Multiple isotope effect study of the acid-catalyzed hydrolysis of formamide

Multiple isotope effects were measured at the reactive center of formamide during acid-catalyzed hydrolysis in water at 25 degrees C. The mechanism involves a rapid pre-equilibrium protonation of the carbonyl oxygen, followed by the formation of at least one tetrahedral intermediate, which does not appreciably exchange its carbonyl oxygen with the solvent (k(h)/k(ex) = 55). The pK(a) for formamide was determined by N-15 NMR and found to be about -2.0. The formyl-hydrogen kinetic isotope effect (KIE) is indicative of a transition state that is highly tetrahedral ((D)k(obs) = 0.79); the carbonyl-carbon KIE ((13)k(obs) = 1.031) is in agreement with this conclusion. The small leaving-nitrogen KIE ((15)k(obs) = 1.0050) is consistent with some step prior to breaking the C-N bond as rate-determining. The carbonyl-oxygen KIE ((18)k(obs) = 0.996) points to attack of water as the rate-determining step. On the basis of these results, a mechanism is proposed in which attachment of the nucleophile to a protonated formamide molecule is rate determining.