Catalytic Reduction of Nitrobenzene and o-Nitrotoluene with Hydrogen in Sulfuric Acid-Methanol Solution

Abstract

The catalytic reduction (5% platinum on carbon carrier and under 1 atm of H₂) of nitrobenzene [1a] and o-nitrotoluene [1b] with hydrogen was carried out in sulfuric acid-methanol solution at a room temperature. The products were aniline [2a], o-toluidine [2b], and o-as well as p-methoxyanilines. o- and p-Methoxyanilines were considered to form in terms of the Bamberger-typeirearrangement of phenylhydroxylamines [9]which are the intermediates formed in the catalytic reduction of nitrobenzenes to anilines. In the reduction of [lb] more 4-methoxy-2-methylaniline [4b] was obtained than 6-methoxy-2-methylaniline [3b] as s rearrangement product. The maximum yield of [4b] was 56% under the typical reaction conditions, while that of [3b] was in every case lower than 1%. The yield of [4b] increased to 70% by the addition of a small amount of dimethyl self oxide to a reaction medium. In the reduction of [la], the total yield of rearrangement products (o- and p-anisidine ([3a], [4a])) was 13% or below, and olp ratiovaried significantly from 0.2 to 12 with an amount of sulfuric acid. Although in the rearrangement of phenylhydroxylamine [9a] in sulfuric acid-methanol solution, almost constant o/p ratio ([3a]/[4a] ratio, O.20-0.22) was observed, this ratio increased to 0.31-0.48 in the presence of Pt-C catalyst. The result indicates that when [9] are adsorbed onto a catalyst surface, the rearrangement to o-position becomes energetically more favorable than that to p-position.