Other metal and acid reductions

Zinc and Acid. A method which, although quite expensive, is often very useful in reducing pertain insoluble and difficultly reducible nitro compounds, such as sulfones and highly ringnsubstituted derivatives of nitrobenzene, is that involving zinc dust and acid. The reaction may be represented as follows  

Usually sulfuric or hydrochloric acid is used but, where a milder acid is required because of sensitivity of the nitro compound or amine, acetic acid may be substituted. Generally, 3.5-7.5 moles of zinc dust is used per mole of nitro compound and excess acid of about 20-50 per cent concentration. 

Normally the nitro compound is slurried in acid and zinc dust added gradually, with stirring, at whatever the optimum temperature may be, usually somewhere between 50 and 100 C. (The use of hot concentrated sulfuric acid results in p-aminophenol rather than aniline.) A variation of this method is to slurry the nitro compound in water together with zinc dust and (at times) an emulsifying agent and to add strong acid slowly. 

This process is technically most feasible for the preparation of those amines which are insoluble at a pH lower than 5.0, so that the amine may be separated from dissolved zinc salts without resorting to a very elaborate isolation scheme such as extraction with organic solvents. 

Tin and Acid. Owing to the comparatively high cost of tin, this method of effecting reductions can hardly be considered of commercial importance In place of tin and hydrochloric acid, a solution of stannous chloride in hydrochloric acid can be employed but there is little advantage, in general, in this procedure, as double the amount of tin must be used to effect the reduction. 

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