ORTHO EFFECT IN BENZALDEHYDE PHENYLHYDRAZONE FORMATION

Ortho-substituted benzaldehyde phenylhydrazone formation, like that for para-substituted benzaldehydes, occurs with rate-determining carbinolamine formation under slightly acidic conditions and with rate-determining dehydration of carbinolamine under basic conditions. The addition of phenylhydrazine to form carbinolamines from these substrates is subject to general acid catalysis by carboxylic acids: Brønsted α is 0.3-0.4. Rate constants have been measured for the separate steps in the reaction of phenylhydrazine with several ortho- and para-substituted benzaldehydes. Rate constants for hydrated proton catalysis for para-substituted benzaldehyde phenylhydrazone formation are well correlated by a dual Hammett substituent parameter treatment: log kH = log k°. + pIσI + PR+σR+.pR is slightly greater than ph indicating that the resonance effect of substituents is somewhat more important than the inductive effect for this reaction. Rate constants for hydrated proton catalysis for ortho-substituted benzaldehyde phenylhydrazone formation are also well correlated by a dual Hammett substituent parameter treatment: log kH = log k°. + pIσI + pR+σR+.R is much larger than pR, indicating that the inductive effect of the substituents is much more important than the resonance one for this reaction. These results account for the order of the ortho/para rate ratios for the benzaldehydes studied. © 1979, American Chemical Society. All rights reserved.