Estimation of the standard reduction potentials of some 1-arylethyl radicals in acetonitrile Annarita Cardinale, Abdirisak A. Isse, Armando Gennaro * Dipartimento di Chimica Fisica, Universit  a di Padova, via Loredan 2, 35131 Padova, Italy Received 17 July 2002; received in revised form 2 August 2002; accepted 2 August 2002 Abstract The redox properties of some arylethyl radicals, which are involved in the electrochemical synthesis of important anti-inflam- matory agents, have been investigated in CH 3 CN by an indirect electrochemical method based on the catalytic reduction of the corresponding arylethyl halides (RX) by electrogenerated aromatic or heteroaromatic anion radicals (D Å ). The reaction between RX and D Å leads to a radical R Å , which can react with D Å either by radical coupling ðk 3 Þ or by electron transfer (k 4 ). Examination of the competition between these reactions, which can be expressed by a dimensionless parameter q ¼ k 4 =ðk 3 þ k 4 Þ, as a function of E 0 D=D Å allows estimation of the reduction potentials of the arylethyl radicals. The standard reduction potentials obtained for the radicals 1-(4-isobutylphenyl)ethyl, 1-(6-methoxy-2-naphthyl)ethyl, and 1-(4-biphenyl)ethyl are )1.64, )1.62, and )1.15 V vs. SCE, respectively. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Free radicals; Standard potential; Arylethyl halides; Redox catalysis 1. Introduction The redox properties of free radicals are of great in- terest for many aspects of organic chemistry. Free rad- icals are, however, typically short-lived, which makes direct measurement of their standard potentials a very difficult task. Several direct or indirect electrochemical methods have been developed to study the redox prop- erties of transient radicals. A description of methods and an extensive compilation of literature data have been recently presented by Daasbjerg et al. [1]. The vast majority of the methods are based on a combination of photochemical and electrochemical techniques. Two photoelectrochemical methods have been used for the determination of the reduction potentials of organic radicals in aprotic solvents. One is the photomodulation voltammetry technique in which transient radicals are generated by modulated photolysis and are detected with a phase-sensitive voltammetry [2–4]. In the other method, the radicals are generated by reduction of an appropriate precursor by thermalised photoejected electrons and detected with a dc polarography [5,6]. Various organic radicals, especially benzyl radicals, have been characterised by these methods in acetonitrile and dimethylformamide (DMF). Lund and co-workers [7–10] developed an indirect electrochemical method and applied it to the estimation of reduction and standard potentials of a large number of radicals in DMF. The method is based on the ho- mogeneous reduction of a radical generating substance RX (an organic halide for example) by a series of elec- trogenerated aromatic or heteroaromatic anion radicals (D Å ). The reaction is well known as the homogeneous redox catalysis and occurs according to the mechanism shown in Scheme 1: Scheme 1 D þ e  ¡ D Å ð1Þ D Å þ RX ! D þ R Å þ X  ð2Þ D Å þ R Å ! DR  ð3Þ D Å þ R Å ¡ D þ R  ð4Þ Electrochemistry Communications 4 (2002) 767–772 www.elsevier.com/locate/elecom * Corresponding author. Tel.: +39-049-8275132; fax: +39-049- 8275135. E-mail address: a.gennaro@chfi.unipd.it (A. Gennaro). 1388-2481/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S1388-2481(02)00447-2