The Nature of Reactions in a Solvent

Many reactions that take place in a solvent do not occur in the gas phase. This statement is generally true for ionic reactions. Homolytic decompositions of neutral molecules can sometimes be observed in both. One reaction that can be studied in the gas phase and in a variety of solvents is the decomposition of N2O5, 

This reaction follows first-order kinetics. It is not unimolecular, however, and occurs by a chain mechanism. Table 9-1 summarizes the activation parameters. The rate constant is nearly the same in the gas phase as in solution, and from one solvent to the next. 

Although not suitable for the gas phase, it can be conducted in many nonpolar and polar solvents. The rate of this reaction is quite sensitive to the solvent. From the least polar solvent (hexane) to the most (nitrobenzene), the rate constant increases 2700 times. 

The rate constants and their activation parameters are given in Table 9-2. As one would expect, the reaction is faster the more polar the solvent. This reaction is likely to have a polar transition state, which would be stabilized in a medium of high dielectric constant. A quantitative correlation will be given in Section 9.4. 

Collisions in the gas phase, whether they result in a reaction or not, are timed somewhat uniformly. In solutions, however, solute pairs undergo multiple collisions within a solvent cage. Once two solute species are in one cage, they are likely to remain neighbors for some time, during which they experience repeated collisions. 

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