The C - H bond energy of benzene

Article abstract of DOI:10.1021/ja00114a023

A flowing afterglow/selected ion flow tube instrument has been used to measure the rates of reaction of amide ion with benzene and phenide ion with ammonia: C₆H₆ + NH₂^- ? C₆H₅^- + NH₃ The ratio of these rate constants gives a free energy change: Δ_reactionG₃₀₀ = -3.58 ± 0.06 kcal mol^-1. Use of the established gas-phase acidity of ammonia gives a value for Δ_acidG₃₀₀(C₆H₆) of 392.9 ± 0.4 kcal mol^-1. From the computed value for Δ_acidS₃₀₀(C₆H₆) of 29.6 ± 1.0 cal mol^-1 K^-1, the enthalpy change, Δ_acidH₃₀₀(C₆H₆) = 401.7 ± 0.5 kcal mol^-1, is derived. The enthalpy of deprotonation of benzene, the C-H bond dissociation energy, and the electron affinity of the phenyl radical are simply related to each other: Δ_acidH₃₀₀(C₆H₆) = DH₃₀₀(C₆H₅-H) + IP(H) - EA(C₆H₅). Since earlier photoelectron experiments have provided a value for the electron affinity for the phenyl radical, EA-(C₆H₅) = 25.3 ± 0.1 kcal mol^-1, the enthalpy of deprotonation can be used to extract a value for the C-H bond enthalpy of benzene at 300 K and the C-H bond energy at 0 K, D₀. These bond energies are used to compute the heats of formation of the phenyl radical at 0 and 300 K: DH₃₀₀(C₆H₅-H) = 113.5 ± 0.5 kcal mol^-1, Δ_fH₃₀₀(C₆H₅) = 81.2 ±0.6 kcal mol^-1; D₀(C₆H₅-H) = 112.0 ± 0.6 kcal mol^-1, Δ_fH₀(C₆H₅) = 84.3 ± 0.6 kcal mol^-1.