The empirical Harnme:t treatment has been applied to the investigation of structure-reactivity relationships for pyridine and pyridine N-oxide carboxylic acids in the reaction with diazodiphenylmethane. The values obtained for the pyridine aza-group (a2 0.878, o3 0.740.0, 1 a 1 05) are in good agreement with the literature values for the alkaline hydrolysis of the corresponding esters. For the N-oxide group, the o values for the 3and 4-positions (as 1.31 2, o4 1 .I 40) indicate that the overall effect is electron withdrawing. The negative value for the 2-position (a2 -0.395) can be explained on the basis of intramolecular hydrogen bonding. It is also shown that the Hammett relationship holds for pyridinecarboxylic acids with a fixed orrho-substituent as well.THE reactivity of pyridine and pyridine N-oxide systems is of much interest in connection with the electronic effects operating in the ring. Numerous investigations, including electrophilic and nucleophilic substitution in the ring and reactivity of side chains, as well as the study of spectra and dipole moments, indicate the complexity of effects caused by nitrogen in the pyridine nucleus and even more so by the N-oxide function in pyridine Noxide, which is both an electron donor and an electron accept or.The reaction of carboxylic acids with diazodiphenylmethane (DDM) has been used by numerous authors for quantitative structure-reactivity ~tudies.l-~ The mechanism of this reaction, suggested by Roberts: was

The reactivity of a series of substituted pyridine-and P oxylpyridine-carboxylic acids with diazodiphenylmethane has been investigated and the empirical Hamrnett trea.ment applied to a study of substituent effects. In general, satisfactory additivity of substituant effects was obtained, and it was possible to correlate with the same regression line the rate c,mstan:s for both investigated series. The calculated reaction canstant (p 0.81 8) differs from that for substi'Jted benzoic acids, ref!ecting the same attenuating effect observed in ;he hydrolysis of substituted pyridine carboxylates. A satisfactory cL>-relatiofi for the PDUM-Phydr relationship vvas obtained with literature data for the hydrolysis of corresponding srr5stituted methyi pyridinecarboxylates.IN oui-previous investigation the reaction of pyridineand A -0xylpyridine ca boxylic acids with diazociiphenylmethane (DDM) was used to study the structuref Presented in part a t the Fifth International Congress of Hdtrocyclic Chemistry, Ljubljana, 1975.

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