Hydrolysis Rates of Formic Esters

From the results in Table 13, the entropy changes for the reaction (where the first formula on each side of the equation represents a OH OH 

In the words of Humphreys and Hammett, there is therefore considerable basis for the generalization that the conversion of an electrically neutral acetic acid derivative to an electrically charged entity, no matter whether this be positively charged or negatively charged, involves a considerably greater decrease in entropy than does the analogous process in the case of a formic acid derivative, whereas the enthalpy effects are likely to be nearly the same for the acetic and formic acid derivatives . 

Humphreys and Hammett have estimated that in solution the entropy of acetic acid or its derivative is about 4-6 e.u. greater than the entropy of formic acid or its corresponding derivative due to the internal freedom of the methyl group. On this basis the authors concluded that the entropy of the acetate ion must be about the same as that of the formate ion, meaning that the internal motion of the methyl group is frozen out in the ionic species. It would appear from the data, however, that the entropy of the activated complex for acetate hydrolysis is more negative than that for formate hydrolysis by another 5 e.u. A possible explanation is that the charge becomes more concentrated in the acetate complex with a resultant increase in solvent electrostriction. 

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