THE C-H BOND-ENERGY OF BENZENE

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- reversible arrow C6H5- + NH3. The ratio of these rate constants gives a free energy change: Delta(reaction)G(300) = -3.58 +/- 0.06 kcal mol(-1). Use of the established gas-phase acidity of ammonia gives a value for Delta(acid)G(300)(C6H6) of 392.9 +/- 0.4 kcal mol(-1). From the computed value for Delta(acid)S(300)(C6H6) of 29.6 +/- 1.0 cal mol(-1) K-1, the enthalpy change, Delta(acid)H(300)(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: Delta(acid)H(300)(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, D-0. 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), Delta(f)H(300)(C6H5) = 81.2 +/- 0.6 kcal mol(-1); D-0(C6H5-H) = 112.0 +/- 0.6 kcal mol(-1), Delta(f)H(0)(C6H5) = 84.3 +/- 0.6 kcal mol(-1).