Solvent electrostriction

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. 

The temperature dependence of the observed rate constants leads to the activation energy parameters Aif" " = 69.6 1.5 kJ mol and = —4.5 9.5 J K mol Negative values of AS are known for Dn + An substitutions. The experimental value of AS is at most slightly positive. This may be due to solvent electrostriction about the hydroxide ion as leaving group. 

Mundnich and Plieninger have studied the rearrangement of hexamethyl-Dewarbenzene to hexamethylbenzene using high pressure techniques. Large rate accelerations were found with AF = -34.6 2.5cm mol" and AK = —22.5 1 cm mol The value found for the volume of activation was independent of solvent polarity and therefore not caused by solvent electrostriction by dipolar types of transition state. 

The activation volumes, AV, have been analyzed for the same reactions with similar conclusions. However, the analysis is more complex than was originally anticipated because of solvent electrostriction effects and the effect of the volume of the leaving group on the volume of the reactant. ... 

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