Studies of the variations in bond dissociation energies of aromatic compounds - II. Substituted bromobenzenes

Abstract

The paper reports the effects of substituents on C— Br bond dissociation energy in substituted bromobenzenes. The following substituents introduced in various positions of benzene ring were investigated: F, Cl, Br, CH₃, C₆H₅, CN and OH. In addition, the studies were extended to bromopyridines and bromothiophene. Assuming that the frequency factor is constant for the series of decompositions, the following values were obtained for the difference ∆D = D(Ph— Br) - D(Ph_s— Br), where Ph_sBr denotes a molecule of substituted bromobenzene: substituent ∆D (kcal/mole) substituent ∆D (kcal/mole) p-F 0-5 m-C₆H₅ 0-8 p-Cl 0-6 o-C₆H₅ 2-7 m-Cl 1-0 p-CN 0-3 o-Cl 1-2 m-CN 0-8 p-Br 0-3 o-CN 0-6 o-Br 1-8 p-OH 3-9 P-CH₃ 0-2 o-OH 3-8 m-CH₃ 0-2 3-bromopyridine -5-0 o-CH₃ 0-8 2-bromopyridine -0-6 p-C₆H₅ 0-2 2-bromothiophene 2-4 The significance of these results is discussed, and a tentative explanation of the observed effects is proposed.