Entropic Contributions to Rate Accelerations in Enzymic and Intramolecular Reactions and the Chelate Effect

It is pointed out that translational and (overall) rotational motions provide the important entropic driving force for enzymic and intramolecular rate accelerations and the chelate effect; internal rotations and unusually severe orientational requirements are generally of secondary importance. The loss of translational and (overall) rotational entropy for 2 --> 1 reactions in solution is ordinarily on the order of 45 entropy units (e.u.) (standard state 1 M, 25 degrees C); the translational entropy is much larger than 8 e.u. (corresponding to 55 M). Low-frequency motions in products and transition states, about 17 e.u. for cyclopentadiene dimerization, partially compensate for this loss, but "effective concentrations" on the order of 10(8) M may be accounted for without the introduction of new chemical concepts or terms.