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Hammett u values1 for the 2-, 3-, and 4-pyridine o aza groups determined from the hydrolysis of pyridylCH,OCCH, methyl acetates,2 (I), (am = 1.1,oas = 1.3,041: = 1.6) are all significantly greater than other reported values3 " (1) for these groups. While some variation in aza a values has been found, especially for the 2-position, there seemed no obvious reason for the considerably enhanced values for all three positions found by Simonetta and Favini for this reaction. As part of a study of factors affecting aza a values, we have investigated this alkaline hydrolysis in 56% (w/w) acetone-water at 25", and obtained results indicating close agreement between a values required for this reaction and for hydrolysis of methyl pyridinecarboxylates.4 The values for aza and nitro groups are as follows :4-8

A determination of the sigma zero constant for the pyridine aza group