Acid-Catalyzed Enolization and Aldol Condensation of Acetaldehyde

Article abstract of DOI:10.1021/ja00298a039

The condensation of acetaldehyde (1) to an equilibrium mixture of aldol (2) and crotonaldehyde (3) is second order in 1.An excess acidity analysis reveals that a water molecule is also involved in the rate-limiting step; the reaction is actually the base-assisted addition of vinyl alcohol to protonated 1, even in concentrated H2SO4.A previous report of a kinetically first-order conversion of 1 to 3 is shown to be due to the presence of a fast-reacting oligomer of 1.The reaction of 1 in D2SO4 leads to partially deuterated 3, a result ascribed to partial conversion of vinyl alcohol to deuterated 1.Hydrogen isotop exchange of 3 was also observed, but at a slower rate.The rates of enolization of 1 were studied by iodination and are consistent with previous results and the proposed mechanism.The interconversion of 2 and 3 is shown to proceed via the enol of 2; in this case the rate-limiting step is water attack on/water loss from protonated 3/2, not proton transfer at carbon.