Journal of the American Chemical Society p. 5118 - 5123 (1988)
Update date:2022-08-11
Topics:
Soerensen, Poul E.
Pedersen, K. J.
Pedersen, P. R.
Kanagasabapathy, V. M.
McClelland, Robert A.
Kinetics of breakdown are reported for the ethyl, methyl, 2-chloroethyl, and 2,2,2-trifluoroethyl hemiacetals of α-bromoacetophenone, the hemiacetals being generated by the aqueous bromination of the appropriate α-alkoxystyrene.Acid catalysis is characterized by a Broensted α value of 0.6 and a β1g (for H+) of 0.3.With these values the transition state has been located on a reaction coordinate diagram for the class e mechanism of acid catalysis and a comparison made with positions of transition states for acetaldehyde and formaldehyde hemiacetals.With changing electrophilicity of the carbonyl there is a consistent shift in the position of the transition state, but the effect is relatively small.The base-catalyzed breakdowns of the ethyl and 2-chloroethyl hemiacetals have β vlues near 0.7, significantly higher than the values associated with analogous formaldehyde and acetaldehyde hemiacetals.The difference can be explained in terms of the reaction coordinate diagram of the class n base mechanism, recognizing the decreased susceptibility of the ketone to nucleophilic addition.With the trifluoroethyl hemiacetal, buffer dilution plots are curved and there is a downward break in the rate-pH profile.These are explained by a mechanism-hemiacetal <=> hemiacetal anion -> product-where at low buffer concentrations or low H+ concentration the deprotonation step is rate-limiting.As shown through a kinetic analysis, rate-limiting proton transfer is a result of a large rate constant for anion breakdown (ca. 8E8 s-1).Thus, at low buffer concentration breakdown occurs more quickly than reprotonation.The curvature arises because the buffer acid provides efficient protonation.At high buffer concentration the deprotonation step is (approaching) a rapid equilibrium preceding rate-limiting breakdown.Comparison with the other two hemiacetals shows that the behavior of the trifluoroethyl compound is predictable on the basis of structure-reactivity correlations.A reaction with rate-limiting proton transfer is enforced by the very short lifetime of the hemiacetal anion.
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