is shown that regardless of the cyanoethyl content, the aqueous CE-CTG solutions are characterized
by non-Newtonian pseudoplastic behavior [17].
Apparent viscosity of the aforementioned solutions at various rates of shear before and after storing
for 72 h are given in Tabs. 6 and 7. Data show that the %N plays a dominant role on the apparent
viscosity of the product at any specific rate of shear. At a constant rate of shear, the apparent
viscosity increases as the %N of the product increases. At a rate of shear 2.8 s-1 the apparent
viscosity increases from 2900 mPas (2.73 %N) to 5650 mPas (3.35 %N). The increase in apparent
viscosity by increasing the %N may be due to the increase in the hydrodynam-ic volume [18]
because of cyanoethyl groups introduced. Results also show that increase in concentration of alkali
has a detrimental effect on the extent of reaction as well as on apparent viscosity; for example, by
keeping the same shear rate (2.8 s-1) and concentration of AN (0.243 mol) the apparent viscosity
drops from 2900 to 2450 mPas by increasing the alkali concentration from 0.625 to 1.25 mol. Tabs.
6 and 7 show the apparent viscosity of the solutions at variable alkali concentrations. Results show
that the apparent viscosity remains stable up to 72 h for all the samples examined. The slight
increase in the apparent viscosity of these derivatives by storing reflects its stability, which may be
due to the increase in the swellability of molecules by the presence of carboxyethyl groups.
5 Conclusion
The optimum reaction conditions for cyanoethylation of CTG are: [CTG] = 0.246 mol; [NaOH] =
0.625 mol; [AN] = 0.608 mol; time = 4 h; temperature = 30 °C. CE-CTG shows good cold water
solubility, solution stability, solu-
tion clarity and increased viscosity. With all these properties the CE-CTG can be exploited in a
much better way for its industrial applications.
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(Received: October 25, 2001)
(Revision received: July 22, 2002)