Rate Constants for Carbanion Protonation

Rate constants for thermodynamically favorable protonation of unstable carbanions are typically very large. These may be determined by direct methods. A description of these direct methods, the most important of which use laser flash photolysis in carbanion generation [29, 30], is outside the scope of this chapter. The indirect methods used to estimate rate constants for carbanion protonation will be described in greater detail, because they provide insight into the nature of the rate determining step for carbanion protonation in water. 

Carbanion protonation in water is a two-step reaction (i) movement of a Bronsted acid into a reactive position, and (ii) proton transfer to carbon. The overall rate constant for carbanion protonation may be limited by either the rate constant for formation of the reactive complex, in which case the overall rate constant for proton transfer can be estimated by using a representative rate constant for the rate-determining transport step, or by the rate constant for proton transfer to carbon. 

The limiting rate constants for thermodynamically favorable protonation of carbanions, and the observations from experiments that provide evidence for these limiting reactions are different, depending upon the type of acid that protonates the carbanion. 

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