The conversion of aniline (otherwise known as aminobenzene) to benzene is a good teaching example of two important reactions in organic chemistry. The first of these is the reaction of aniline with nitrous acid to form a unusual type of chemical known as a diazonium salt. Diazonium salts are useful since they can be made into dyes and food colourings. In the conversion modelled here, however, the salt is next treated with acid to remove the diazonium group and produce the plain benzene molecule. This reaction also is useful and occurs in the synthesis of some organic compounds.
Prepare a solution of aniline in aqueous hydrochloric acid inside a reaction flask. Cool this solution to 0 degrees Celsius. Note that aniline has the molecular formula C6H5NH2 (where C is carbon, H is hydrogen and N is nitrogen) so it is essentially equal to benzene (C6H6) with one hydrogen removed and the group -NH2 inserted in its place.
Add drop wise, with stirring, a solution of sodium nitrite (NaNO2) in water. The combination of sodium nitrite and acid will produce nitrous acid (HONO) in the reaction flask. The nitrous acid will react with the aniline already in the flask to change the -NH2 group into -N2+Cl- and so produce the corresponding chemical known as a diazonium salt (C6H5N2+Cl-). Keep this solution below 5 degrees Celsius to maintain the unstable diazonium compound.
Add drop wise, with stirring, an aqueous solution of hypophosphorous acid (H3PO2) to the solution of the diazonium salt. The reaction of hypophosphorous acid with the diazonium salt will remove the diazonium group and replace it with a hydrogen, generating benzene (C6H6). Aniline has now been converted to benzene via the intermediate diazonium compound.
This procedure is given here in generic form without exact quantities or concentrations of chemical reagents since it is meant to simply illustrate the necessary chemical reactions for this conversion process. The exact quantities and concentrations of chemicals required, as well as specific rates of addition, will depend on the amount of aniline to be converted and other factors.
These reactions involve toxic, corrosive and dangerously reactive chemicals and should only be attempted by trained professionals who understand the hazards involved.