ESTIMATION OF THE PK(A) FOR SOME HYDROCARBONS AND ALDEHYDES AND SOLVATION ENERGIES OF THE CORRESPONDING ANIONS

Thermochemical cycles are used to transform previously obtained standard potentials of carbon-centred radicals R . into pK(a) values for the corresponding weak hydrocarbon and aldehyde acids RH and into differences in solvation energies, Delta Delta G degrees(sol), of the anions R(-) and the radicals R . in N,N-dimethylformamide (DMF). For saturated hydrocarbons (e.g., methane and ethane) the pK(a) is high and relatively constant ranging from 48 to 54. The unsaturated compounds (e.g., triphenylmethane and propene) are stronger acids having values of pK(a) in the order of 29 to 41 owing to the possibility of conjugation in the corresponding anions. A similar grouping of acidities is found for aliphatic and aromatic aldehydes. Even though the uncertainty in the absolute values of pK, is estimated to be as much as +/- 5 units the consistency in the relative values is high. The values obtained in DMF are compared with corresponding results in dimethyl sulfoxide and acetonitrile and the relationship between absolute and relative acidities in these three aprotic solvents is discussed. The numerical values of Delta Delta G degrees(sol) for the non-conjugated carbanions are high and relatively independent of the size of the alkyl group whereas they are smaller for the conjugated carbanions and decrease with an increase in the size of the anion. The trend in the Delta Delta G degrees(sol) values is discussed.