Reduction of azo compounds to amines

Reduction of azo compounds to amines, with fission of the N=N double bond, assumes practical importance because so many azo compounds are readily accessible, even on an industrial scale, by coupling of diazonium salts with reactive molecules. In this way a large number of primary amines can be obtained free from isomers, which is not always easy to achieve by other 

However, the benzidine and semidine rearrangements may occur in acid solution if the hydrazo stage is not further reduced sufficiently fast, and in such cases it is preferable to carry out the reduction in an alkaline medium. 

When reducing azo compounds it is generally possible to use the solutions or pastes that result directly from the coupling reaction, although it is advisable to purify the azo compound before catalytic hydrogenation. 

The reagents most commonly used for reducing azo compounds are sodium dithionite,137 zinc,138 tin(n) chloride,139 hydrogen iodide,140 and activated hydrogen. 

Reduction by sodium dithionite always gives good yields and is preferred here over all other methods because the technique required is simple. Two moles of sodium dithionite are added to a warm (40-60°) neutral or alkaline, aqueous or alcoholic solution of one mole of the azo compound, and the solution is warmed for a short time (until decolorized). 

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