Halogenation of carbon acids

General base catalysed halogenation of carbon acids often involves a rate-determining proton transfer from carbon, and halogenation is a technique which is frequently used for obtaining rates of proton transfer. The technique has been developed particularly by Bell [34] to measure rates of proton transfer from ketones. Other carbon acids have been studied, however, for example, esters [35], sulphonates [36], nitro-paraffins [37], and cyanocarbons [38]. Under chosen conditions which may vary depending upon the substrate, the mechanism, illustrated for acetone in eqns. (19) and (20), 

The halogenation of ketones is also general acid catalysed. The mechanism usually consists of a rapid pre-equilibrium protonation of the carbonyl group followed by a slow proton transfer from carbon to the base catalyst [41]. The enol thus produced reacts rapidly with halogen. The overall mechanism is similar to mechanism (7) described earlier and the observed rate coefficient is a product of the equilibrium constant for protonation of the carbonyl group and the rate coefficient for the proton transfer from carbon, and therefore does not refer to a single proton transfer step. 

---