Journal of the American Chemical Society p. 1677 - 1684 (1991)
Update date:2022-08-11
Topics:
Coleman, Charolotte A.
Murray, Christopher J.
Rate constants and structure-reactivity coefficients for the breakdown of acetaldehyde and acetaldehyde-d4 hemiacetals were determined in water and deuterium oxide by trapping the acetaldehyde formed with α-effect nucleophiles. General-base catalysis by substituted acetate and cacodylate ion catalysts represents equilibrium ionization of the hemiacetal CL3CL(OL)OR (L = H or D) to form the hemiacetal anion, CL3CL(O-)OR followed by rate-determining general-acid catalysis of the cleavage of the hemiacetal anion to form acetaldehyde and ROL. Solvent isotope effects for the catalytically active proton kpBH/kpBD = 0.9-2.5 do not change significantly with changes in the pK of the catalyst or the leaving group alcohol. The increase in the secondary αβ-deuterium isotope effects kαβH/kαβD = 1.21-1.30 with decreases in the pK of the leaving group alcohol can be described by the interaction coefficient pyy′ = ?ρn/-?pK1g = -0.069. The increase in Br?nsted β = 0.48-0.72 with decreases in the pK of the leaving group alcohol in water can be described by the interaction coefficient pxy′, = ?β/-?pK1g = 0.090 and in D2O by pxy′ = 0.078. The interaction coefficients and the observation of both solvent and secondary deuterium isotope effects are consistent with a coupling between proton transfer to the leaving group oxygen and changes in hybridization about the central carbon in the transition state for cleavage of the hemiacetal anion. The results are discussed in the context of proposals for stable hydrogen-bonded protons in concerted acid- and base-catalyzed reactions in water.
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