A practical guide for estimating rates of heterolysis reactions.

Chemists are well trained to recognize what controls relative reactivities within a series of compounds. Thus, it is well-known how the rate of ionization of R-X is affected by the stabilization of the carbocation R(+), the nature of the leaving group X(-), or the solvent ionizing power. On the other hand, when asked to estimate the half-life of the ionization of a certain substrate in a certain solvent, most chemists resign.

Suppression of common-ion return by amines: a method to measure rates of fast S(N)1 reactions.

Rate constants for solvolyses of benzhydryl chlorides, which take place on the 10 ms to minute time scale, have been determined in aqueous acetone and acetonitrile by conductometry, using conventional conductometers as well as stopped-flow techniques. Secondary and tertiary amines were used to suppress ion recombination (common-ion return) thus giving access to the ionization rate constants k(1). The observed common-ion rate depressions can be rationalized by the correlation equation for electrophile-nucleophile combinations, log k(20 degrees C) = s(E + N), where electrophiles (here: carbocations) are characterized by the parameter E and nucleophiles (here: chloride anions and solvents) are characterized by N and s.