-R-!;&.-----. -----.&.----.t- - NISCAIRnopr.niscair.res.in/bitstream/123456789/18099/1/IJCA 43A(2) 258-264.pdf · hydrolysis of methyl acetate, ethyl acetate, Il-butyl acetate, ethyl

Indian Journal of Chemi stry Vol. 43A. February 2004. pp. 258-264

Effect of organized assemblies. Part I-Effect of variation in chain length of carboxy and alkoxy end of esters on the

hydrolysis of simple aliphatic esters

Ranjib Padhi , Shreelata Mishra, Pramila K Misra* & B K Sinha

Centre of SllIdies in Surface Science and Technology. Department of Chemistry, Samba lpur University, Jyoti Vihar 7680 19, India

Received 27 No vell/bel' 2002; revised 10 Novell/bel' 2003

The hydrol ys is o f some simple aliphatic esters having variation in number of carbon atoms in their alkoxy and carboxy pan has been carried out in presence of ionic and non-ionic micelles to investigate the effect of the subsequent change in chain length o n the mechani sm of hydrolysi s of esters. The effect of variation o f temperature, concentration of salt, substrate, surfactant , and acid has also been examined. The results have been asc ribed to the role of the charge o f surfactants and partitioning of the substrate into the surfactanr aggregates.

In continuation to our earlier work on hydrolysis of schiff bases 1·3 and esters4

.5 in presence of organized

assemblies the present work is an attempt to investigate the role of variation in chain length of carboxy and alkoxy end of the simple aliphatic esters on the mechanism of ester hydrolysis.

Materials and Methods SDS (CDH, New Delhi), CT AB (CDH, New Delhi)

and the esters (ethyl formate, methyl acetate and ethyl acetate, ethyl propionate, butyl acetate, Sisco Chem, Mumbai) were purified by standard methods. Triton­X-IOO (CDH, New Delh i) was used without further puri fication.

Salts were of AR grade and were dried in oven to constant weight before use. Salts solutions and Baryta soiution were prepared in triple distilled water as described by method reported earlier4

.5

.

The pseudo-first order rate constants were determined by reported methods4

.5. The volume change at infinite time was measured by refluxing the sample at 60-70°C for an hour and then reading was taken after 24 hrs . Free energy change (,1C,),

enthalpy of activation (D.H"#), 6.S ~ (entropy of activation) and activation energy (En) were evaluated from the rate data at four different temperatures using standard equations.

Results and Discussion The acid catalyzed hyd ro lysi s involves protonation

of an ester oxygen atom followed by the rate determining cleavage of C-O bond forming an acyl

carbonium ion intermediate that reacts with water to give carboxylic acid (Scheme 1). Scheme I has been proposed assuming that the hydrolys is of ester in presence of surfactants (beyond their CMC) occurs both in micellar aggregates and in bulk water phase.

o R'

A-H +R'-O- ~ -R-!;&.-----. H -----.&.----.t-R l [ Trar.s ilon5late ~ J

I 51"'" A-+R'-OH+R-c+

I o

R-C + H 0- R-C -OH+H+ I 2 I o 0

Scheme 1

ElTect of surfactant The pseudo-first order rate constants for the

hydrolysi s of methyl acetate, ethyl acetate, Il-butyl acetate, ethyl formate and ethyl propionate in presence of varying concentrations of cationic surfactant (cetyltrimethylammonium bromide, CT AB), anionic surfactant (sodium dodecyl sulphate, SDS), and non-ionic surfactant (Triton-X-lOO) beyond critical micelle concentration (CMC) are given in Tables 1-3 respectively. The hydrolysis of butyl acetate cannot be carried out beyond 0.004 M of [CT AB] due to appearance of turbidi ty in the solution.

It is observed that with increase in surfactant concentration the rate of acid hydrolysis of ester is retarded by CT AB micelles and enhanced by SDS

PADHI el al.: EFFECT OF CHA IN LENGTH ON ESTER HYDROLYSIS 259

Table I- Effect of variati on o f [CTAB] on the rate of ac id cata lyzed hydrolysis of different esters . ([ethyl formate] = 0.248 M, [methyl acetate]=0.25I M, [eth yl acetate]=0.204 M, [ethyl propionate] = 0.175 M , [II-butyl acetate]= 0.175 M, [HCI] = 0.45 M)

[CTAB] Ethyl formate Methyl acetate Ethyl acetate Ethyl propionate II -B utyl acetate inM k,x I O~ s- I. (35°C) k,x lO' s- I.(30°C) k,x lO' S- I.(30°C) k,x 105 s- I.(35°C) k,x 105 S- I (35°C)

0.000 IH I 8.32 7.66 7.67 2.43 0.001 2. 15 0.002 13.3 1 8.01 6.7 1 7.29 2. 11 0.003 1.85 0.004 12.78 7.83 6.39 6.93 1.52 0.006 12.32 6.77 0.008 12.09 7.54 6.26 0.0 10 11.5 1 7.33 6.49 0.020 9.97 7.11 6. 14 6.14 0.030 9.59 5.87 5.89 0.040 5.61 0.050 6.91 5.60 5.31 0.070 6.74 5.20

Table 2-Effect of variati on of [SDSj on the rate of acid catalyzed hydrolysis of esters. ([ethyl formatej=0.248 M, [methyl acetate]=0.25I M, [ethyl acetate]=0.204 M, lethyl propionate]=0.1 75 M, [n-butyl ace tate] =0.175 M , [HCI] =0.45 M)

[SDS] Ethyl formate Methyl acetate Ethyl ace tate Ethyl propi onate II-Butyl acetate M k,x 104 S- I (300C) k,x IO'S- 1 (30°C) k,x IO' S- 1 (30°C) k,x 10' S- I (30°C) k,x lO' S- I (30°C) , (40°C)

0.00 13.8 1 8.32 7.66 7.67 1.92 4.95 0.01 15 .35 8.5 1 8.22 9.2 1 4.02 12.70 0.02 16.63 8.93 8.44 10.22 6.93 15.70 0.03 18.42 9. 14 8.95 11 .5 1 8.37 2 1.90 0.04 19.94 9.3 1 9.59 12.27 11.20 23 .30 0.05 2 1.48 9.58 9.98 13.93 13.80 23.80 0.06 23.03 9.92 10.23 14.57 16.80 26.80 0.07 20.72 9.51 9.59 11 .5 1 15.59 34.30

Table 3-EfTect of varia ti on of [Tri ton-X-I OOJ on the rate of ac id catal yzed hydrolys is of esters ([ethyl forlllate]=0.248 M, [methyl acetate] = 0.251 M, rethyl acetate]=0.204 M, [ethyl propionate] = 0.175 M, rll-butyl acetate]= 0.175 M, [HCI] =0.45 M)

[Triton-X-IOO] Ethyl formate Methyl acetate in mll50ml k,x l04

5-1 (35°C) k,x lO' S- I (35°C)

0.00 13.8 1 13.25 0.40 13.43 13.13 0.50 14.07 13.00 0.60 13.8 1 12.97 0.70 13.43 13. 10 0.80 13.43 13. 15 0.90 13.8 1 13.00 1.00 13.20 l.l0 13.8 1

micelles. Tn presence of TX-IOO micelles no significant change was observed . These rate acceleration or inhibition of acid hydrolysis of esters in ionic micellar solutions in the present case can be attributed to (i) the distribution of the substrate between micellar phase and bulk water phase as a result of electrostatic and hydrophobic interactions of the substrate with the surfactant aggregate and (ii ) different rates of hydrolysi s of the substrate in the micellar phase and in the bulk solution .

Ethyl acetate Ethyl propionate Butyl acetate k,x lO' S- I (40°C) k,x IO-" S- I (40°C) k,xI0-5 S- I (40°C)

16.6 1 13.8 1 4.95 15.35 13.8 1 4.98 15.35 13.42 4.89

12.8 1 4.90 15.35 13.42 4.97

13.8 1 4.91 4.94

15. 35 13.8 1 4.87 13.42

The ester molecules get partitioned to the micellar phase, oriented themselves appropriately so as to have favourable hydrophobic interaction of their alkyl chain with the micellar core and that of the >CO group of esters with the stern layer. Depending on the surface charge of the micelles the potenti ally electrophilic acidic proton is either oriented at or near the mice llar surface hydrated by surrounding water molecules or away from it thereby favouring/preventing the formation of the transition

260 INDIAN J CHEM, SEC A, FEBRUARY 2004

state (Scheme 1), The electrostatic stabilization of the positively charged transition state by anionic mice lles and destabilization of the same by cationic micelle are responsible for the observed change in the rate of hydrolysis , The gradual decrease of the reaction rate on increasing CT AB concentration in all the cases (Table 1) may be attributed to the increase in number of micelles wherein more number of ester molecules are entrapped leading to decrease in the effective concentration of esters in the bulk , The acidic proton cannot approach the ester molecule to initiate the hydrolysis due to electrostatic repul sion between the proton and positive head groups of these micelles, Similar effect of cationic surfactants on ac id catalyzed hydrolysis of p-nitro phenyl propionate has been reported6

, In the presence of SDS, the rate of hydrolys is of all esters increases with a gradual increase in the concentration of SDS from 0,0 I to 0,06 M (Table 2) . With further increase of [S DS], the rate decreases in case of all esters, except butyl acetate at 40°C which retains its increasi ng trend. The rate enhancement occu rs due to the increase in effective concentration of H+ at the SDS micellar surface which can attack on carbonyl carbon atom of esters present close to the in terface. The transition state (acyl carbonium ion) is also stab ilized due to favourable col umbic interaction . The gradual incorporation of more and more es ter molecules into the micelles causes the rate en hancement till saturation. The maximum value of rate constant (k,) has been observed in all cases at about 0.06 M [SDS]. Beyond 0.06 M [SDS] the rate decreases probably due to the dec rease in effective concentration of [H+] at the miceliar surface with increase in the number of micellar aggregates in the reaction mi xture. The ester localized in one mi celle can' t in teract wi th H+ present in other micelle7

. The maxi mu m li mit possibly shifts to higher concentration of surfactants at higher temperature and hence the decrease in the rate of hydrolysis is not observed for butyl acetate at 40°C.

Menger and Portnoy using the distribu tion model (Scheme 2) and taking kinetic steps involved into consideration, have derived an expression (Eq. 1), which has been successfully applied to micelle catalyzed unimolecular and bimolecular reactions8

.

Product

Scheme 2

Here Dn = micellar aggreagete, S= Substrate (ester) , D"S = micelle-substrate complex, K., = association constant, klV and kill are the first order rate constan ts for the hydrolysis in bulk phase and micellar pseudo phase respectively.

1

where k obs= observed rate constant, CD=concentration of surfactant, N=aggregation number of surfactant molecules , K." k", and kill are mentioned in Scheme 2.

The kinetic data of all the esters with varying concentrations of SDS up to 0.06 M are fitted to the

__ l _ _ Vs 1 (CD - CMC)

above equation . Plots of k -k

H ' obs

are linear in all the cases. From the intercept and slope of each of the plot , the value of Ks / N is calculated using the values of N for SDS to be 62. The binding constants (Ks) and kill for all the esters are calculated and are given in Table 4. The val ues of Ks indi ca te that there is a substantial binding of all esters molecules with SDS micell es. The kill values are re markabl y larger due to increase in the effect ive concentration of both reactants in a small volume of micellar phase.

The effect of the non-i oni c micelles on the rate of hydro lys is is fo und to be negligible (Table 3). Generally non-ionic surfactants either decrease or have insi gnificant effects on the rate of hydrolytic reactions of carboxylic esters9

•

With increase in temperature the rate is fo und to increase in all the cases (Table 5). The hi gh negative entropy of activation suggests more o rdering of the trans ition state complex . However, there is not much dev iation in the 6.G* values due to the change in the medi um of the reaction.

Tab le 4-Association constants and ra te constants fo r acid catalyzed hydrolys is of es ters in SDS mice ll ar pseudophase. {[ethyl formate] = 0.248 M, [ethyl acetate I = 0.25 1 M. [ethyl acetate]= 0.204 M, [ethyl propionate] =0.175 M . [butyl aceta te] = 0.175 M, [HCI] =0.45 M (i n all cases)}

Esters Assoc iati on constants (Ks)

Rate co nstants in micellar phase (k",) in S-I

Ethyl formate 6.6 1x 102 38.50x I 0-4 (30°C) Methy l acetate 18.92x I 02 I 0.52xl 0-" (30°C) Ethyl acetate 4.40x 102 17.59x I0-5 (30°C) Ethyl propionate I3 .09x I02 20. 14x I0-5 (35°C) Butyl acetate 2 .43x 102 27. 12x I0-5(30°C)

--~------------~--~----~

PADHI e' al.: EFFECT OF CHAIN LENGTH ON ESTER HYDROLYSIS 261

Table 5-Effect of temperature on the rates of acid catalyzed hydrol ysis of esters ([ethyl formate]=0.248 M. [methyl acetate]=0.25 I M. [ethyl acetate]= 0.204 M, [ethyl propionate]=O. 175 M, [butyl acetate] = O. 175 M, [HCI]=0.45 M)

[Esters] k, * lOX S- I

25°C 30°C 35"C 40°C

[Surfactant] = 0.00 M Ethyl formate 8.44 10.74 13.8 1 18.42 Methyl acetate 6.63 8.32 13.25 15 .33 Ethyl acetate 6.30 7.66 12.2 1 16.61 Ethyl propionate 3.82 5.22 7.67 13.81 Butyl acetate 3.07# 1.92 2.42 4.95

[CTAB] = 0.02 M Ethyl formate 6.90 7.67 11 .51 15.35 Methy l acetate 5.75 7. 11 12.35 13.85 Ethyl acetate 5.29 6.14 10.56 14.86 Ethyl propionate 2.94 4.47 6.14 11.97 Butyl acetateS 2.31 # 1.62 1.86 2.56

[SDS] = 0.03 M Ethy I formate 9.59 12.79 18.42 24.55 Methy l acetate

. 8. 12 9.58 16.27 18.66

Ethyl acetate 7.67 8.95 13.43 19.9 Ethyl propionate 5.91 8.44 I 1.51 17.89 Buty l acetate 18.6# 8.37 14.2 21.9

x=4 for formate, =5 for acetate and propionate, *[SDS]=0.05 M, #Temp.= 37.5°C. $ [CTAB]=O.003 M.

It is seen that with constant [acid], the pseudo-first order rate constant, k, increases with increase in [ester] in the presence and absence of surfactants. The observed kinetics depend on the value of equi librium constant for the formation of substrate-proton complex and the relative concentration in H+ and substrate, and therefore, the rate enhancement due to increase in [H+] is justified Cfable 6). This is an example of specific hydrogen-ion catalys is 10.

However, at a fixed concentration of SDS increase in [ester] leads to decrease in observed rate constant in case of acetates and increase in case of formate and propionate. Behme et al. II have also made simi lar observation in the acid hydrolysis of orthoester. Increase in surfactant concentration beyond a certain value has also caused rate retardation . This dependence of substrate and surfactant concentration on the rate are consistent with the saturation of the micelle by the substrate and saturation of the substrate by the micelle.

The variation in the rate constants due to the presence of salt depends on both the mechanism and the structure of the substrate. The observed retardation in the rate in the absence of surfactant and presence of NaCI (Tables 7) may be attributed to the salt-in of uncharged esters 12. Addition of NaBr retards the rate of hydrolysis initially and then remains constant.

Ell' !'J.H" -!'J.S" !'J.C" kJ mol- I kJ mol- I at 30°C J mol- I at 30°C kJ mol- I at 30°C

40. 19 37.67 177 .57 9 1.47 46.23 43.71 178.88 97.91 52.25 49.73 159.7 1 98.12 65.6 1 63. 10 11 8.80 99.10 47.63 45.11 184.54 101.02

43.37 40.85 169.86 92.32 49.48 46.96 169.47 98.3 1 56.32 53.80 148. 11 98 .68 70. 13 67.61 105. 17 99.48 24.66 22.14 262.53 101.69

49.38 46.87 145.76 91.03 46.90 44.38 175.50 97.56 48 .83 46.31 169.70 97 .73 56.31 53.79 145.51 97.88 49. 16 46.64 164.78 96.57

In case of SDS micelles, charge neutralization of micellar surface occurs due to increased concentration of counterions. As a resul t of which electrostatic stabilization of the transition state is reduced and catalytic effect is decreased. Counterions displace the reactive species from the micellar surface. Presence of salt also decreases the number of micelles 13. Hence rate retardation is observed in these cases. In presence of CT AB micelles the rate is further retarded on adding salts. This may be due to the presence of excess chloride ions that cause a decrease in the catalytic effect of the proton . Plots of kJ vs [NaCI] in absence and presence of surfactants are found to be linear.

To investigate the effect of carbon chain length of ester on reactivity , the length of the carbon chain has been varied in both alkoxy and carboxy end of ester and the rate constant for hydrolysis are determined at constanr concentration of esters (0.175 M) and acid (0.45 M) (Table 8). The order of reactivity is methyl acetate > ethyl acetate > butyl acetate. These observations are also explicable in terms of the extent of incorporation of the esters into the micellar core due to hydrophobic interactions and destabilization of the transition state due to increase in bulkiness in the alkoxy part. The longer is the hydrophobic chain length, the larger is the incorporation into the micelle and hence less is the probability of the contact of the

262 INDI AN J CHEM. SEC A, FEBRU ARY 2004

Table 6- Vari ati on of (ester] and [acid] on the rate of acid hydrolysis of various esters: (i) et hyl formate at temperature 35°C. (ii ) methyl acetate at 30°C and 35°C respectively , (iii ) ethyl aceta te at 30°C, (iv) ethyl propionate at 35°C. (v) buty l acetate at 30"C

Ester [Esterj. M [HClj , M k/x I 0-4, S- I

[Surfactant]=O.OO M [CTA B]=0.02 M [SDS]=O.03 M

0.173 0.45 0.198 0.45 0.223 0.45

Ethy I formate 0.248 0.45 0.248 0.30 0.248 0.35 0.248 0.40

0.25 10 0.45 0.3765 0.45 0.5020 0.45

Methyl acetatc 0.6275 0.45 0.2510 0.45 0.2510 0.50 0.2510 0.55 0.2510 0.60

0.204 0.45 0.306 0.45 0.408 0.45

Ethyl acetate 0.510 0.45 0.204 0.35 0.204 0.40 0.204 0.50 0.204 0.55

0.140 0.45 0.1 57 0.45

Ethyl propionatc 0.175 0.45 0.198 0.45 0. 175 0.40 0. 175 0.50 0.175 0.55

0.100 0.45 0.200 0.45 0.300 0.45 0.400 0.45 0.175 0.10

Butyl acetate 0.175 0.20 0. 175 0.30 0.175 0.40 0.175 0.50 0.1 75 0.60

esters with the catalytic proton. In general the order of reactivity of ethyl es ters with respect to hydrolytic rates is as follows : forma te> acetate> propionate.

The micellar medium provides a multi-pocket env ironment to the substrate where the polarity of the pockets varies from highly non-polar hexane type medium to polar aq ueous medium. Mi shra el 01. 14

have quantitatively evaluated the polarity of various pockets of the medium by investigating the photophysical processes of some tailor made dyes in

8.06 6.90 10.55 9.2 1 8.06 12.79 10.36 8.82 14.07 13.8 1 9.97 l 8.42 8.63 8.44 r 1.5 1 11 .51 8.63 l3.43 12.66 9.05 15.35

8.32 7.11 9.58 8.6 1 7.45 9.16 9.02 8.0 1 8.44 9.53 8.33 8.02 13.25 12.35 16.27 16.3 1 14.72 19.23 19. 18 16.88 21.6 1 20.80 19.1 8 23. 14

7.66 6.14 8.95 8.36 6.90 7.83 11.l2 8.21 7.32 14.62 9.59 7.10 4.65 2.95 5.75 5.10 9.59 6.90 10.20 9.59

5.75 4.88 8.2 1 6.90 5.48 lO.36 7.67 6. 14 11.5 1 8.06 7. 18 5.75 4.60 9.58 8.75 6.56 14.37 11.5 1 8.2 1 8.93

1.57 1.34 9.87 2.68 1.96 8. 13 3.72 2.53 7.63 4.62 3.24 6.5 1 1.2 1 1.08 3.27 1.34 1.14 4.36 1.53 1.23 5. 17 1.76 1.36 7. 12 1.99 1.59 9. 18 2.08 1.63 10.29

cationic and anionic micelles. With increasing hydrophobic group, the substrate finds a suitable polar pocket for its resid~nce. The polarity (dielec tric constants) has an asymmetric trend around the micelle. In the present study CTAB constitutes the cationic mi celles providing a cation ic interface devoid of protons. The effective concentration of [H+] in the bul k thus increases. The decrease in the rate constants of the subst rate in the presence ' f CTA B micelles indicates the partitioning of some subs trates into the

PADHI et a/.: EFFECT OF CHAIN LENGTH ON ESTER HYDROLYSIS 263

Table 7-Effe\:t of [salt] on the acid cata lyzed hydrol ys is of (i) methyl acetate ([methyl acetale]= 0.251 M, [HCI] =0.45 M. temp.=30°C). (ii) ethyl acetate ([ethyl acetate]== 0.204 M , [HCI] =0.45 M , temp.=30°C), (iii) ethyl propionate ([ethyl propionate]= 0.175 M, [HCI I =0.45 M, temp.=35°C), (iv) butyl acetate([butyl acetate]= 0.175 M , rHCI] =0.45M, temp.=35°C), (v) (ethyl formate ([ethyl formate]= 0.248 M. [HCI] =0.45 M, temp.=35°C).

Ester [NaCI],M k,x I 04 s- I

[Surfactant]=O.OO M [CTAB]= 0.02 M [SDS] = 0.03 M

0.00 13.8 1 11.51 18.42

Ethyl formate 0.40 12.66 9.74 15.33 0.50 11.51 9.21 14.07 0.60 9.59 8.80 13.43

0.00 8.32 7. 11 9.58 Methyl acetate 0.40 7.95 6.63 9.31

0.50 7.83 6.50 9. 11 0.60 7.73 6.31 8.93

0.00 7.66 (7.66)* 6.14 8.95 Ethyl acetate 0.40 7.13 (5.75)* 4 .60 7.67

0.50 6.92 (5.75)* 4.18 7.32 0.60 6.7 1 (5.75)* 3.83 7.04

0.00 7.67 11.51 6.14 Ethyl propionate 0.40 6.33 10.22 5.46

0.50 5.75 9 .21 5.29 0.60 5.31 8.44 5.11

0.00 2.43 8.37 1.86 Butyl acetate 0.40 2.11 7.14 1.64

0.50 1.92 6.47 1.51 0.60 1.75 6.03 1.43

* = NaBr

Table 8- Comparison of pseudo-first order rate constants of hydrolys is of different esters ([Ester] = 0.175 M, [Acid] = 0.45 M, temp = 35°C)

Esters

[Surfactant] = 0.00 M

Ethyl formate 85.0 Ethyl acetate 8.00 Ethyl propionate 7.67 Methyl acetate 6.80 Butyl acetate 2.42

* [CTAB]=O.003M

micelle. Considering klCTAl3l/klwilhoul SlirfaClanll as a monitor of partitioning the substrate from aqueous medium to the micelle, it is seen that the value of this ratio decreases with increasing number of carbon atoms indicating an increase of partitioning of the substrate into the micelle with increase of carbon atoms. The trend of reactivity in the micelle remains the same as that without surfactant. It also corroborates the proposition that most of the reaction takes place in the bulk in the CTAS micellar systems.

In the presence of SDS micelles the effective proton concentration is more at the surface due to favourable electrostatic interaction. The partitioning indicator kl SDS lklwilhOlH ,"rfac,antlis found to increase exponentially with increase in the number of carbon atoms indicating most of the reaction occurring at the

k,x IO', S- I

ICTAB] = 0.02 M ISDS] = 0.03 M

72.5 114.0 6.57 12.80 6.14 15. 12 5.85 9.30 1.86' 14.20

micellar interface. The exponential rise of partitioning indicator reveals the increase in partitioning of the substrate to the micelle.

Acknowledgement The authors thank UGC(DRS) and DST(FIST) for

financial support of this work and Dr. S K Mi ~h ' t,

Department of Chemistry , Sambalpur University lor useful discussion .

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264 INDIAN J CI-IEM , SEC A, FEBRUARY 2004

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