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The experimental recording of spectrophotometric data suitable for detailed kinetic analysis and reaction mechanism determination is covered early in the chapter with examples of experiments and subsequent data treatment on a variety of different chemical systems. Several new methods to distinguish between single-step and complex reaction mechanisms are described in detail. The fundamental organic reactions covered in more detail were limited to recent work on four general reaction types. These include (1) proton transfer reactions of nitroalkanes and related compounds, (2) S N 2 reactions of CH 3 -X where X is the leaving group, (3) electrophilic aromatic substitution, and (4) hydride transfer reactions of NADH/NAD + model systems. In all cases possible, discussion of the experimental studies was complemented by computational studies on the same reaction systems. The experimental kinetic studies all involved short-lived intermediates and could only distinguish the complex mechanisms involved from the mechanism with a single transition state. Computational studies confirmed the complex nature of the mechanisms and provided structures for the likely intermediates in the experimental reactions. The energies of the noncovalently bonded intermediates were highly dependent on the computational method and therefore could not confirm the apparent Gibbs free energies of the transition states.

Publisher Summary This chapter focuses on linear sweep and cyclic voltammetry. Polarography refers to measurements at the dropping mercury electrode while measurements at stationary electrodes are referred to as linear sweep voltammetry (LSV). This chapter discusses measurements at stationary electrodes and illustrates typical experimental apparatus for conducting LSV or cyclic voltammetry (CV). The cell shown is the simplest version and contains the electrolyte in a single compartment into which all three electrodes are inserted. The electrolytic current (I) flows through the electrolyte between the working (W) and counter (C) electrodes. The electrode potential of interest (E) is that between W and the reference electrode (R). The LSV current–voltage curve, often called an LSV wave, for a reversible charge-transfer reaction is presented. The theoretical treatment of mass transfer in LSV and CV assumes that only diffusion is operative. The chapter also discusses the theoretical LSV and CV response in terms of the Faradaic current––that is, the current arising from an electrode reaction––and has referred to the electrode potential as the potential difference between the working and reference electrodes. Because the reactions of electrode-generated intermediates frequently involve sequential steps, either heterogeneous or homogeneous, it is convenient to use a short-hand notation to describe the processes. Quantitative studies using LSV and CV can be carried out for both heterogeneous charge-transfer kinetics and the kinetics of homogeneous chemical reactions coupled to charge transfer at electrodes.

Publisher Summary All organic reactive intermediates readily undergo electron transfer reactions; electrochemical methods play an important role in the study of their chemistry. The measurement of the electrode potential for the formation of the intermediate can lead directly to the standard free energy of the process. The kinetics of the reactions of intermediates, formed in exceedingly low concentrations can be deduced from the electrode response of the substrate from which the intermediate is derived by an electron transfer. This chapter describes the study of reactive intermediates by electrochemical methods. The study of reactive intermediates generated at electrodes is organic electrochemistry. In volume reactions, the electrode functions as an electron source or sink and specific interactions between the electrode and substrates or intermediates are weak or insignificant. In this case, the remaining possible influence of the electrode is because of the electric field in its immediate vicinity. During surface reactions the substrate, the intermediates, or both, interact strongly with the electrode and diffusion is restricted. The reactions of intermediates generated under these conditions differ substantially from those in homogeneous solution. Electrode potentials measured under such conditions cannot be equated to thermodynamic potentials for the formation of the intermediates. Electrode measurements involve low substrate concentrations; therefore, reactive impurities have to be held to a very low level. The measurement of an electrode potential is the most direct method of obtaining the free energy change or the equilibrium constant of a reaction.

Publisher Summary The dimerization of cation radicals of simple benzene derivatives is usually accompanied by the formation of the corresponding biphenyl derivative. This, by virtue of the extended conjugation is more easily oxidized, and frequently only intractable tars are found as final products. If the reaction medium is carefully selected, so that the biphenyl cation radicals are stable, useful synthetic procedures can be developed. Two mechanisms are probable for the dimerization of cation radicals of anisoles and related compounds. They are termed “radical–radical dimerization” (RRD) and “radical–substrate coupling” (RSC). The oxidation of aromatic hydrocarbons and alkylaromatic compounds in media of low nucleophilicity results in the formation of dimers. The cation radicals of triphenylamines form dimers in high yield when generated in acetonitrile. The products are tetraphenylbenzidines.