FORMATION, STABILITY, AND REACTIVITY OF RADICAL CATIONS OF 1-BROMO-N-CHLOROALKANES IN AQUEOUS-SOLUTION - A PULSE-RADIOLYSIS STUDY

Hydroxyl radicals are able to form solute radical cations in acidic aq ueous solutions of 1-bromo-n-chloroalkanes (n = 1-6). Depending on the value of n, the bromine centered radical cation stabilizes on coordin ation with an unoxidized bromine atom from another molecule (intermole cular) or with an unoxidized chlorine atom of the same molecule (intra molecular). With n = 2, 5, and 6, only dimer radical cations (lambda(m ax) 430-450 nm) are formed through intermolecular coordination, wherea s, with n = 1, 3, and 4, radical cations are stabilized both by intra- and intermolecular coordination, forming intramolecular radical catio ns (lambda(max) = 380 nm) or dimer radical cations (lambda(max) = 425- 440 nm) at low and high solute concentrations, respectively. Cl-2(.-) is unable to undergo an electron transfer reaction with 1-bromo-2-chlo roethane whereas SO4.- is able to react with 1-bromo-2-chloroethane wi th a bimolecular rate constant of 8.3 x 10(6) dm(3) mol(-1) s(-1). The dimer radical cation of 1-bromo-2-chloroethane is a strong one-electr on oxidant and is able to undergo electron transfer reactions with a n umber of molecules with high rate constant values (10(9) dm(3) mol(-1) s(-1)). The dimer radical cation decays by a deprotonation mechanism, and the stability constant is determined to be 147 dm(3) mol(-1) at 2 5 degrees C. Quantum chemical calculations of the strength of the thre e-electron bond between two heteroatoms at a semiempirical level with AM1 parametrization show good correlation with experimental results. G ood correlation, between experimental results and theoretical calculat ions, is also observed for variation of the net atomic charge over bro mine, the ionization potential (IP) of the molecule, and the difficult y of oxidation of various alkyl halides.