Substituent effects on the ionization reaction of beta-mesylate phenethyl radicals.
A series of beta-methanesulfonate phenethyl radicals bearing a range of electron donating and withdrawing aromatic substituents were generated and studied in a variety of solvent mixtures using nanosecond laser flash photolysis. Rate constants for the formation of the corresponding styrene radical cation via heterolytic loss of the beta-mesylate leaving group were measured using time-resolved absorption spectroscopy. The ionization reaction was investigated in a variety of solvents and solvent mixtures including 1,1,1,3,3,3-hexafluoro-2-propanol, 2,2,2-trifluoroethanol, acetonitrile, methanol and water. The influence of substituent electronic effect and solvent polarity on the kinetics of the beta-heterolysis reaction are discussed and assessed using the sigma+ Hammett parameter and Y(OMs) values, respectively. The small magnitude of m calculated for the formation of the 4-methoxystyrene radical cation by ionization of the mesylate group (m = 0.33) in aqueous methanol mixtures is compared to values obtained for the formation of the same radical cation via loss of chloride and bromide where m = 0.56 and m = 0.45, respectively.
Lancelot
Sandy F
SF
Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3.
Cozens
Frances L
FL
Schepp
Norman P
NP
D001555:Benzene Derivatives / Q000737:chemistry*; D002412:Cations; D005609:Free Radicals / Q000737:chemistry; D007700:Kinetics; D007834:Lasers; D008698:Mesylates / Q000737:chemistry*; D010782:Photolysis; D012997:Solvents / Q000737:chemistry; D013053:Spectrophotometry / Q000379:methods; D020058:Styrene / Q000737:chemistry
D001555:Benzene Derivatives; D002412:Cations; D005609:Free Radicals; D008698:Mesylates; D012997:Solvents; D020058:Styrene