Piria reaction

What is Piria reaction?

The Piria reaction, named after its discovery by Piria in 1851, is a two-step process used to prepare aromatic amines and amino sulfonic acids. This involves the reduction of aromatic nitro compounds using bisulfite followed by hydrolysis in mineral acid.

Piria reaction - general reaction scheme
Piria reaction

The Piria reaction results in sulfonation of the aromatic ring with the sulfonic group typically located at the ortho- or para-position towards the nitro group. However, in some cases, such as 4-nitrophenol, the sulfonic group enters the meta-position, possibly through 1,4-addition of NaHSO3 to the -N=C-C=C- system present in the quinone-oxime form of the intermediate. It has been proposed that the nitro group is initially reduced to a nitroso group, and the sulfonation occurs at or beyond this stage.

The aromatic nitroso intermediate is further reduced to sulfaminic acid salt, as supported by the isolation of such a molecule during reduction without successive acidic hydrolysis or by independent hydrolysis of sulfaminic acid into amine and sulfuric acid. The reaction rate is higher when sodium sulfite or partially neutralized NaHSO3 is used instead of sodium bisulfite, and when the reaction is carried out in dilute solution. In the case of α-nitronaphthalene, amino-naphthalene disulfonic acid was isolated in a higher yield than the other two products normally found in a regular Piria reaction.