Journal of the American Chemical Society p. 4675 - 4690 (1987)
Update date:2022-08-10
Topics:
Soerensen, Poul E.
Jencks, William P.
Rate constants and structure-reactivity parameters for general acid and general base catalysis of the reactions of acetaldehyde with water and alcohols at 25 deg C and ionic strength 1.0 (KCl) were determined by trapping the carbonyl group formed upon cleavage of acetaldehyde hydrate or hemiacetal with semicarbazide or hyrazine.The Broensted coefficients of α=0.50-0.59 for general acid catalysis by a class e mechanism are larger than those for the corresponding reactions with formaldehyde.The difference is described by a normalized interaction coefficient pxy'=0.067=δα/-δ?=δρ/δpKHB.The increase in α with decreasing pKa of the alcohol is described by the coefficient pxy=0.020=δα/-δpKlg=δβlg/-δpKHB.A small increase in βlg for the acetaldehyde compared with formaldehyde reactions corresponds to a normalized coefficient pyy'=0.014=Δβlg/-δ?=δr/-δpKlg.These coefficient are consistent with a concerted reaction mechanism and a largely diagonal reaction coordinate on an energy contour diagram that is defined by the structure-reactivity parameters; there may be a larger vertical than horizontal component to the reaction coordinate.A stepwise mechanism is excluded by a requirement for rate constants that would have to be in the range 1E13-1E18 s-1 for proton transfer and for decomposition of the dipolar addition species R<*>.Extrapolated rate constants for the cleavage of R<*> in the range 1E20-1E23 s-1 suggest that the concerted mechanism is enforced by the absence of a significant lifetime for R<*>.The general base catalyzed reactions show a decrease in βlg and an increase in β for acetaldehyde compared with formaldehyde that correspond to normalized values of pyy'=-0.052=δβlg/-δ?=δρ/-δpKlg and pxy=0.042=δβ/-δρ=δ?/-δpKBH.The increase in Broensted β values with decreasing pKa of the leaving group and the increase in βlg with decreasing catalyst pKa are described by an interaction coefficient pxy'=0.07=δβ/-δpKlg=δβlg/-δpKBH, and upward curvature in the dependence of log k on the pKa of the leaving group (an "anti-Hammond effect") is described by a value of py'=-0.20=δβlg/-δpKlg.These interaction coefficients and a value of px=0=δβ/-δpKBH from the linear Broensted plots correspond to a reaction coordinate that is rotated 57 deg clockwise from the vertical on an energy contour diagram that is defined by the structure-reactivity parameters, with β for proton transfer on the x axis.The results support a fully concerted reaction mechanism with an important component of proton transfer in the transition state.Rate constants for "water-catalyzed" reaction are consistent with those for buffer bases; there is no evidence for a cyclic mechanism involving proton transfer through water.Hydroxyde ion catalysis of the reaction represents specific base catalysis.The equilibrium constant for acetaldehyde hydration was found to be Kb=1.2+/-0.1 at 25 deg C, ionic strength 1.0.
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