Simple and competitive catalytic hydrogenation of nitrobenzene, allyl benzyl ether and benzyl crotyl ether in alkaline alcoholic media

The catalytic hydrogenation of nitrobenzene into aniline, in ethanolic solu tions modified by different additions of KOH, was studied. Platinum catalys ts supported on graphite were used. In ethanolic solutions, different dimer reaction intermediates (azoxybenzene, azobenzene, hydrazobenzene), strongl y adsorbed on the catalyst, were formed thus inducing a decrease of selecti vity in aniline. The best selectivity in aniline was obtained in a neutral or slightly alkaline solution. The competitive hydrogenation of nitrobenzen e with benzylic ethers (allyl benzyl ether: C6H5-CH2-O-CH2-CH=CH2 and benzy l crotyl ether: C6H5-CH2-O-CH2-CH=CH-CH3) was also studied in the same expe rimental conditions, as a function of the acidity of the solution (obtained by addition of KOH or H2SO4). In an acid solution the group C=C was more e asily hydrogenated than the NO2 group whereas in an alkaline solution the o pposite result was obtained. The hydrogenation rate of the crotyl group was lower than that of the allyl group and became equal to zero in strong alka line solutions. This result can be explained by a steric effect and/or an e lectronic donor effect of the methyl group. (C) 1999 Elsevier Science B.V. All rights reserved.