Aromatic Reactions: Sandmeyer Reaction (Cu-X)

Diazonium molecules react with copper(I) catlysts (CuBr/CuCl/CuI/CuCN) to form aryl halides (or benzonitriles). This reaction is known as the Sandmeyer reaction and is useful as in intermediate step in converting aniline molecule to aryl halides (or benzonitriles):

Mechanism

The reaction mechanism is depicted below:

In the first step, the free radical electron from copper attacks the positively charged nitrogen on the diazonium moiety.

In the second step, the bond between the benzene ring and nitrogen is split evenly, sending one electron to the benzene carbon and one electron to the nitrogen. This completes the nitrogens octet and it is removed from the molecule. Simultaneously, the now positively charged copper atom is attacked by a free halide molecule, forming the copper(II) halide (bromide in this example).

In the third step, the copper(II) halide reacts with the free radical on the benzene ring, pulling one of the halides off the copper atom, thereby regenerating the copper(I) catalyst and forming the aryl halide.

The reaction and mechanism proceed identically whether CuBr, CuCl, or CuI are used. In the case of CuCN, it would follow the same mechanism but the electron attacks that are occuring with the halides would be occuring on the negatively charged carbon atom on the C≡N moiety.

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