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

Citation
C. Ouvrard et al., The first basicity scale of fluoro-, chloro-, bromo- and iodo-alkanes: some cross-comparisons with simple alkyl derivatives of other elements, J CHEM S P2, (7), 1999, pp. 1357-1362
Citations number
39
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2
ISSN journal
03009580 → ACNP
Issue
7
Year of publication
1999
Pages
1357 - 1362
Database
ISI
SICI code
0300-9580(199907):7<1357:TFBSOF>2.0.ZU;2-J
Abstract
Formation constants (K-f) are determined for the 1 : 1 hydrogen-bonded comp lexes between 4-fluorophenol and 42 halogenoalkanes, in CCl4 at 298 K, by F TIR 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 similar to Br similar to I. Attempts at correlations of pK(HB) vs . 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 correc ted to -1.15 per chlorine atom). The main substituent effects explaining th ese pK(HB) variations are for a given halogen: (i) field-inductive effects (polyhalogenoalkanes), (ii) resonance effects (cyclopropyl bromide), and (i ii) polarizability effects (alkyl halides). The steric effects of bulky alk yl substituents do not appear to be significant since the basicity increase s with the lengthening and branching of the alkyl groups (from methyl to 1- adamantyl). For a given halogen, pK(HB) increases linearly with the infrare d shift, Delta v(OH), produced by hydrogen bonding. However, as the group 1 7 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 gr oup 17.