Relative rates of oxidation of N-methylacridan (1a), 2-methoxy-N-methylacridan (1b), and 3-methoxy-N-methylacridan (1c) have been investigated as a probe of transition state structure in hydride transfer reactions. Oxidants investigated cover a 2 × 107-fold (with 1a) to 2 × 108-fold (with 1c) range of reactivity, and include the π acceptors 1,4-benzoquinone (BQ), p-chloranil (CA), 2,3-dicyano-1,4-benzoquinone (DCBQ), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and tetracyanoethylene (TCNE) in acetonitrile (AN), BQ in 50:50 (v/v) AN–water, and the one-electron oxidant tris(2,2′-bipyridyl)cobalt-(III), [Formula: see text], in AN, all at 25 °C. For all of the π acceptors except TCNE the order of reactivity, 1c > 1b > 1a, (DH) indicates a transition state resembling the corresponding acridinium ion product (D+), while the order of reactivity with [Formula: see text], 1b > 1c > 1a, indicates a transition state resembling the acridan radical cation [Formula: see text]. The order of reactivity observed with TCNE, 1b > 1c > 1a, is tentatively interpreted as indicating a transition state resembling the corresponding 9-acridanyl radical (D•).
