Ethyl benzoate, hydrolysis

On the basis of the general mechanism for acid catalyzed ester hydrolysis shown in Figure 20 4 write an analogous sequence of steps for the spe cific case of ethyl benzoate hydrolysis... 

A number of correlations involving linear relationships between pre-exponential factors and activation energy are widely quoted for reactions on surfaces, as we shall see in the next chapter. This behavior, commonly termed the compensation effect implies a linear relationship between A7/ and A5. An exact linear relationship between A77 and TAS means that there is no variation in AG for the series. An example of this for a liquid-phase reaction is given by Fairclough and Hinshelwood for ethyl benzoate hydrolysis [R.A. Fairclough and C.N. Hinshelwood, J. Chem. Soc., 1573 (1937)]. 

In an extension of the work described m the preceding section Bender showed that basic ester hydrolysis was not concerted and like acid hydrolysis took place by way of a tetrahedral intermediate The nature of the experiment was the same and the results were similar to those observed m the acid catalyzed reaction Ethyl benzoate enriched m 0 at the carbonyl oxygen was subjected to hydrolysis m base and samples were isolated before saponification was complete The recovered ethyl benzoate was found to have lost a por tion of Its isotopic label consistent with the formation of a tetrahedral intermediate... 

Fig. 4.2. Conelation of acid dissociation constants of benzoic acids with rates of alkaline hydrolysis of ethyl benzoates. [From L. P. Hammett, J. Am. Chem. Soc. 59 96 (1937).]...

Quantitative Hydrolysis of Ethyl Benzoate -The quantitative estimation of an ester Ijy hycliolysis is conducted as follows a standaid half-normal solution of alcoholic potash is prepared by dissolving 7 grams of caustic potash m about an equal weight of water and diluting to 250 c.c. with absolute alcohol. The liquid is alloived to stand ovei night in a stoppered... 

The rates of alkaline hydrolysis of ethyl benzoate, ethyl 2-thenoate, and 3-thenoate are very similar. Also, the acid constants of benzoic acid (pAfl = 4.20) and 3-thenoic acid (pKa " 4.08) are of the same magnitude, whereas 2-thenoic acid (piCa = 3.49) is stronger. 

Dibenzoylmcthane has been prepared by the hydrolysis of dibenzoyl acetic acid 1 by the slow spontaneous decomposition of acetyl dibenzoyl methane 2 by the action of metallic sodium,3 sodium ethylate,3 sodium methylate,4 alchoholic potash,4 or sodamide 5 on mixtures of acetophenone and ethyl benzoate and by the action of alcoholic potash,6 sodium methylate,7 or sodium ethylate 8 on benzalacetophenone dibromide. 

LFER. Consider the rate of base hydrolysis of a series of ethyl benzoates given by d[C2HsOH]/dt = LfdALQ COOQHsHOH"]. Show that a plot of log Jfcx versus log Ka, where Ka is the ionization constant of the parent benzoic acid, should be linear, and relate its slope to that given by a conventional Hammett plot of log k versus a. 

Yukawa and coworkers (1972)84 determined a0 values from the rate constants for alkaline hydrolysis of m- and p-substituted-benzyl benzoates in 70% (v/v) aqueous acetone at 25 °C. ap° values for SOMe and S02Me were found to be 0.573 and 0.749 respectively. These were compared with 0.564 and 0.721, respectively, for values determined from the rate constants of alkaline hydrolysis of substituted ethyl benzoates in 85% aqueous ethanol. From these values there is no evidence for any — R cross-conjugative effect of SOMe as a substituent in the benzoate moiety, which is eliminated when it is in the benzyl. However, both the values for SOMe are substantially higher than most of the ap values for SOMe which we have surveyed previously. For S02Me the order ap° > if significant,... 

The intermediates 74 and 76 can now lose OR to give the acid (not shown in the equations given), or they can lose OH to regenerate the carboxylic ester. If 74 goes back to ester, the ester will still be labeled, but if 76 reverts to ester, the 0 will be lost. A test of the two possible mechanisms is to stop the reaction before completion and to analyze the recovered ester for 0. This is just what was done by Bender, who found that in alkaline hydrolysis of methyl, ethyl, and isopropyl benzoates, the esters had lost 0. A similar experiment carried out for acid-Catalyzed hydrolysis of ethyl benzoate showed that here too the ester lost However, alkaline hydrolysis of substimted benzyl benzoates showed no loss. This result does not necessarily mean that no tetrahedral intermediate is involved in this case. If 74 and 76 do not revert to ester, but go entirely to acid, no loss will be found even with a tetrahedral intermediate. In the case of benzyl benzoates this may very well be happening, because formation of the acid relieves steric strain. Another possibility is that 74 loses OR before it can become protonated to 75. Even the experiments that do show loss do not prove the existence of the tetrahedral intermediate, since it is possible that is lost by some independent process not leading to ester hydrolysis. To deal with this possibility. Bender and Heck measured the rate of loss in the hydrolysis of ethyl trifluorothioloacetate- 0 ... 

Figure 20. Relations between AG and between E values for two related reactions alkaline hydrolysis of substituted methyl and ethyl benzoates (188, 228).

This is borne out by a comparison of the rates of base-catalysed hydrolysis (cf. p. 238) of m-N02 (5), and of m-Me (6), substituted ethyl benzoates with that of the unsubstituted ester a reaction in which the slow, and hence rate-limiting, step is initial attack on the ester by eOH (p. 239) ... 

We have already had some discussion of a reaction with a +ve p value, reaction 13 in the list (p. 364), the base-catalysed hydrolysis of m- and p-substituted ethyl benzoates (11) ... 

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