The detection and fate of bromine atoms and beta-bromo radicals generated via the photochemical debromination of vicinal dibromides in acetonitrile.

Abstract

The photolysis of vicinal dibromoalkanes in acetonitrile produces bromine atoms and $\beta$-bromoalkyl radicals. Bromine atoms react with halide ions to produce easily detectable and long-lived radical anions $\rm (BrX\sp{\sbsp{\!-}{\bullet}}).$ The reaction of photolytically produced bromine atoms with iodide, bromide, chloride, and fluoride occurs with rate constants of (1.4 $\pm$ 0.8) $\times$ 10$\sp $ M$\sp{-1}$ s$\sp{-1}$, (1.6 $\pm$ 0.3) $\times$ 10$\sp $ M$\sp{-1}$ s$\sp{-1}$, (1.4 $\pm$ 0.1) $\times$ 10$\sp $ M$\sp{-1}$ s$\sp{-1},$ and (1.0 $\pm$ 0.1) $\times$ 10$\sp $ M$\sp{-1}$ s$\sp{-1}$, respectively in acetonitrile. The formation of the Br$\sb2\sp{\sbsp{\!-}{\bullet}}$ radical anion was found to have an activation energy of $1.9\pm 0.4$ kcal/mol and a logA factor of $11.6\pm 0.3.$ These radical anions have extinction coefficients of $\rm (13\pm 1)\times 10\sp3\ M\sp{-1}\ cm\sp{-1}$ (BrI$\rm\sp{\sbsp{\!-}{\bullet}}),\ (9\ \pm\ 1)\ \times\ 10\sp3\ M\sp{-1}\ cm\sp{-1}\ (BrCl\sp{\sbsp{\!-}{\bullet}}),$ and $\rm (6.9\pm 0.7)\times 10\sp3\ M\sp{-1}\ cm\sp{-1}\ (BrF\sp{\sbsp{\!-}{\bullet}})$ at 360 nm in MeCN. The formation of the $\rm BrX\sp{\sbsp{\!-}{\bullet}}$ radical anions can be used to probe the dynamics of the decays of $\beta$-bromoalkyl radicals. For example, the lifetimes of the $\beta$-bromoethyl and $\beta$-bromocyclopentyl radicals were determined to be 145 ns and 100 ns, respectively. The halides were found to reduce the $\beta$-bromoalkyl radicals with rate constants in the $\rm 10\sp9\ M\sp{-1}\ s\sp{-1}$ regime. For example, the bromide, chloride, and fluoride reduction rate constants were determined for the reaction with the $\beta$-bromocyclopentyl radical to be $\rm (3.2\pm 0.3)\times 10\sp9\ M\sp{-1}\ s\sp{-1},\ (2.0\pm 0.2)\times 10\sp9\ M\sp{-1}\ s\sp{-1},$ and $\rm (1.3\pm 0.1)\times 10\sp9\ M\sp{-1}\ s\sp{-1},$ respectively. The activation energies and log(A) factors were determined for the temperature dependence of the bromide reduction of the $\beta$-bromocyclopentyl radical to be $3.3\pm 0.5$ kcal/mol and $11.8\pm 0.5$ and for the fluoride reduction of the $\beta$-bromoethyl radical to be $4.0\pm 0.5$ kcal/mol and $12.2\pm 0.5.$