The kinetics of reactions in solution. Part II.—The decomposition of trinitrobenzoic acid in various solvents

There are very few reactions the rate of which can be measured both in the gaseous state and in solution. Several bimolecular reactions have been investigated in solution, and since the rate of a bimolecular gas reaction can be calculated from the equationInk =In Z — E/RT, where E is heat of activation and Z the collision number, the rate of reaction in solution can be compared with that of the hypothetical corresponding gas reaction. The observed velocity constants in solution have usually been found to be smaller by several powers of 10 than the calculated values. On the other hand, as shown in the previous paper, in the one example where direct comparison has been possible, namely, the decomposition of chlorine monoxide, the rate in solution in carbon tetrachloride is the same as that in the gas phase. Thus it is evident that the retardation of reactions by certain solvents is a specific action and need not occur in an “ideal” solvent. The rates of unimolecular gas reactions cannot be calculated, and direct comparison has hitherto been possible only in two examples. The equationInk = 31•69 — 24,710/RT has been found to represent the rate of decomposition of nitrogen pentoxide in the gaseous state and in a series of eight similar, chemically rather inert, solvents. In nitric acid or in propylene dichloride, however,In Z increases by several units and E becomes 28,300 calories. Daniels regards as “normal” those solvents which do not produce an alteration in E, and as “abnormal” those which cause a deviation from the value characteristic of the gaseous state. The solvents in which the decomposition of nitrogen pentoxide could be studied were naturally not very varied in character, since most liquids would be attacked chemically. Thus the impression which the results tend to convey, namely, that “normal” or "ideal" behaviour is more common with unimolecular reactions than with bimolecular reactions, may be an illusory one. In the only other known example, the isomerisation of pinene, the rate of reaction in the gas, in the liquid, and in carbon tetrachloride is the same.