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KINETICS OF ALKALINE HYDROLYSIS OFISOAMYLACETATEBhaswati Ghosh a , Sudeshna Bag a , Parthasarathi Ray a , SekharBhattacharjee a & Basab Chaudhuri aa Department of Chemical Engineering , Calcutta University ,Calcutta, IndiaPublished online: 25 Jan 2007.

To cite this article: Bhaswati Ghosh , Sudeshna Bag , Parthasarathi Ray , Sekhar Bhattacharjee& Basab Chaudhuri (2005) KINETICS OF ALKALINE HYDROLYSIS OF ISOAMYLACETATE, ChemicalEngineering Communications, 192:6, 787-794, DOI: 10.1080/009864490521246

To link to this article: http://dx.doi.org/10.1080/009864490521246

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Kinetics of Alkaline Hydrolysis of Isoamylacetate

BHASWATI GHOSH, SUDESHNA BAG,PARTHASARATHI RAY, SEKHAR BHATTACHARJEE,AND BASAB CHAUDHURI

Department of Chemical Engineering, Calcutta University,Calcutta, India

The theory of mass transfer accompanied by chemical reaction has been used todetermine the kinetics of alkaline hydrolysis of isoamylacetate. The rate constantfor the alkaline hydrolysis is around 9.52 cm3=(mole)(sec). This is much lower thanthe rate constant for alkaline hydrolysis of n-amyl acetate, which is about85 cm3=(mole)(sec). The presence of an electrolyte like sodium chloride in theaqueous phase reduced the rate of extraction to some extent. The rates of extractionwith sodium chloride in the aqueous phase and the corresponding rate constantvalues have been reported.

Keywords Liquid–liquid; Kinetics; Hydrolysis; Isoamylacetate; Stirred Cell;Electrolytes

Introduction

The theory of mass transfer accompanied by chemical reaction has been widely used todetermine the kinetics of heterogeneous liquid-liquid chemical reactions (Doraiswamyand Sharma, 1984; Westerterp et al., 1984).

The alkaline hydrolysis of acetate esters of lower molecular weight alcohols(typically, C4, C5, etc.) is known to conform to a regime between very slow and slow,that is, both mass transfer resistance across the liquid-liquid interface and the resist-ance associated with the intrinsic chemical reaction in the bulk aqueous phasebecome relevant when the alkaline hydrolysis is conducted at ordinary temperatures.Alwan et al. (1983) studied the rate of extraction of n-amylacetate (NAA) withalkaline hydrolysis in the aqueous phase and reported the rate constant data forthe reaction. The authors showed that the alkaline hydrolysis of NAA conformedto a reaction regime between very slow and slow as expected. Isoamylacetate(IAA) is an isomer of NAA and one expects that the rate constant for the alkalinehydrolysis of IAA will be close to that for NAA. In the absence of any experimentaldata for the alkaline hydrolysis of IAA such an assumption is not unreasonable. Buthard experimental evidence is always better than logical assumptions in chemistry.Therefore, we attempted to study the alkaline hydrolysis of IAA. In the presentarticle we report results of our work. To the best of our knowledge no systematicinvestigation on the alkaline hydrolysis of IAA in heterogeneous liquid-liquid modeof operation appears to have been done.

Address correspondence to Basab Chaudhuri, Department of Chemical Engineering,Calcutta University, 92 A. P. C. Road, Calcutta 700 009, India. E-mail: basabc@vsnl.net

Chem. Eng. Comm., 192:787–794, 2005Copyright # Taylor & Francis Inc.ISSN: 0098-6445 print/1563-5201 onlineDOI: 10.1080/009864490521246

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Relevant Theoretical Consideration

The basic reaction between IAA and sodium hydroxide can be written as

CH3COOC5H11 þNaOH�!C5H11OHþ CH3COONa

In view of the limited solubility of IAA in the aqueous alkali solution, the ester andthe alkali solution exist as two distinct phases and both phases must contact eachother for the reaction to occur. Hence both the rate of mass transfer of IAA acrossthe liquid-liquid interface and the rate of chemical reaction in the bulk aqueousphase will enter the overall rate expression.

If the concentration of IAA in aqueous alkali solution is [A0], the volumetric rateof mass transfer is given by kLa([A

�]� [A0]) and that is equal to the rate of alkalinehydrolysis of the ester in the aqueous phase, given by k2 [A0] [B0]. By equating thetwo expressions the unknown concentration [A0] can be determined.

By making the above mass balance, the rate of extraction accompanied bychemical reaction of IAA can given by the following expression:

RAa ¼ �d½B0�=dt ¼ ½A��ð1=kLaþ 1=k2½B0�Þ�1 ð1Þ

where a is the interfacial area per unit aqueous phase volume. In Equation (1) [A�] isthe solubility of IAA in aqueous alkali solution. Since the ester reacts with aqueousalkali, it is difficult to experimentally measure its solubility in aqueous alkali sol-ution. It is, however, possible to estimate the solubility of IAA in alkali solutionby knowing its solubility in aqueous solution (in the absence of any alkali) and someother parameters. Van Krevelen and Hoftizer originally proposed such a method forthe estimation of solubility of gases in electrolyte solutions (Danckwerts, 1970); laterNanda and Sharma (1966) extended the method for liquid - liquid systems. The ratioof the solubility of IAA in aqueous alkali, [A�], to that in aqueous solution, [Aw], isgiven by the relation (the so-called Sechenov equation):

logð½A��=½Aw�Þ ¼ �KsI ð2Þ

In Equation (2) I is the ionic strength of the electrolyte solution and Ks is a para-meter that is itself the sum of contributions due to cations, anions, and ester. Basi-cally, Ks ¼ iþ þ i� þ iester, where i denotes contributions of various species inaqueous solution.

If the extraction experiments are conducted in a stirred cell having a flat liquid -liquid interface and the mass transfer coefficient of the stirred cell is known, the rateconstant k2 can readily be calculated by using Equation (1) from the experimentallydetermined values of RAa. Alwan et al. (1983) used this method for the determi-nation of rate constant for the alkaline hydrolysis of NAA.

Equation (1) may be used slightly differently to determine simultaneously therate constant and the mass transfer coefficient. We may recast Equation (1) asfollows:

½A��=RAa ¼ 1=kLaþ 1=k2½B0� ð3Þ

If the extraction experiments are conducted in a number of dilute alkali solutionswhose physical properties such as viscosity and density do not vary considerably,and the rates of extraction are determined, we will have a number of [A�]=RAavalues. For dilute solutions having almost identical physical properties kLa will

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remain practically unchanged. Under these conditions, if a plot of [A�]=RAa versus1=[B0] is constructed, we will get a straight line, the slope of which will give us 1=k2and the intercept of which will give 1=kLa. The notable feature of this method is thatwe do not require accurate values of kLa (usually determined from a separate set ofexperiments involving physical extraction) in order to determine the rate constant.Moreover, a reasonable value of kLa is an indirect validation of the experimentalresults. To the best of our knowledge, no researcher has so far reported the use ofthis technique for analyzing results of liquid-liquid reactions of the type studied here.

Materials

IAA, sodium hydroxide pellets, and sodium chloride of AR grade were procuredfrom S. D. Fine Chem. Pvt Ltd, Mumbai, India and were used as such withoutany further purification. Distilled water was used for preparing all aqueous solu-tions. Fresh solutions were made prior to every experiment.

Experimental Procedure

Experiments were conducted in a stirred cell with a flat liquid-liquid interface atambient temperature. A beaker of diameter of 5.5 cm was used as a stirred cell.A six-bladed turbine impeller of diameter 3 cm was used for stirring the aqueousphase. In all experiments the aqueous alkali solution was first transferred into thestirred cell and the impeller was then positioned just below the top surface of theaqueous solution. The ester was then transferred on top of the aqueous phase andthe reaction started by switching on the impeller. The impeller position in the aque-ous phase was adjusted so that maximum mixing of the aqueous phase without for-mation of any ripple or disturbance at the interface could be achieved. The speed ofagitation was maintained at 40� 2 revolutions per minute.

The alkaline hydrolysis of IAA resulted in the formation of isoamylalcohol andacetic acid. The alcohol was extracted into the organic phase and the acetic acidreacted with aqueous alkali and reduced its strength. In order to ensure that therewas essentially no mass transfer resistance in the organic phase (because of dilutionof the ester with the alcohol formed in the hydrolysis), the conversion of the esterwas not allowed to exceed 5mol% in any of the experiments reported here.

Analysis

Solubility data of IAA in aqueous solution and in aqueous sodium chloride solutionare not reported in the literature. We determined them experimentally and the valuesare reported in Table I. In the same table solubility values of n-propylacetate, n-butylacetate, isobutylacetate, and NAA in aqueous solutions are reported. These data areprovided for the sake of comparison. The solubility of IAA in aqueous sodiumhydroxide solution cannot be measured experimentally because of the hydrolysisreaction; it was therefore, determined by using Equation (2). The Ks value isreported in the footnote to Table I.

The progress of the two-phase hydrolysis of IAA was determined by noting thefall in normality of the aqueous alkali solution. Standard acid solutions were usedfor titration of the alkali. A 10mL sample from the aqueous phase was withdrawn

Kinetics of Alkaline Hydrolysis of Isoamylacetate 789

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and analyzed for the alkali content. In some cases, particularly when low titer valueswere obtained, the volumes of the aliquots were larger.

Results and Discussion

Properties of the Aqueous Phase

It was mentioned earlier that a plot of [A�]=RAa against 1=[B0] has been constructedin the present work in order to find simultaneously the rate constant for the hydroly-sis and the mass transport coefficient relevant for the hydrodynamic conditions ofthe experiment. In order that such a plot can be constructed, the physical propertiesof the aqueous phase must remain reasonably constant. We used aqueous alkalinesolutions having strengths varying from N=20 to N=5 for determining the rates ofhydrolysis of IAA. In Table II are reported the density and viscosity of the differentalkali solutions used in the work. We find that the solution properties, including theviscosity, that influence the mass transport coefficient remain practically constant asthe concentration of alkali (NaOH) in the aqueous phase is varied.

Table I. Solubility of isoamylacetate and other esters in water and sodium chloridesolutions at 28� 2�C

Solute Solvent Solubility� 105 (mol)=mL)

Isoamylacetate Water 1.91n-Amyl acetate Water 1.38n-Butyl acetate Water 6.14Iso-butylacetate Water 6.98n-Propylacetate Water 14.7Isoamylacetate 5% NaCl 1.64Isoamylacetate 10% NaCl 1.46Isoamylacetate 15% NaCl 1.28Isoamylacetate 20% NaCl 1.13

ks value used for determining the solubility of IAA in aqueous sodium hydroxidesolution ¼ 0.280.

Table II. Properties of the aqueous alkali solutions

Solution Density (g=mL) Viscosity (centipoises)

N=5 NaOH 1.25 0.96N=10 NaOH 1.00 0.799N=15 NaOH 0.999 0.781N=20 NaOH 0.998 0.7585% NaClþNaOH 1.03 1.08910% NaClþNaOH 1.07 1.19815% NaClþNaOH 1.11 1.35620% NaClþNaOH 1.15 1.563

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Rates of Hydrolysis of IAA with Various Alkali Solutions

Table III gives the rates of alkaline hydrolysis of IAA with alkali solutions of differ-ent concentrations and also with alkaliþ sodium chloride solutions. For the firstfive concentrations, the plot of [A�]=RAa versus 1=[B0] was constructed and is shownin Figure 1. There is good linearity of the data; the equation given below representsthe data:

ð½A��=RAaÞ ¼ ð0:105Þð1=½B0�Þ þ 0:0849 � 104

The rate constant obtained from the plot is 1=0.105, that is, 9.52 cm3=(mole)(sec).This is reported in Table III. The intercept of the plot gives the reciprocal of thevolumetric mass transfer coefficient; its value is 1.17� 10�3 sec�1. The stirred cellused in the work had a diameter of 5.5 cm; the aqueous phase volume was 70mL.Therefore, the interfacial area for the hetreogenous system was 0.339 cm2=cm3. Bydividing the volumetric mass transfer coefficient by interfacial area we get the masstransfer coefficient, 3.47� 10�3 cm=sec. This is a very reasonable value at the rel-evant hydrodynamic conditions and is an indication of the correctness of the experi-ments. It is to be noted, however, that a small error in analysis by titration will tendto manifest itself in a relatively big way as the left-hand side of Equation (3) containsthe term [A�]=RAa (the numerator and denominator are experimentally determined,involve dilute solutions, and are sources of error). That is the limitation of thepresent metnod.

When we compare the rate constant for the alkaline hydrolysis of IAA with thatof NAA, we find that the rate constant for IAA is around nine times lower than thatfor NAA. A qualitative explanation for the reduced value of the rate constant forIAA may be given. Perhaps the steric hindrance associated with the structure ofIAA as compared to NAA is the reason for the reducted rate constant. The relativerates for alkaline hydrolysis of methyl acetate, ethyl acetate, and isopropyl acetateunder otherwise uniform conditions are, respectively, 1, 0.47, and 0.10, as reportedby Finar (1973). The decrease in the relative rates, as the carbon chain attached tothe acetate group becomes longer, is not consistent. It is pertinent to add that suchvalues are not reported for higher esters.

A comparison of the resistances associated with the hydrolysis for very dilutealkali solution also indicates that about 70% resistance is associated with intrinsicchemical reaction and the rest, 30%, is due to mass transfer across the liquid-liquidinterface under the experimental conditions

Table III also gives the values of the rates of hydrolysis of IAA and the rateconstants when the aqueous phase contained an added electrolyte (in this case,sodium chloride). The addition of sodium chloride into the aqueous phase changedits physical properties to some extent (data given in Table II). The mass transfercoefficient obtained from the plot was modified in order to suit the changedconditions as per the procedure described by Ghosh et al. (2001). The mass trans-fer coefficient values are also reported in Table III. The rate constants were calcu-lated by using Equation (1) for the data points obtained in the presence of sodiumchloride in the aqueous phase. An examination of the rate and rate constant valuesgiven in Table III reveals that both the rates of hydrolysis and the rate constantsare to some extent redued due to the presence of sodium chloride in the aqueousphase. The hydroxide ion attacks the ester as a nucleophile, and its action isperhaps suppressed by the presence of chloride ion in the aqueous phase due to

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Table

III.

RatesofhydrolysisofIA

Aandtherate

constantvalues

at28�2� C

Initial

concentration

ofalkali

(mol=mL)�103

Finalconcentration

ofalkali

(mol=mL)�103

Rate

of

hydrolysis

(mol=mL

sec)�109

Averagealkali

concentration

(mol=mL)�103

Solubility

ofester

(mol=mL)�105

Mass

transfer

coefficient

(cm=sec)�103

Second-order

rate

constant

cm3=(m

ol)(sec)

0.180

0.139

11.49

0.16

1.727

0.111

0.078

9.23

0.095

1.801

0.090

0.060

8.16

0.0756

1.873

3.48

9.52

0.062

0.037

6.94

0.0495

1.886

0.054

0.034

5.55

0.044

1.889

0.1171þ5%

NaCl

0.1006

4.58

0.1089

1.629

3.27

3.45

0.094þ10%

NaCl

0.087

1.94

0.0905

1.44

3.04

1.76

0.094þ15%

NaCl

0.086

2.22

0.088

1.26

2.75

2.46

0.098þ20%

NaCl

0.091

1.94

0.0945

1.10

2.46

2.36

Tim

eofexperim

ents

foralldata

reported

¼3600seconds.

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anion-anion interaction. Higher sodium chloride concentrations, however, did notreduce the rate constants drastically.

Conclusions

A systematic investigation of the alkaline hydrolysis of IAA has been accomplished,and the rate constant for the hydrolysis has been determined. The rate constant isaround 9.52 cm3=(mol)(sec) for an aqueous alkali solution without any additionalelectrolyte being present. The presence of sodium chloride in the aqueous phase doesalter the rate constant significantly. The mean value of the rate constant (taking intoconsideration all the data for sodium chloride) is around 2.5 cm3=(mol)(sec). Therate constant for the hydrolysis of IAA with aqueous sodium hydroxide is muchlower than that for NAA (85 cm3=(mol)(sec)) under otherwise uniform conditions.Our work also confirms that the hydrolysis of IAA conforms to the ‘‘diffusion-con-trolled slow reaction regime.’’

Nomenclature

a interfacial area, cm2=cm3

[A�] solubility of ester in aqueous alkali, mol=cm3 aqueous phase[Aw] solubility of ester in aqueous solution, mol=cm3 aqueous phase[B0] concentration of aqueous alkali, mol=cm3

I ionic strength, mol=LkL mass transfer coefficient, cm=seck2 second-order rate constant, cm3=(mol)(sec)Ks iþ þ i� þ iester, L=molRA specific rate of extraction, mol=(cm2)(sec)t time, sec

Figure 1. A Plot of [A�]=RAa vs. 1=[B0] to determine the second-order rate constant.

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References

Alwan, S., Hiraoka, S., and Yamada, I. (1983). Extraction rate of n-amyl acetate with alkalinehydrolysis in the aqueous phase, Chem. Eng. Commun., 22, 317–328.

Danckwerts, P. V. (1970). Gas-Liquid Reactions, McGraw-Hill, New York.Doraiswamy, L. K. and Sharma, M. M. (1984). Heterogeneous Reactions: Analysis, Exam-

ples, and Reactor Design, John Wiley, New York.Finar, I. L. (1973). Organic chemistry, 6th ed. 250, Longman, London.Ghosh, B., Mukherjee, D. C., Bhattacharjee, S., and Chaudhuri, B. (2001). Alkaline hydroly-

sis of isoamylformate: Effect of dissolved electrolytes on kinetics, Can. J. Chem. Eng., 79,148–155.

Nanda, A. K. and Sharma, M. M. (1966). Effective interfacial area in liquid-liquid extraction,Chem. Eng. Sci., 21, 707–714.

Westerterp, K. R., van swaaij, W. P. M., and Beenackers, A. A. C. M. (1984). Chemical Reac-tor Design and Operation, John Wiley, New York.

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