Kinetics and mechanism for the formation of phenylhydrazone from benzaldehydes, naphthaldehydes, and formyl-1,6-methano[10]annulenes

Abstract

Formation of phenylhydrazone from benzaldehyde, 4-chlorobenzaldehyde, 1-naphthaldehyde, 2-naphthaldehyde, 2-formyl-1,6-methano[10]annulene in 50% aqueous ethanol (v/v) at 25.0 °C and ionic strength 0.20_M occurs in two steps, (i) the formation of an aminomethanol intermediate and (ii) the dehydration of the aminomethanol to give the products of reaction. Formation of the aminomethanol intermediate is the rate-determining step below pH 5–6 and dehydration of the aminomethanol is the rate-determining step above pH 5-6. Both the reactions are subject to hydronium ion catalysis. The values of the catalytic constants are very similar with all the aldehydes. Formation of the aminomethanol is subject to general acid catalysis by carboxylic acids. The α Broønsted exponents for catalysis of the intermediate formation reaction for the formyl-1,6-methano[10]annulenes are higher than those for the benzaldehydes and smaller than those for the naphthaldehydes. Comparison of the general behaviour of the formyl-1,6-methano[10]annulenes with the benzaldehydes and naphthaldehydes provides strong evidence of the aromatic character of 1,6-methano[10]annulene ring.