The rates of acid-catalysed hydrolyses of a range of glycopyranosides (see Table l), and of one glycofuranoside, have been measured, mostly a t three or more temperatures ; the activation parameters have been calculated.The steric and electronic effects of changes in the sugar configuration, and in the nature and configuration of the aglycone, upon the rates of hydrolysis are discussed; the most significant changes are found in the entropy of activation and an explanation of these is proposed.For hydrolysis of all the pyranosides studied the entropies of activation are positive and relatively large, supporting an A-1 mechanism, whereas for the furanoside it is negative, suggesting an A-2 mechanism. Independent evidence for the A-1 mechanism, based on the products of hydrolysis of methyl a-and P-D-ghcopyranoside, is presented.

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+0-44" +0.29" -0.03" (const.) Compound (11) (c 0.6) Time (hr.

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