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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 --&gt; 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.

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