Showing papers in "Advances in Physical Organic Chemistry in 1984"

Kinetics and mechanisms of reactions of organic cation radicals in solution

Abstract: 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.

Magnetic Field and Magnetic Isotope Effects on the Products of Organic Reactions

Abstract: Publisher Summary This chapter focuses on the magnetic field and magnetic isotope effects on the products of organic reactions. The chapter discusses reactions of radical pairs and examines the spin states of radical pair systems as archetypes. The crucial condition for observation of magnetic field effects is a competition between two processes: one of the processes must be magnetic field dependent and the other is magnetic field independent. As for magnetic field effects, magnetic isotope effects are generally very small or negligible in homogeneous solution, but may become significant in microheterogeneous environments, such as micelles. The chapter illustrates an example of a magnetic isotope effect involving 17 O, occurring in the thermolysis of endoperoxides of aromatic compounds. The decomposition of certain endoperoxides produces singlet molecular oxygen and the yield of singlet oxygen is magnetic field dependent. Such effects have been observed in many different kinds of chemical systems, and are readily interpreted in terms of the well-established principles of chemically induced magnetic polarization.

The Photochemistry of Aryl Halides and Related Compounds

Abstract: Publisher Summary This chapter discusses the photochemistry of aryl halides and related compounds. Due to the widespread use of substituted aryl halides as pesticides, electrical insulators, moth proofing agents, etc. and the environmental problems which the use of these compounds pose, it seems worthwhile reconsidering the photochemistry of these compounds. Furthermore, the photoinduced decomposition of aryl halides is used extensively in synthesis, and much of this latter work has been recently reviewed. It is well established that on photolysis, many aryl halides undergo homolysis to generate aryl radicals and halogen atoms. The question as to whether the excited singlet or triplet state or indeed both excited states are responsible for reaction, is of considerable importance. Thus, if reaction occurs from the excited singlet state, the homolytic process may well be in competition with an electrocyclic reaction. The precise mechanism for photoinduced homolysis of aryl-halogen compounds is dependent upon both the nature of the halogen substituent and the aryl group.