The Oxidation of Trimethylamine by OH Radicals in Aqueous Solution, as Studied by Pulse Radiolysis, ESR, and Product Analysis. The Reactions of the Alkylamine Radical Cation, the Aminoalkyl Radical, and the Protonated Aminoalkyl Radical

Max-Planck-Institut für Strahlenchemie, Stiftstraße 34—36. D-4330 Mülheim a .d . Ruhr. West Germany Z. Naturforsch. 41b, 505—513 (1986); received O ctober 29, 1985 Radiation Chemistry, Amines, Electron Transfer. Hydrogen Abstraction, Reaction Kinetics Hydroxyl radical reactions with trimethylamine in aqueous solution lead to the formation of the aminoalkyl radical (C H ,)2NCH; (A ) and its conjugated acid (CH 3):HNCH2 (A H t ) as well as to the alkylamine radical cation (CH3)3N t (N t). These radicals are transformed into each other by hydrolytic reactions, e.g. — H + H + (CH,),HNCH; c — -> (c h i n c h ; < -> (c h ,)3n + ( A H t ) h " ( Ä) —H + ( N + ) pK, = 3.6 pK, = 8.0 Radicals A H t are more acidic (pK., ~ 3.6) than the radicals N t (pK, ~ 8.0). Consequently, N T predominate over A H t under quasi equilibrium conditions (e.g. in the presence of phosphate buffer) and are the only species observed by ESR in acid solutions. Reacting with the protonated amine. OH radicals abstract hydrogen at nitrogen and at carbon with comparable ease. Reaction of OH radicals with the free amine may initially also generate N t, beside H-abstraction at carbon. Radicals A ’ absorb more strongly at 260 nm (f = 3390 dm ’mol 'em ') than the radical cation N T (f = 950 dm’m o r 'em '). Radical A' has reducing properties whereas radicals A H t and N t have oxidizing properties and hence can be monitored with p-nitroacetophenone (the reducing radi­ cals), and Fe(CN)64“ , N,N'-tetramethyl-/?-phenylenediamine and 2,2'-azinobis-(3-ethyl-benzthiazoline-6-sulphonate) (the oxidizing radicals). These radicals mainly (> 85% ) disproportion­ ate. one of the products being formaldehyde.