The polarographic reduction of conjugated hydrocarbons: VI. Comparison of Hückel's and Wheland's m.o. approximation with experimental half‐wave potentials of various alternant and non‐alternant hydrocarbons

Abstract On the basis of the results reported in the foregoing papers IV and V and the data obtained by Bergman 2 ) from the reduction of various aromatic hydrocarbons in ethylene glycol mono‐methyl ether Maccoll's relationship between the experimental halfwave potential and the root a of the m.o. secular equation corresponding to the lowest non‐occupied π‐electronic level has been investigated in more detail. It is shown that the half‐wave potentials of 38 alternant and non‐alternant hydrocarbons being reduced according to the mechanism inferred in paper IV satisfy the relationship: The mean value of the resonance parameter γ in e.v. for various experimental: magnified image The half‐wave potentials of the univalent anions of various conjugated hydrocarbons determined by the equilibrium R′ + e ⇋ R″ in 96% dioxan‐water and the half‐wave potential of the radical triphenylmethyl fit the relationship. In this case the mean value of γ following from Hückel's and Wheland' m.o. approximation becomes −2.82 ± 0.13 and −2.57 ± 0.11 e.V., respectivelly. In accordance with the discussion given in paper V the half‐wave potentials of various l‐n‐diarylpolyenes show a slight departure from the linear relationship. The addition of an electron to non‐planar hydrocarbons may involve a decrease in the mutual hindrance between the integral parts of the system, since the lenght of the carbons bonds in the negative ions formed, will be greater than in the original hydrocarbons. Owing to this “resonance stabilisation” the difference between the experimental half‐wave potential and the one computed with the aid of the linear relationship assuming a planar structure, is sometimes appreciably larger (dibiphenylene‐ethene) or smaller ( cis 1,2‐diphenylethene and cis, cis 1,4‐diphenylbutadiene‐1,3) than we might expect from the non‐planarity of the reducible hydrocarbon.