The SN mechanism in aromatic compounds. XXXII. Reactivity of some perhalogenobenzenes

Abstract

Substituent effects in a series of pentafluorophenyl compounds (C₆F₅X) ranging from powerful activating to powerful deactivating substituents are considered, both as regards orientation and rates, and shown to be simply and reliably predicted by considering the change as made up of two components, viz. from C₆F₆ to C₆F₅H and from C₆F₅H to C₆F₅X. The same principle applies generally to C₆Hal₅X compounds, and the necessary orientation and rate data are obtainable from the existing large body of substituent effects in aromatic S_N reactions. Comparisons are thus also easily possible with substituent effects in aromatic S_E reactions, and more generally. The reasons for predominant para direction for almost all groups, except the more powerful deactivating substituents which are meta directing, and the expectation of a few borderline cases, emerge clearly from discussion. While orientation is largely known already, there have been very few precise rate determinations apart from the present work. Some brief consideration of the admixture of dioxan with protic solvent is given. The reactivities of hexafluorobenzene and hexachlorobenzene are compared. The substantially greater reactivity reported with the former is regarded as due solely to the greater mobility of fluorine than chlorine, with first row nucleophdes in protic solvents; whereas in contrast it is predicted that the C₆Cl₅ group is slightly more activating for aromatic S, reactions than the C,F, group. The relative reactivities of C₆F₆ and C₆H₅F, and C₆Cl₆ and C₆H₅Cl are also compared. Rate ratios are c.10⁸; ΔΔE? c.15 kcal mole^-1; and Δlog₁₀B c.+1.