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The hydrolyses of some sterically crowded benzoate esters in sulfuric acid. The excess acidity method at different temperatures

Canadian Journal of Chemistry (Impact Factor: 0.96). 02/2011; 57(22):2960-2966. DOI: 10.1139/v79-481

ABSTRACT The excess acidity method has been used to analyze the observed acid-catalyzed hydrolysis rate constants for methyl benzoate, methyl para-toluate, methyl ortho-toluate, and methyl 2,6-dimethylbenzoate, over a wide sulfuric acid concentration range, at several different temperatures. Enthalpies and entropies of activation in the aqueous standard state are reported, with slope parameters m≠ also given are the and m* values found for the protonation of these compounds. The mechanistic changeover from AAc-2 to AAc-1 hydrolysis occurs at lower acidity with increasing methyl substitution, mainly due to the decrease in activation enthalpy in the transition state for the AAc-1 process, caused by release of steric strain and increased mesomeric interaction. The AAc-2 hydrolysis involves two water molecules, and is energetically favourable and entropically unfavourable. The AAc-1 reaction is difficult energetically, but this is offset by the large positive activation entropies found.

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