We report the first example of acceleration of a bimolecular reaction by increased solvent viscosity. In a
series of hydrocarbon solvents of graded viscosity, the rates of two cycloadditions, the Diels−Alder dimerization
of cyclopentadiene and the 1,3-dipolar cycloaddition of diphenyldiazomethane with ethyl phenylpropiolate,
rise with increasing viscosity to about 1 cP and then fall thereafter. We interpret this rise as viscosity-induced
acceleration up to ∼1 cP, which cannot be accounted for by current kinetic theory. Several examples from
the literature are reinterpreted in these terms, providing further support for our concept.
Activation volumes ∆V * are presently obtained by measuring the effect of pressure on the reaction rate. It is implicitly assumed that the entire response of rate to pressure is volume related, i.e., acceleration by high pressure reveals shrinkage as the reactants progress to the transition state, and vice versa. However, we now demonstrate that high pressure accelerates some bond-making reactions in an additional, nonvolume-related way, through its elevation of solvent viscosity. Diels-Alder reactions, 1,3-dipolar cycloadditions, and Claisen rearrangements are accelerated by rising viscosity and are therefore subject to viscosity-associated acceleration at raised pressures. This gives rise to the illusion of volume shrinkage along the reaction coordinate toward the transition state, which we term phantom ∆V * . Thus the true ∆V * for these reactions, while negative, is less negative than previously believed. Corrections in ∆V * , calculated from experimental rate-viscosity plots, range up to 61%.
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