Catalytic Reduction of Nitrobenzene and o-NitrotoIuene with Hydrogen in Sulfuric Acid-Methanoi Solution

The catalytic reduction (5% platinum on carbon carrier and under l atm of H₂) of nitrobenzene [1 a] 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-type-rearrangement 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 re arrangement product. The maximum yield of [4b] was 56% under the typical reaction con ditions, 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 o xide to a reaction medium. In the reduction of [1a], the total yield of rearrangement products (o-and p-anisidine ([3a], [4a])) was 13% or below, and olp ratio varied 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.