Aromatic Reactions: Aniline Diazotization to Diazonium

Aniline molecules react with nitrous acid (HNO2) and sulfuric acid (H2SO4) to form the corresponding diazonium molecule. Visually, an extra nitrogen atom is triple bonded to the original aniline nitrogen while having the two attached H atoms removed. This reaction preceedes the Sandmeyer reaction, which then converts the diazonium ion to the respective aryl halide (or benzonitrile):

Mechanism

The reaction mechanism is depicted below:

In the first step, the HNO2 molecule is protanated by the H2SO4 acid, leading to the formation of H2NO2.

In the second step, the free electrons from the aniline-nitrogen attack the H2NO2, leading to the H2O group leaving and the NO moiety binding to the aniline-nitrogen.

In the third and fourth steps, water molecules pull the hydrogen atoms of the original aniline-nitrogen atom. H2O or HSO4- can perform this (thereby regenerating the H2SO4 for use later).

In the fifth and sixth steps, H2SO4 is attacked by the free electrons on the O atom, leading to the formation of an H2O moiety.

In the seventh and final step, the aniline-nitrogen forms a triple bond with the adjacent nitrogen, leading to the H2O moiety leaving the molecule.

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