Structural Aspects of the Kinetics of Four-Center Reactions in the Vapor Phase

Abstract

The data on the unimolecular elimination of HX from gas-phase alkyl halides (Cl, Br, I) are examined critically in terms of the recent and less controversial results on iodides. Despite difficulties in the bromide and chloride systems due to chains and surface effects, these systems all show the same effects of α and β substitution. Using available or estimated thermodynamic data for the elimination reaction, the rate constants for the bimolecular addition reactions (HX+olefin→RX) are evaluated. Here, for the exothermic direction, the regularities are even more striking. The activation energies decrease by about 5 kcal on going from HCl to HBr and by another 5 kcal going to HI. They also decrease by about 7 kcal for the substitution for H of an α-Me and by about 1–2 kcal on substitution of a β-Me group. An equally striking aspect of these substituent effects is the directive effect of the α-Me group in locating the X atom. In the iodide systems the selectivity is 1000:1 in favor of formation of the i-RI over n-RI. This is directly observed and for the RCl and RBr systems, the same order of magnitudes can be calculated from kelim and Keq. It is shown that these data are compatible with a very polar transition state. Ion-pair transition states are shown to require too high activation energies. It is estimated that a ``semi-ion pair'' transition state in which the olefin and HX are each polarized by about ±½ε can give semiquantitative correlation of the data. Empirically, almost any electrical properties of the molecules plot very well against the observed activation energies for either common HX or common olefin. It is shown also that the positive inductive property of the α-Me group can be understood in terms of a simple electrostation polarization effect.