Hydrolysis mechanisms for the acetylpyridinephenylhydrazone ligand in sulfuric acid.
J Org Chem 2000;
65:3781-7. [PMID:
10864765 DOI:
10.1021/jo000085m]
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Abstract
The excess acidity method was applied to rate data obtained for 2-acetylpyridinephenylhydrazone hydrolysis in strong acid media using various aqueous/organic solvents, and it was observed that the reaction rate decreases with increasing permittivity of the medium. Two hydrolysis mechanisms are indicated. Below 0.6 M H(2)SO(4), no hydrolysis was observed; between 0.6 and 6.0 M H(2)SO(4), the substrate hydrolyzes by an A-S(E)2 mechanism and switches to an A-2 mechanism at higher acidity. This change of mechanism was justified on the basis of the syn and anti rotational conformers of the diene -N((1))=C-C=N((2))- group; the greater stability of the former can be explained by the formation of hydrogen bonds between the proton and the N((1)) and N((2)) nitrogen atoms, giving rise to a very stable five-membered ring. If the syn conformer is predominant, then no hydrolysis is observed; above 0.6 M, the attack of a second proton gives rise to a balance between the syn and anti forms, the latter being responsible for the hydrolysis of the hydrazone group.
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