The first basicity scale of fluoro-, chloro-, bromo- and iodo-alkanes: some cross-comparisons with simple alkyl derivatives of other elements

Abstract

Formation constants (K_f) are determined for the 1∶1 hydrogen-bonded complexes between 4-fluorophenol and 42 halogenoalkanes, in CCl₄ at 298 K, by FTIR spectrometry. A base parameter, pK_HB = log K_f, has been defined which measures, for the first time, the relative hydrogen-bond acceptor strength of the halogens. Comparison with the basicity of group 15 and 16 elements shows that halogens are very weak bases, in the order: N  O > P > S > Se > F > Cl ≈ Br ≈ I. Attempts at correlations of pK_HBvs. electronegativity or vs. hardness lead to general scatter diagrams which may be partially resolved into separate trends within a given group of the periodic table. The pK_HB scale of halogenoalkanes extends from +0.26 for 1-fluoroadamantane to –0.70 for 1,1,1-trichloroethane (statistically corrected to –1.15 per chlorine atom). The main substituent effects explaining these pK_HB variations are for a given halogen: (i) field-inductive effects (polyhalogenoalkanes), (ii) resonance effects (cyclopropyl bromide), and (iii) polarizability effects (alkyl halides). The steric effects of bulky alkyl substituents do not appear to be significant since the basicity increases with the lengthening and branching of the alkyl groups (from methyl to 1-adamantyl). For a given halogen, pK_HB increases linearly with the infrared shift, Δν(OH), produced by hydrogen bonding. However, as the group 17 is descended, IR shifts do not parallel pK_HB. An increasing sensitivity of IR shifts to polarizability is suggested when going from group 15 to group 17.