Unimolecular Reactions of CH₂BrCH₂Br, CH₂BrCH₂Cl, and CH₂BrCD₂Cl: Identification of the Cl−Br Interchange Reaction

Abstract

The recombination reactions of CH(2)Br and CH(2)Cl radicals have been used to generate vibrationally excited CH(2)BrCH(2)Br and CH(2)BrCH(2)Cl molecules with 91 kcal mol(-1) of energy in a room-temperature bath gas. The experimental unimolecular rate constants for elimination of HBr and HCl were compared to calculated statistical rate constants to assign threshold energies of 58 kcal mol(-1) for HBr elimination from C(2)H(4)Br(2) and 58 and 60 kcal mol(-1), respectively, for HBr and HCl elimination from C(2)H(4)BrCl. The Br-Cl interchange reaction was demonstrated and characterized by studying the CH(2)BrCD(2)Cl system generated by the recombination of CH(2)Br and CD(2)Cl radicals. The interchange reaction was identified from the elimination of HBr and DCl from CH(2)ClCD(2)Br. The interchange reaction rate is much faster than the rates of either DBr or HCl elimination from CH(2)BrCD(2)Cl, and a threshold energy of congruent with43 kcal mol(-1) was assigned to the interchange reaction. The statistical rate constants were calculated from models of the transition states that were obtained from density functional theory using the B3PW91 method with the 6-31G(d',p') basis set. The model for HBr elimination was tested versus published thermal and chemical activation data for C(2)H(5)Br. A comparison of Br-Cl interchange with the Cl-F and Br-F interchange reactions in 1,2-haloalkanes is presented.

Unimolecular HBr and HF Elimination Reactions of Vibrationally Excited C2H5CH2Br and C2D5CHFBr: Identification of the 1,1-HBr Elimination Reaction from C2D5CHFBr and Search for the C2D5(F)C:HBr Adduct

“…This chemical activation study of C 3 H 7 Br* complements an earlier study − of C 2 H 5 Br*. The 12-fold reduction of the rate constant for C 3 H 7 Br* upon substitution of a methyl group for a H atom is consistent with expectations of statistical theory.…”

“…However, the transition-state model for C 2 H 5 Br also gave a similar overestimate of the Arrhenius pre-exponential factor. The geometry of the 2,1-HBr transition state shown in Figure is typical for four-centered HX (X = F, Cl, Br) reactions ,,, with a 30% extension of the C–Br distance relative to the parent molecule. The long distance for the heavy Br atom causes an increase in the moments of inertia, and the average ratio of the two conformers, ( I x I y I z ) ‡ /( I x I y I z ), is 2.16 , which contributes a factor of 1.47 to the Arrhenius and chemical activation rate constants.…”

“…This conclusion is in strong contrast to the elevation of threshold energies for both HX elimination reactions by several kcal mol −1 for CH 2 FCH 2 Cl and CH 2 FCH 2 Br. 1,25 The MP2 and M06-2X calculated 2,1-HBr threshold energies for C 3 H 7 Br and C 3 H 6 FBr are 57.3 and 52.4 and 52.4 and 49.6 kcal mol −1 , respectively. As expected, the M06-2X results are closer to the experimental assignments, but both predict a reduction in threshold energy with F atom substitution.…”

“…Pyrex vessels with contents were irradiated by the output of an Oriel 200 W high-pressure mercury lamp. In addition to the desired product C 2 H 5 CH 2 Br, self-recombination radical reactions gave C 4 H 10 and CH 2 BrCH 2 Br*; the latter was observed as CH 2 CHBr since the half-quenching pressure for CH 2 BrCH 2 Br* is 112 Torr . Small contributions from radical disproportionation reactions were ignored.…”

NaI‐Catalyzed Oxidative Amination of Aromatic Sodium Sulfinates: Synergetic Effect of Ethylene Dibromide and Air as Oxidants

“…Based on the outcomes of these control reactions, a plausible reaction mechanism is proposed (Scheme ). First, the I/Br exchange reaction with EDB produced 1‐bromo‐2‐iodoethane that after elimination of bromide via intramolecular S N 2 substitution, generated a cyclopropa‐iodinium cation ( I ) which was immediately trapped by an iodine anion to form ethene and I 2 . The reaction of ArSO 2 Na ( III ) with I 2 generates the reactive sulfonyl iodide ( IV ) .…”