Kinetics and mechanism of the Duff reaction

Abstract

The Duff reaction of β-naphthol (I) with hexamine in glacial acetic acid at 100° gives 2,2′-dihydroxy-1-naphthylidene-1′-naphthylmethylamine (II), but at lower concentrations of starting materials 2-hydroxy-1-naphthaldehyde (IV) is the main product. The rate law for the latter reaction is expressed as: d[IV]/d t = [I] m [hexamine] n ( m ≳ 1, n ∼ ·4). 2-Hydroxy-1-naphthylmethylamine (III) also reacts with hexamine to produce IV, the rate being higher than that of β-naphthol with hexamine. The rate law is a similar type: d[IV]/d t = k′[III] m′ [hexamine] n′ ( m′ ≳ 1, n′ ∼ 0·3). The reaction of this amine III with formaldehyde alone also yields IV and this reaction has an induction period. The addition of n-butylamine or ammonia to the system of III-HCl and formaldehyde eliminated the induction period and increased the rate considerably. The addition of tri-n-butylamine to the same system shortened the induction period, but the rate constant was little increased, while addition of β-naphthol had no appreciable effect on the rate. A mechanism involving the fast aminomethylation of β-naphthol and then the rate-determining dehydrogenation of the amine to the imine, followed by the rapid hydrolysis of the imine is proposed and discussed.