Hexaminium salt, Sommelet reaction

Cognate preparations. p-Nitrobenzaldehyde. This preparation is an example of the Sommelet reaction in which the hexaminium salt is isolated. Dissolve llg (0.13mol) of hexamethylenetetramine in 70ml of chloroform (CAUTION) and add 11.4 g (0.067 mol) of p-nitrobenzyl chloride or 14.4 g of p-nitrobenzyl bromide (Expt 6.28). Heat the mixture under reflux on a steam bath for 4 hours a precipitate gradually separates. Replace the reflux condenser by a condenser set for distillation and distil off about 35 ml of solvent. Add 35 ml of acetone, cool in ice, collect the precipitate by suction filtration and dry it in the air. Heat the hexaminium salt thus obtained under reflux for 1 hour with 100 ml of 50 per cent acetic acid then add 100 ml of water and 25 ml of concentrated hydrochloric acid and continue the refluxing for 5-10 minutes. Cool the solution in ice, collect the crystals of p-nitrobenzaldehyde and dry them in a vacuum desiccator. The yield is 6.4 g (63%), m.p. 106 °C. The p.m.r. spectrum is noted in Expt 6.117. 

The Sommelet reaction, also known as the Sommelet oxidation, is the oxidation of benzyl halides (1) to the corresponding aldehydes (3) using hexamethylenetetramine (HMTA) followed by hydrolysis of the hexaminium salt 2.1-3 This reaction is closely related to the Delepine amine synthesis, the Duff aldehyde synthesis, and the Krtihnke reaction (see Section 7.10.3). 

The Delepine reaction or the Del6pine amine synthesis is the closest variation of the Sommelet reaction. The Delepine amine synthesis is the acid hydrolysis of the hexaminium salt 2 with strong acid in ethanol, giving rise to benzylamine 4 with the formaldehyde being removed as volatile formaldehyde diacetal.13-15 In essence, it may be considered as half of the Sommelet reaction or the interrupted Sommelet reaction. The Delepine reaction works well for active halides such as benzyl, ally halides, and a-halo-ketones. 

According to Angyal s recommendation,1 the optimal Sommelet reaction may be run without isolation of the hexaminium salt under such conditions 0.1 mole of halomethyl compound is heated under reflux for one or two hours with 30 g of hexamine in 90 mL of 50% acetic acid, or with 16.5 g of hexamine in 90 mL of 60% ethanol. 

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