Unimolecular HBr and HF Elimination Reactions of Vibrationally Excited CHCHBr and CDCHFBr: Identification of the 1,1-HBr Elimination Reaction from CDCHFBr and Search for the CD(F)C:HBr Adduct.

Chemical activation experiments and computational methods have been used to study the unimolecular reactions of CHCHBr and CDCHFBr with 90 and 93 kcal mol of vibrational energy, respectively. The four-centered elimination reactions of HBr and DBr are the dominant reactions; however, 2,1-DF, 1,1-HBr, and 1,1-HF reactions are also observed from CDCHFBr. The main focus was to search for the role of the CD(F)C:HBr adduct in the 1,1-HBr elimination for comparison with carbene adducts in 1,1-HX(Y) elimination from RCHXY (X,Y = Cl and F) molecules. Models of transition states and molecules from electronic structure calculations were used in statistical calculations of the rate constants to assign threshold energies for each reaction based on the experimental rate constants. The threshold energy for 2,1-HBr elimination from 1-bromopropane is 50 kcal mol, which is in basic agreement with thermal activation experiments. Comparison of the 2,1-DBr and 2,1-HBr rate constants permits discussion of the kinetic isotope effects and the effect of F atom substitution on the threshold energy for 2,1-HBr elimination. Although CDCD═CDF from 1,1-HBr elimination of CDCHFBr followed by D atom migration is an experimentally observed product, dissociation of the CD(F)C:HBr adduct may be the rate-limiting step rather than crossing the barrier associated with the transition state for 1,1-HBr elimination. The calculated dissociation energies of CH(X)C:HF adducts are 9.9, 9.3, and 9.0 kcal mol for X = F, Cl, and Br, and the values for CH(F)C:HX are 9.9, 6.4, and ∼4.9 kcal mol.