Journal of the American Chemical Society p. 6472 - 6481 (1980)
Update date:2022-08-12
Topics:
Palmer, John L.
Jencks, William P.
The observed secondary α-deuterium isotope effects for catalysis by acetate ion of the cleavage of formaldehyde hemiacetals increase from k2H/k2D = 1.23 to 1.28 to 1.34 with decreasing pK of the leaving alcohol in the series ethanol, chloroethanol, and trifluoroethanol.The pH-independent reaction shows a smaller isotope effect of 1.15-1.14 for the ethyl and chloroethyl hemiacetals.These reactions involve general-base catalysis of alcohol attack in the addition direction and the kinetically equivalent cleavage of the hemiacetal anion with general-acid catalysis by acetic acid or the proton in the cleavage direction.The results indicate that the amount of C-O cleavage in the transition state increases with decreasing pK of the alcohol and increasing pK of the acid catalyst, corresponding to a negative coefficient pyy'=δρn/-δpK1g = δβ1g/-δ? and a positive coefficient pxy = δρn/-δpKHA = δα/δ?.These results provide additional support for a concerted reaction mechanism with an important role of proton transfer in the transition state.Qualitative and semiquantitative characterizations of the transition state are presented in terms of reaction coordinate diagrams that are defined by the structure-reactivity parameters.The properties of the transition state suggest that the reaction is best regarded as an electrophilic displacement on the oxygen atom by the proton and by the carbonyl group in the cleavage and addition directions, respectively.The large secondary isotope effect of k2H/k2D = 1.63 for cleavage of the chloroethyl and trifluoroethyl hemiacetals catalyzed by hydroxide ion indicates a late transition state for alkoxide expulsion from the hemiacetal anion.
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