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The catalytic hydrogenation of nitrobenzene into aniline, in ethanolic solutions modified by different additions of KOH, was studied. Platinum catalysts supported on graphite were used. In ethanolic solutions, different dimer reaction intermediates (azoxybenzene, azobenzene, hydrazobenzene), strongly adsorbed on the catalyst, were formed thus inducing a decrease of selectivity in aniline. The best selectivity in aniline was obtained in a neutral or slightly alkaline solution. The competitive hydrogenation of nitrobenzene with benzylic ethers (allyl benzyl ether: C6H5–CH2–O–CH2–CHCH2 and benzyl crotyl ether: C6H5–CH2–O–CH2–CHCH–CH3) was also studied in the same experimental conditions, as a function of the acidity of the solution (obtained by addition of KOH or H2SO4). In an acid solution the group CC was more easily hydrogenated than the NO2 group whereas in an alkaline solution the opposite 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 alkaline solutions. This result can be explained by a steric effect and/or an electronic donor effect of the methyl group. |
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