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A solution of acetaldehyde in methanol reacts to form a hemiacetal, but if catalytic acid is added to the mixture, not only does the rate of reaction increase but another product is formed. This product is an ‘acetal’. In the presence of acid, hemiacetals can undergo an elimination reaction, losing the oxygen atom that once belonged to the parent aldehyde’s carbonyl group of the hemiacetal. The two stages in the formation of acetal are shown.
G. W. Meadows and B. D. Darwent, Can. J. Chem., 1952, 30, 501–506.