Formation, Stability, and Reactivity of Radical Cations of 1-Bromo-n-chloroalkanes in Aqueous Solution: A Pulse Radiolysis Study

Article abstract of DOI:10.1021/j100032a022

Hydroxyl radicals are able to form solute radical cations in acidic aqueous solutions of 1-bromo-n-chloroalkanes (n=1-6).Depending on the value of n, the bromine centered radical cation stabilizes on coordination with an unoxidized bromine atom from another molecule (intermolecular) or with an unoxidized chlorine atom of the same molecule (intramolecular).With n=2, 5, and 6, only dimer radical cations (λ_max=430-450 nm) are formed through intermolecular coordination whereas, with n=1, 3, and 4, radical cations are stabilized both by intra- and intermolecular coordination, forming intramolecular radical cations (λ_max=380 nm) or dimer radical cations (λ_max=425-440 nm) at low and high solute concentrations, respectively.Cl2^.- is unable to undergo an electron transfer reaction with 1-bromo-2-chloroethane whereas SO4^.- is able to react with 1-bromo-2-chloroethane with a bimolecular rate constant of 8.3 * 10⁶ dm³ mol^-1 s^-1.The dimer radical cation of 1-bromo-2-chloroethane is a strong one-electron oxidant and is able to undergo electron transfer reaction with a number of molecules with high rate constant values (10⁹ dm³ mol^-1 s^-1).The dimer radical cation decays by a deprotonation mechanism, and the stability constant is determined to be 147 dm³ mol^-1 at 25 deg C.Quantum chemical calculations of the strength of the three-electron bond between two heteroatoms at a semiempirical level with AMI parametrization show good correlation with experimental results.Good correlation, between experimantal results and theoretical calculations, is also observed for variation of the net atomic charge over bromine, the ionization potential (IP) of the molecule, and the difficulty of oxidation of various alkyl halides.