Kinetic isotope effects as probes of the mechanism of reaction of 1-naphthylcarbene with cyclohexane and toluene

The rates of reaction of 1-naphthylcarbene (1-NC) in hydrocarbon solution have been measured with excimer laser flash photolysis of 1-naphthyldiazomethane. The kinetic data were obtained by monitoring the growth of the 1-naphthylmethyl radical (1-NCH) at 370 nm. The observed kinetic deuterium isotope effect for the reaction of 1-NC with cyclohexane (cyclohexane-d/sub 12/) in 2,2,4-trimethylpentane (k/sub H//k/sub D/ = 1.32 +/- 0.17) and the high CH insertion to H-atom abstraction product ratio are consistent with largely singlet reactivity for 1-NC. In toluene, where addition to the aromatic ring is favored over hydrogen atom abstraction, the measured inverse isotope effect (k/sub H//k/sub D/ = 0.52 +/- 0.05) also argues for predominantly singlet reactivity and a small singlet-triplet energy gap. The Arrhenius activation parameters measured for the reaction of NC with cyclohexane and toluene were E/sub act/ = 2.43 +/- 0.19 kcal/mol, log (a, s/sup -1/) = 7.70 +/- 0.15 and E/sub act/ = 1.93 +/- 0.38 kcal/mol, log (A, s/sup -1/) = 7.28 +/- 0.30, respectively.