Acidities and Homolytic Bond Dissociation Energies of the Acidic C-H Bonds in Radical Cations

Article abstract of DOI:10.1021/jo00041a020

The homolytic bond dissociation energies (BDE_HA+*) of the acidic C-H bonds in radical cations of the type HA+* have been estimated by using the equation BDE_HA+* = BDE_HA + 23.1ox(A*) - E_ox(HA)>.The BDE_HA values of the acidic H-A bonds in neutral molecules were estimated from pK_HA and E_ox(A-) values in DMSO.Most E_ox(A-) and E_ox(A*) values were obtained from the reversible oxidation potentials of the conjugate bases of HA, and most E_ox(HA) values were obtained from the irreversible oxidation potentials of the neutrals by cyclic voltammetric measurements.The method has been applied to obtain BDE_HA+* values for thirteen 9-(dialkylamino)fluorenes, two 2-benzoyl-N,N'-dialkylperhydropyrimidines, two α,α-bis(alkylamino)acetophenones, three triphenylmethanes, two diphenylmethanes, 9,10-dihydroanthracene, four xanthenes, toluene, four fluorenes, and three arylacetonitriles. ( The acidities of the corresponding radical cations have also been estimated.) For most of the carbon-centered radical cations stabilized by one or two adjacent dialkylamino groups, the BDE_HA+* values were estimated to be in the 22 - 40 kcal/mol range: i.e., they are 30 - 50 kcal/mol lower than the corresponding BDE_HA values in the neutral parents.These relatively low BDE_HA+* values were correlated with the stabilities of the cations formed in reactions of the type HA^+* -> H^* + A⁺.On the other hand, the BDE_HA+* values for substituted aromatic hydrocarbons were found to be controlled primarily by the stabilities of the radical cations, HA^+*, rather than by the stabilities of the product cations.Aromatic hydrocarbons containing two or more aryl groups usually gave radical cations with BDE_HA+* values in the 30 - 40 kcal/mol range whereas those with only one aryl group gave BDE_HA+* values in the 40 - 50 kcal/mol range.