Ethyl acetate, hydrolysis rate

Specific rate coefficients (related to unit amount of acid centres) were approximately the same for solid catalysts as well as for HC1 [474]. However, when a montmorillonite clay activated by adsorption of protons on its surface was used as the catalyst in ethyl acetate hydrolysis [475], a higher specific rate coefficient (about 1.8 times at 25°C) was found for the reaction catalysed by adsorbed protons than by dissolved acid, this result being explained by the authors by an increase of activation entropy in the former case. 

This sieve effect cannot be considered statically as a factor that only determines the amount of accessible acid groups in the resin in such a way that the boundary between the accessible and non-accessible groups would be sharp. It must be treated dynamically, i.e. the rates of the diffusion of reactants into the polymer mass must be taken into account. With the use of the Thiele s concept about the diffusion into catalyst pores, the effectiveness factors, Thiele moduli and effective diffusion coefficients can be determined from the effect of the catalyst particle size. The apparent rates of the methyl and ethyl acetate hydrolysis [490] were corrected for the effect of diffusion in the resin by the use of the effectiveness factors, the difference in ester concentration between swollen resin phase and bulk solution being taken into account. The intrinsic rate coefficients, kintly... 

Fig. 20. Effect of degree of crosslinking (% DVB) of a standard ion exchanger on the diffusivities, Def (cm2 min-1), and the selectivity ratio, S = efs/ efAc ( ef = effective rate coefficient, S = sucrose, Ac = ethyl acetate). Data were obtained by rate measurements and Wheeler—Thiele analysis of simultaneous sucrose and ethyl acetate hydrolysis at 70°C [508],...

The rate of the reaction depends on the pH of the solution. If it is around or higher than the p.JCa of the thiol, thiolate anion will be formed and this opens the epoxide much faster than does the unionized thiol. The nucleophile is regenerated by the oxyanion produced in the rate-determining step. A more familiar example is the base-catalysed hydrolysis of esters we have mentioned several times in this chapter. The full pH-rate profile (Chapter 13) for the hydrolysis of a simple ester such as ethyl acetate shows just two straight lines meeting each other (and zero rate) at about neutrality. Ethyl acetate hydrolysis occurs by SAC or SBC only. 

Chemical heterogeneities present in soils, sediments, and aquifers undoubtedly have an effect on rates of pollutant degradation. Other sources of surface catalysis not discussed here include Bronsted acidity of surface sites, that become apparent as surfaces become dehydrated (El-Amamy and Mill, 1984). Surface and pore structure may play a role in the catalysis of phosmet hydrolysis by montmorillonite (Sanchez-Camazano and Sanchez-Martin, 1983) and in the catalysis of ethyl acetate hydrolysis by zeolites (Nam-ba et al., 1981). 

In one of the earliest kinetic studies of an organic reaction earned out m the nine teenth century the rate of hydrolysis of ethyl acetate m aqueous sodium hydroxide was found to be first order m ester and first order m base... 

Hydrolysis of vinyl acetate is catalyzed by acidic and basic catalysts to form acetic acid and vinyl alcohol which rapidly tautomerizes to acetaldehyde. This rate of hydrolysis of vinyl acetate is 1000 times that of its saturated analogue, ethyl acetate, ia alkaline media (15). The rate of hydrolysis is minimal at pH 4.44 (16). Other chemical reactions which vinyl acetate may undergo are addition across the double bond, transesterification to other vinyl esters, and oxidation (15—21). 

In contrast to the hydrolysis of prochiral esters performed in aqueous solutions, the enzymatic acylation of prochiral diols is usually carried out in an inert organic solvent such as hexane, ether, toluene, or ethyl acetate. In order to increase the reaction rate and the degree of conversion, activated esters such as vinyl carboxylates are often used as acylating agents. The vinyl alcohol formed as a result of transesterification tautomerizes to acetaldehyde, making the reaction practically irreversible. The presence of a bulky substituent in the 2-position helps the enzyme to discriminate between enantiotopic faces as a result the enzymatic acylation of prochiral 2-benzoxy-l,3-propanediol (34) proceeds with excellent selectivity (ee > 96%) (49). In the case of the 2-methyl substituted diol (33) the selectivity is only moderate (50). 

The crude ketal from the Birch reduction is dissolved in a mixture of 700 ml ethyl acetate, 1260 ml absolute ethanol and 31.5 ml water. To this solution is added 198 ml of 0.01 Mp-toluenesulfonic acid in absolute ethanol. (Methanol cannot be substituted for the ethanol nor can denatured ethanol containing methanol be used. In the presence of methanol, the diethyl ketal forms the mixed methyl ethyl ketal at C-17 and this mixed ketal hydrolyzes at a much slower rate than does the diethyl ketal.) The mixture is stirred at room temperature under nitrogen for 10 min and 56 ml of 10% potassium bicarbonate solution is added to neutralize the toluenesulfonic acid. The organic solvents are removed in a rotary vacuum evaporator and water is added as the organic solvents distill. When all of the organic solvents have been distilled, the granular precipitate of 1,4-dihydroestrone 3- methyl ether is collected on a filter and washed well with cold water. The solid is sucked dry and is dissolved in 800 ml of methyl ethyl ketone. To this solution is added 1600 ml of 1 1 methanol-water mixture and the resulting mixture is cooled in an ice bath for 1 hr. The solid is collected, rinsed with cold methanol-water (1 1), air-dried, and finally dried in a vacuum oven at 60° yield, 71.5 g (81 % based on estrone methyl ether actually carried into the Birch reduction as the ketal) mp 139-141°, reported mp 141-141.5°. The material has an enol ether assay of 99%, a residual aromatics content of 0.6% and a 19-norandrost-5(10)-ene-3,17-dione content of 0.5% (from hydrolysis of the 3-enol ether). It contains less than 0.1 % of 17-ol and only a trace of ketal formed by addition of ethanol to the 3-enol ether. 

It is otherwise for complex reactions, for which the rate equation may or may not be simply related to the overall stoichiometric reaction. For example, the rate equation for the alkaline hydrolysis of ethyl acetate, which is a complex reaction (see Section 1.2),... 

The rate constant for the acid-catalyzed hydrolysis of ethyl acetate is... 

The rate of hydrolysis in the presence of resins increases with the number of catalytically active ions. In some reactions, the reaction rate is a linear function of the quantity of catalyst added [26,34]. Figure 1 shows the effect of varying catalyst concentration on the rate of hydrolysis of ethyl acetate. Higher values of q are shown with the larger amount of catalyst. 

J mol ). This is additional evidence in favor of rate limitation by inner diffusion. However, the same reaction in the presence of Dowex-50, which has a more open three-dimensional network, gave an activation energy of 44800 J mol , and closely similar values were obtained for the hydrolysis of ethyl acetate [29] and dimethyl seb-acate [30]. The activation energy for the hydrolysis of ethyl acetate on a macroreticular sulphonated cationic exchanger [93] is 3566 J mol . For the hydrolysis of ethyl formate in a binary system, the isocomposition activation energy (Ec) [28,92] tends to decrease as the solvent content increases, while for solutions of the same dielectric constant, the iso-dielectric activation energy (Ed) increases as the dielectric constant of the solvent increases (Table 6). 

Example. A solution of ethyl acetate in pH 9.5 buffer (25°C) was assayed in triplicate several times over a 20-hour period. The data obtained are presented in Table 2. The results were plotted on semilogarthmic graph paper as shown in Fig. 3. Calculate the psuedo-first-order rate constant for the hydrolysis of ethyl acetate at pH 9.5 (25°Q. 

The hydrolysis of ethyl acetate (B) with an alkaline hydroxide (A] non-aqueous solutions is believed to have the third order rate equation... 

This agrees with A. E. Leighton s statement that when sodium carbonate is present in water used in steam boilers, the boiled water contains sodium hydroxide. J. Shields and K. Kolichen have measured the degree of hydrolysis of aq. soln. of sodium carbonate the former from measurements on the rate of hydrolysis of ethyl acetate, the latter from the decomposition of diacetone-alcohol, CH3.CO.CH2.C(CH3)2OH 2CH3.CO.CH3. The percentage hydrolysis of soln. of sodium carbonate of different cone, by the two methods are of the same order of magnitude ... 

O 16.14% crysts (from acet, benz or ethyl acetate), mp 206.5-07.5° 214.5-15.5°, de-pending on cryst size, rate of heating solv used fairly sol in hot benz CCI4 diffc sol in eth, acet, eth acet acet ac insol in ale was obtd by ozonolysis of 1,1-diphenyl-l-alkenes In CCI4 and hydrolysis of the ozonide No expin of this compd occurred when heated to its mp, but when heated for 5 mins at 214-15°, it decompd comjietely to benzo-phenone... 

Activation parameters that have been measured for Aal1 reactions are generally consistent with a unimolecular rate-determining step. The volume of activation for the hydrolysis of f-butyl acetate in 0.01 M HCI at 60°C is zero, within experimental error70. No significant change in rate is observed from atmospheric pressure up to 2 kbar, although this increase in pressure almost doubles the rate of hydrolysis of ethyl acetate in 0.1 M HCI at 35°C. 

Finally, an important study by Lane38 shows that the 180-excbange and hydrolysis reactions of ethyl acetate in aqueous sulphuric acid at 25°C depend in a closely similar way on the activity of water. Lane s plot of log kobsl [BH+] against logaH o has a slope of 2.07 for hydrolysis, while the similar plot of the data for 180-exchange has a slope of 1.84. Lane interprets this as evidence that two molecules of water are involved in the transition state for each reaction. Since the two reactions are similar in detail it seems more than ever likely that they involve a common rate-determining step. 

The effects of added salts are shown in Fig. 8. Sodium chloride has a small positive effect on the hydrolysis rate, and sodium chloride and sodium perchlorate have a similar, rather larger, effect on the rate of lactone formation. This is the expected result, for many salts increase the protonating power of the medium as measured by Hammett s acidity function116, and thus assist acid-catalyzed reactions. Sodium perchlorate, unusually, has a small negative effect on the hydrolysis rate. Qualitatively similar results have been found by Bunton et al,56, who studied the effects of added salts on the acid-catalyzed hydrolysis of ethyl acetate. Added lithium and sodium chloride assist the Aac2 hydrolysis of ethyl acetate, but the perchlorates have essentially no effect. In each case the effect is a little more positive than for y-butyrolactone hydrolysis, and, in particular, chloride anions appear to assist Aac2 hydrolysis more effectively than do the perchlorates. 

Lane38 measured the rate of hydrolysis of ethyl acetate at 25°C in 11 -79% sulphuric acid by both spectrophotometric and dilatometric techniques. He also measured spectrophotometrically the concentration of the conjugate acid of the ester in solution by measuring the absorptivity at 190 nm, and extrapolating to zero time, and by the change in the chemical shift of the acetyl protons. He found a linear relation between the ionization ratio and Hammett s acidity function... 

Fig. 9. Rate coefficients for reactions of protonated ethyl acetate. The open circles represent data for hydrolysis. Closed circles are for the oxygen-exchange reaction of the carbonyl- 80 labelled...

The acid-catalyzed hydrolysis of orthoesters is very much faster than that of esters. The second-order rate coefficient for the hydrolysis of ethyl acetate is of the order of 10"4 1-mole-1-sec1 at 25°C, whereas that for the hydrolysis of ethyl orthoacetate103 is of the order of 104 l-mole-1-sec 1, and that for the breakdown of a monoalkyl orthoester must be faster still. If the breakdown of the tetrahedral intermediate is partially rate-determining in acid-catalyzed ester hydrolysis, therefore, its concentration must be very small that is, the equilibrium for its formation must be highly unfavourable. This... 

Fig. 13. Approximate pH-rate profiles for hydrolysis of A, ethyl acetate B, phenyl acetate and C, 2,4-dinitrophenyl acetate, all at 25°C. Data from Skrabal188, Kirsch and Jencks300 and Tommila...

Fig. 14, Linear free energy relationships between the rate coefficient for neutral hydrolysis at 25°C. and the pKa of the conjugate acid of the leaving group, for a series of acetate esters. Data for substituted phenyl acetates (ref. 191), and for ethyl acetate (ref. 187).

This sensitivity to substitution of neutral hydrolysis means that the pH-independent reaction gradually becomes more important than the hydroxide reaction at the high pH end of the region, and becomes much more rapidly more important than acid-catalyzed hydrolysis at low pH. Thus from Fig. 13, the acid-catalyzed reaction can be seen to be significant for the hydrolysis of ethyl acetate between pH 4 and 5, and for phenyl acetate about pH 2 but for 2,4-dinitrophenyl acetate the acid-catalyzed reaction is not detectable at pH 1, and is presumably important only in relatively strong acid. It seems certain that this fast neutral hydrolysis is at any rate a partial explanation for the low efficiency of acid catalysis in the hydrolysis of very weakly basic esters, such as the trifluoroacetates and oxalates, in moderately concentrated acid (see p. 145). 

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