Conjugation and hyperconjugation: a survey with emphasis on isovalent hyperconjugation

Abstract

Various aspects and examples of conjugation and hyperconjugation are surveyed, and the desirability of an explicit classification of both conjugation and hyperconjugation into two major types, isovalent and sacrificial (ordinary) is emphasized. The existence of a third type, pluvalent conjugation, is also mentioned. Further, the desirability of a subclassification of isoconjugation and isohyperconjugation each into three sub-types (dative, non-dative, and homodative, in orderof increasing conjugative stabilization) is pointed out, with examples. Sacrificial conjugation and hyperconjugation are usually non-dative, but some examples of dative conjugation and hyperconjugation are cited. The intrinsic jointness of the inductive and mesomeric effects (both of which are forms of resonance) in homodative isoconjugation and isohyperconjugation is emphasized. Homodative alkyl isohyperconjugation in carbonium ions is discussed extensively, with especial reference to McCaulay's Conference paper. Some hitherto incompletely published theoretical evidence, supported by experimental spectroscopic evidence, that the ethylene positive ion is partially twisted in its ground state, is presented; the twist is due to incipient isohyperconjugation. Definitions of the term hyperconjugation are considered with some care. It is suggested that the Baker-Nathan effect (in so far as it refers to differences between C-H and C-C hyperconjugation) should not itself be described as hyperconjugation, but rather as differential hyperconjugation. It is pointed out that there are no obvious theoretical reasons for expecting radical differences in stabilization energy or in magnitude of electron between C-C and C-H hyperconjugation. Finally, the writer wishes to apologize if he has sometimes gone astray in this paper because of the superficiality of his acquaintance with the vast literature of organic chemistry.