The effect of solvent on reaction velocity. IV.—The rate and critical increments of some chlorination reactions

Moelwyn-Hughes has recently shown that the observed rates of a number of reactions in solution approximate to those calculated from the collision frequency of the reacting molecules and the critical increment of the reaction. At first sight, this fact brings reaction velocity in solution in line with reaction velocity in bimolecular homogeneous gas reactions, where a sufficient hypothesis to explain the observed reaction rates, is that reaction occurs whenever two molecules collide with a combined kinetic energy equal to, or greater than, the critical increment. There are, however, a number of reactions in solution, considered by Moelwyn-Hughes, which have a relatively low critical increment for which the calculated rate is several thousand times greater than that observed. The conclusion drawn is that the observed critical increment of these reactions is false. The reactions which show this anomalous behaviour are reactions of the type where quaternary ammonium salts are formed. In such reactions relatively nonpolar molecules are forming a salt. A reaction of the converse type where polar molecules are forming molecules of a less polar nature is the interaction of acetic anhydride and ethyl alcohol. Here also it is found that the calculated rate is many thousand times greater than that observed.