The kinetic acidity of halomethyltrimethylammonium salts

The kinetic acidities of members of a series of halomethyltrimethylammonium salts (Me3N+CH2X, BF4-) have been measured under conditions of -OD/D2O at 50 degrees C, with the rate constants being found to increase X=Cl < Br < I . The observed trend is rationalized on the basis of the polarizabilities a nd electron repulsion factors of the respective halogen substituents. The o bserved rate constants indicate a lowering of the Delta G double dagger for deuterium/hydrogen exchange of 12-13 kcal mol(-1) due to the presence of t he alpha-halo substituents. The kinetic acidity of chloromethyldimethylsulf onium BF4- was approximate to 10(6) that of the analogous chloromethylammon ium salt.