Hydrogenation under supported aqueous-phase

In contrast, liquidiliquid phase-transfer catalysis is virtually ineffective for the conversion of a-bromoacetamides into aziridones (a-lactams). Maximum yields of only 17-23% have been reported [31, 32], using tetra-n-butylammonium hydrogen sulphate or benzyltriethylammonium bromide over a reaction time of 4-6 days. It is significant that a solidiliquid two-phase system, using solid potassium hydroxide in the presence of 18-crown-6 produces the aziridones in 50-94% yield [33], but there are no reports of the corresponding quaternary ammonium ion catalysed reaction. Under the liquidiliquid two-phase conditions, the major product of the reaction is the piperazine-2,5-dione, resulting from dimerization of the bromoacetamide [34, 38]. However, only moderate yields are isolated and a polymer-supported catalyst appears to provide the best results [34, 38], Significant side reactions result from nucleophilic displacement by the aqueous base to produce hydroxyamides and ethers. 

The reaction is carried out in the vapour phase in a multi-tubular reactor packed with a catalyst of mercuric chloride on an activated charcoal support. The reaction is highly exothermic and cooling is applied to keep the temperature at 100—180°C. The pressure used is atmospheric. The gases from the reactor are cooled and washed with aqueous sodium hydroxide to remove unreacted hydrogen chloride. The product is then liquefied by cooling to —40°C and pure vinyl chloride is obtained by fractional distillation. Provided pure reactants are used, this preparation of vinyl chloride is clean and easily accomplished. Vinyl chloride is a colourless gas (b.p. —14°C) with a pleasant, sweet odour. It is conveniently handled, under slight pressure, as a liquid, which may be stored without the addition of a polymerization inhibitor. 

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