Indian Journal of ChemistryVol. 28A, November 1989, pp. 953-957

Kinetics ·ofacid catalysed hydrolysis of methyl, ethyl andbutyl acetates in amide, salt and surfactant environments

Prabhat K Das Gupta & S P Moulik"Department of Chemistry, Jadavpur University, Calcutta 700 032

Received 9 September 1988; revised 12 December 1988; accepted 23 January 1989

The rate of the acid catalysed hydrolysis of ethyl acetate decreases in the presence of urea, substitutedureas, thiourea, acetamide and sodium chloride. The amides affect the rate through significant deactiva-tion of the catalyst, H + ion; sodium chloride does this by decreasing the activities of both H + ion and wa-ter as well as by lowering the polarity of the medium. Micellar solutions of sodium dodecyl sulphate(SDS) and cetyltrimethylammonium bromide (CTAB) increase the rates of hydrolysis of methyl, ethyland butyl acetates, the effect of the former being greater than that of the latter. At 10 times cmc of SDS,the rate enhancement for methyl and ethyl acetates at 310K are 1.07 and 2.12 folds respectively, and5.05 folds for butyl acetate at 304K; butyl acetate is the most affected ester. The micelles lower the acti-vation energy and act as catalyst in the hydrolytic cleavage of the esters. The rate constants and the acti-vation parameters of butyl acetate hydrolysis in SDS medium undergo sharp changes at [SDS] equal to itscmc.

Urea (pKb= 13.82) in aqueous medium increasesthe pH and pK of weak acids 1.2 and retards the rateof acid hydrolysis of esters by reducing the H+ ionactivity":", Urea breaks water structure+" and un-dergoes self aggregation at higher concentration".We have reported that reduced catalytic activity ofH + ion in the acid hydrolysis of methyl acetate isdue to the association of urea and substituted ureaswith H+ ions, mainly through its, carbonyl oxygen.Title investigation is an extension of our earlierwork.

Urea hinders the association of hydrophobes be-cause of its demicellization property'<!''. Using sim-ple electrostatic considerations, Hartley!' showedthat cationic micelles enhance the rate of nucleo-philic anions with uncharged substrates, while an-ionic micelles retard it but catalyse the reactions in-volving cations. Kinetically we have studied the ef-fects of both cationic and anionic surfactants, cetyl-trimethylamrnonium bromide (CTAB) and sodiumdodecyl sulphate (SDS) on the rates of hydrolysis ofmethyl and ethyl acetates in the presence of urea.The acid catalysed hydrolysis of butyl acetate hasbeen also investigated in SDS environment.

Materials and MethodsUrea, methylurea, ethylurea, thiourea and sodium

chlo ride were of AR (BDH) grade and their puritiesascertained before use. Acetamide (GR, E Merck),ethyl acetate and butyl acetate (AR, BDH) wereused as received. Hydrochloric acid (GR, E Merck)was used for the hydrolysis of the esters. CTAB and

SDS (GR, E Merck) were the same samples usedearlier!". All solutions were prepared fresh prior touse. If not stated otherwise, all measurements weretaken at 310 ± 0.02K (for detailed experimentalprocedure, we refer to our earlier publication '), Theestimated standard deviations in the rate constantswere within ± 1%. This, imparted deviations in the~ Gf, ~ Ht and ~St to the maximum extents of ±0.5%, ± 0.5% and ± 1.0% respectively.

Results and DiscussionThe deactivating effect of the additives (Table 1)

on ethyl acetate hydrolysis are in the order: methyl-urea =:: ethylurea > urea > acetamide. Of all theadditives, the effect of urea has been elaboratelystudied; the rate constants decrease exponentiallywith the increase in [urea]. The ratio of the rate con-stants in the presence of an amide (k) and that in itsabsence (ko), is taken as a measure of the deactivat-ing influence of the amide at the respective [acid](Fig. 1). Though the amides increase the dielectricconstant of the medium, the rates remain un-changed. The bonding of H + ions with the oxygen ofamide CO in retarding the rate is much more ascomparedto the effect of the increased dielectric constant ofthe medium in increasing the rate. Thiourea dec-reases the rate to some extent, though the effect isless pronounced than that of urea. This is owing tothe lesser strength of bonding of H + ions with sul-phur atom of thiourea than that with oxygen atom ofthe amide CO due to the lesser electronegativity of

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INDIAN J CHEM, SEe. A, NOVEMBER 1989

Table I-Effects of additives on rate constants of acid hydrolysis of ethyl acetateand methyl acetate at 310K

[Additive) mol dm ? 0 0.2 0.5 1.0 2.0 3.0 4.0 6.0(A) Ethyl acetate in 0.5 mol dm"? HCl

k(urea) 3.64 2.66 2.14 1.53 0.99 0.40 0.32k(methylurea) 1.59 0.69k(ethylurea) 1.57 0.82k(acetamide) 2.70 2.56 1.18 1.12

k(thiourea) 2.24 2.18k(NaCl) 2.73 1.96

k(NaCI in 2.0 mol dm-3 0.73 0.56urea)

[Urea) mol dm-3 0 1.0 2.0 4.0

(B) Ethyl acetate in 0.3 mol dm-3 HCl

k(0.1 mol dm"? SDS) 6.66 2.89 1.76 0.96

k(0.1 mol drn"? CTAB) 5.15 2.53 1.43 0.61

[Urea) mol dm " 0 1.0 2.0 4.0

(C) Methyl acetate in 0.3 mol dm-3 HClk

(0,1 mol dm-3 SDS) 6.81 2.90 1.63 0.80k

(0.1 mol dm-3 CTAB) 6.39 2.60 1.54 0.76k

(2.0 mol dm ? NaCl) 6.22 1.57

k(1.0 mol dm-' NaCl) 1.68

k= Rate constant x 103 min-I; kfor methyl and ethyl acetate in 0.3 mol dm-3 HCl are 6.35 x 103 and 3.14 x 103 min-I respectively.

'·0

..l<-,.l<

OL- ~L- ~ ~~o 2·0 4·0 -6·0

[Additiva],mol dm-J

Fig. I-Dependence of klko on additive concentration. [0,~,.and 0 represent urea, methyl urea, ethylurea, and acetamide re-

spectively)

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the former. We believe that urea competes with theester for the H + ion and the retardation is a conse-quence of the reduced [H+] for catalysis. We haveshown earlier' that the klko can be a measure of therelative activity of the H+ ion in urea medium. Therelative activity values closely agree with those re-ported by Bull et al.', recent pH metric measure-rnents? also corroborate this. A negative salt effecton the rate of hydrolysis has been observed (Table1). The combined effect of urea and NaCI is morethan the influence of the individual species. Resultsgiven in Table 1 show that the rate constant at 2 moldm? urea is 0.99 x 10-3 min-I, whereas in combi-nation with 1 and 2 mol dm ? NaCl, the values are0.73 x 10-3 and 0.53 x 10-3 min-I respectively. Atthese two salt concentrations, the rate constants in 2mol dm? urea medium (0.99 x 10-3 min-I) are ap-proximately reduced by 25% and 50% respectively.

DAS GUPTA et al.: KINETICS OF HYDROLYSIS OF METIIYL, ETHYL & BUTYL ACETATES

The retarding effects of 1 and 2 mol dm"? NaCI onthe rate constant in the absence of urea (3.64 x 10-3min-I) are also approximately 25% and 50% re-spectively (vide row 6, columns 5 and 6, Table 1).The deceleration effects of the salt are thereforeproportional in all situations. NaCI not only dec-reases the dielectric constant of the medium but alsothe activities of both H + ion and water. The effect ofadded NaCl parallels that of urea. Same trend hasbeen observed also in the case of methyl acetate hy-drolysis. Though quantitative partitioning of such aneffect will be interesting, it is likely to be a complexproblem.

In the absence of the catalyst (H + ions) the sur-factants cannot initiate the hydrolysis of the esters.In the presence of acid, both SDS and CTAB in-crease the rate of hydrolysis of ethyl acetate (Table1). The magnitude of increase of the rate of hydroly-sis is higher in the presence of SDS than that in thepresence of CTAB. Similar effects have been ob-served, though to a lesser degree, in the case of me-thyl acetate. The concentrations of the surfactantsused were 10 times and 100 times the micellar con-centrations of SDS and CTAB respectively (cmcs ofSDS and CTAB are taken to be 10.0 and 1.0 mmoldrn>' respectively'<), The effects of micelles on therates of reaction have been well demonstrated-":".Several factors, such as the adsorption of the sub-strates in the Stern layer of the micelles, effectivemicellar -charge, the local dielectric constant, andthe local water structure are important factors forthe micellar catalysis. Ethyl acetate can be prefer-ably solubilised (adsorbed) in the Stem layer zone ofthe micellar surface. Due to the electrostatic attrac-tion, anionic micelles of SDS preferentially holdmore H+ ions than the cationic micelles of CTAB.In the combined environment of the surfactants andurea the rate acceleration and deceleration effects ofthe micelles and urea respectively acted in opposi-tion, the overall values. are presented in Table 1. Incontrast to the effect of urea, the rate constants inthe presence of SDS and CTAB are greater. The ef-fect of urea includes its normal effect of deactivatingthe H + ion and destroying the micelles. The ratiosof the rate constants klko at various [urea] at con-stant concentration of the surfactants (Table 1) aredifferent at corresponding [urea] in the absence ofsurfactants. This suggests deviation from the normalaction of urea in micellar media. How demicelliza-tion effect of urea is linked with its deactivating ef-fect 011 H + ion in influencing the rate of hydrolysis isdifficult to ascertain at present. The activation ener-gy of the hydrolysis is hardly affected by urea addi-tion", it will be subsequently seen that micelles canaffect the activation energy of the process. Urea and

o 0+ II , test I +

H30 + R-C-OR ~ R-C- 9 - R'+ H20

H

o H 0 H 0II /Sl~w I / "

H20+R-C-O -R-C-O+~R-C-O+ H +R'OH

"R' 1+ "R' 2/0"

H H

••• (1)

··(2)

o 0II t tn st /I

R-C-O H2+H20 -R-C-OH + H30+

SCH::ME1.

... {l)

micelles therefore affect the rate by different me-chanisms. The acid-catalysed hydrolysis of carbox-ylic esters essentially occurs by the mechanism 16

shown in Scheme 1.

The results of hydrolysis of ethyl acetate in thepresence of fixed [H +] and tempeature show thaturea, substituted ureas and acetamide slow downthe rate-determining step (2). The following threepossibilities may account for the rate decrease: (a)Urea and other additives change the reaction pathby modifying the activated complex; (b) water parti-cipation in the step (2) might be hampered by thepresence of additives, through their influence on theactivity of water and its structure; and (c) urea due toits ability to fix H+ ions effectively decreases the ef-fective activity of the catalyst H30+ ions present inthe system which in turn affects the formation of thecomplex.

Values of the activation parameters as determinedboth in the presence and absence of additives for thehydrolysis of methyl acetate and ethyl acetate re-ported earlier" rule out the possibility (a) above. Thepossibility (b), if it be of any consequence cannot ex-plain such a large decrease in the rate. The entropyof activation.' is not affected by urea to any signifi-cant extent. Therefore, possibility (c) seems mostplausible. The > C = 0 group of the amide is thecentre of attachment of H + ions of the strong acid 17.

The rate influencing property of the amides is essen-tially through competition with the ester for the H +ions",

The rate constants and the energetics of the hy-drolysis of butyl acetate at 304 K and 318 Kin 1.0mol dm? HCI and in the presence of SDS are pre-sented in Table 2. The rate constants progressivelyincrease in the presence of SDS at both the tempera-tures; their ratios with and without the surfactant atthe studied concentrations are also presented inTable 2. The more or less comparable values at boththe temperatures advocate minimum effect of tem-perature in the studied range on the physicochemi-

955

INDIAN J CHEM, SEe. A, NOVEMBER 1989

Table 2-Rate constants, their ratios with and without SD~, and energetics of acid hydrolysis ofbutyl acetate in SDS at 304K in 1.0 mol dm-3 HCl

SDS (mol dm=') 304K 318K E. !:.Hf !:.Gf -!:. Sfx 104 (kl mol "] (kJmol-l) (kJmol-l) (Jmol-IK-I)

kx 104 min-I kllce k»: 104 min-I klko0 2.87 8.52 62.42 59.89 105.5 149.91 4.81 1.68 14.40 1.69 62.90 60.37 104.2 144.110 7.7~ 2.69 23.40 2.75 63.54 61.01 103.0 138.0100 9.58 3.34 29.71 3.49 64.92 62.39 102.4 131.7

500 11.46 3.99 35.30 4.14 64.53 62.00 102.0 131.5900 14.07 4.90 42.92 5.04 63.98 61.45 101.4 131.6

1000 14.49 5.05 44.60 5.24 64.49 61.96 101.4 129.61300 15.80 5.51 48.44 5.69 64.26 61.73 101.2 129.7

OL-~ L- ~ ~o 0·01 0·05 0·1

[SOSl,mol dm-J

Fig. 2-Dependence of rate constants on SDS concentration. [0,304K; ., 318K)

cal nature of the SDS micelles. The rate constantsand the energetic parameters are plotted in Figs 2and 3; they show breaks at 10 mmol dm-3, whichcorresponds to the cmc of SDS (literature values liein the range 6-10 mmol drrr ").Characteristic effectof micelles on the kinetic process is evidenced 18.

The energy of activation increases in the presence ofSDS upto its cmc, thereafter it decreases; this meansthat micelles positively catalyse the reaction. It hasbeen discussed that in the absence of micelles, theretardation by urea and NaCl is caused not by af-fecting the activation path but by decreasing the ac-tivities of both H+ ion and water. In SDS medium,favourable activated complex is formed in the Stemlayer of the micelles. Comparatively higher increaseof rate constants in the concentration range belowerne than above supports premicellar aggregation ofSDS, the limiting tendency of the rate constants andthe ~ Gr in the post-micellar concentration rangemeans deactivation by micelle-micelle interaction.The increased entropy of activation ~ Sr with SDS

956

"t-oe.....xo

LU

";

"140 ";-0e:A-<l

130 I

66

150

r

[SDS1,mol dm-J

Fig. 3-Dependence of energy and entropy of activation of hy-drolysis of butyl acetate in the presence of Slxs at 304K

supports increased degree of freedom of the activat-ed complex towards conversion into products. In-termicellar interaction deactivates the complex inthe post erne region making the ~ Sr to tend to a li-miting value; the effect is minimal below erne, whereisolated premicellar aggregates offer higher rate ofincrease of ~Sr. The ~sr is a measureof the insta-bility of the complex yielding products and the dis-order of the environment; since micelles assist hy-drolysis, the ~sr values increase with [surfactant).

AcknowledgementOne of the authors (P K D G) is thankful to the

UGC, New Delhi, for the award of a teacher fellow-ship. Kind suggestions from Dr P K Bhattacharyaare much appreciated.

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Chem, 23A(1984) 192.4 Bull H B, Breese G L, Ferguson G L & Swenson C A, Arch

Biochem Biophys, 104 (1964) 297.

DAS GUPTA et al.:KINETICS OF HYDROLYSIS OF METHYL, ETHYL & BUTYL ACETATES

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13 Nogami H, Awazu S & Nakajima N, Chern pharm Bull, 10(1962) 503.

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Kagaku, 24 (1967) 87; Roux G & Viallard A, Can J Chern,54 (1976) 3658.

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