Displacement reactions involving the amino group

Until recently no satisfactory procedure for the direct conversion of a primary aliphatic amine into an alkyl halide was available. This contrasts sharply with the ready conversion of primary aromatic amines into aryl halides via the diazonium salt (Section 6.7, p. 922). However a useful method has not been discovered which is not only applicable to the synthesis of simple alkyl and aralkyl chlorides, bromides, iodides and fluorides, but could clearly be of value when it is required to replace an amino group in a naturally occurring amine by a halo- 

The outline procedure involves the initial reaction of the 2,4,6-triphenyl-pyrylium halide with the primary amine to yield the corresponding 2,4,6-triphenylpyridinium halide (see Section 8.4.1, and also Section 5.15.3, p. 768) this reaction proceeds either at room temperature in a suitable solvent, or more efficiently under reflux in benzene with azeotropic removal of water. Pyrolysis of the pyridinium halide under controlled conditions then yields the alkyl (or aralkyl) halide in good yield. The mechanism of the reaction in this case is probably of the Sjv2 type. 

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