MECHANISTIC STUDIES OF THE S(RN)1 MECHANISM FOR AROMATIC SUBSTITUTION: 1. SPECTROSCOPY AND KINETICS OF ELECTRON CAPTURE BY AROMATIC SUBSTRATES, 2. ABSOLUTE RATE CONSTANTS FOR REACTIONS OF PHENYL RADICAL WITH NUCLEOPHILES (RADICAL ANIONS

Irradiation of thiophenoxide ion and 2-naphthoxide ion in methanol or t-butanol solution by a pulsed nitrogen laser caused release of solvated electrons. Transient solvated electron absorption spectra in t-butanol and methanol were observed. Rate constants for solvated electron capture were measured for a number of substituted benzenes, benzonitriles, biphenyls and naphthalenes. These electron capture rate constants ranged between 1.7 * 10('8) and 7.6 * 10('9) M('-1)s('-1) and are at or near the diffusion controlled limits in these solvents. We also directly observed transient absorption spectra for a few radical anions resulting from electron capture. We report the absorption spectra and decay rate constants of the following unstable aromatic radical anions under ambient conditions: 4-fluorobenzonitrile, 4-chorobenzonitrile, 2-chlorobenzonitrile, biphenyl and 4-chlorobiphenyl. The decay rate constants for these radical anions ranged between 3 * 10('6) s('-1) and 1 * 10('8) s('-1).;The second phase of our studies focussed on the second propagation step of the S(,RN)1 mechanism. We used direct competition experiments to evaluate relative reactivities of several nucleophiles (diphenylphosphide ion, diphenyl phosphinite ion, diethyl phosphite ion, dimethyl phosphite ion, pinacolone enlate ion and thiophenoxide ion) and phenyl scavengers (thiophenol, dimethyl phosphonate and trimethyl phosphite) towards phenyl radical in liquid ammonia at reflux and in dimethyl sulfoxide at 35(DEGREES)C. Rate constants for reactions between phenyl radical and our scavengers are known.('1) Reported herein are the absolute rate constants for reactions of these nucleophiles with phenyl radical. Rate constants for attachment to phenyl radical ranged between 1.1 * 10('7) and 4.6 * 10('9) M('-1)s('-1) in refluxing liquid ammonia and between 1.1 * 10('7) and 1.9 * 10('9) M('-1)s('-1) in dimethyl sulfoxide. These.;rate constants are at or near the diffusion controlled limits in these solvents.;('1)R. G. Kryger, J. P. Lorand, N. R. Stevens and N. R. Herron, J. Am. Chem. Soc., 99, 7589 (1977).