Free radicals overview

Halliwell, B. and Gutteridge, J.M.C. (1990). Role of free radicals and catalytic metal ions in human disease an overview. Meth. Enzymol. 186, 1-85. 

In the following, an overview of the experimental approaches is presented, including the production and detection methods of free radicals and the techniques for studying free radical photodissociation in the molecular beam. The photochemistry of the free radical systems investigated recently will then be discussed in detail. 

In recent years, the amount of research time devoted to materials chemistry has risen almost exponentially and sulfur-based radicals, such as the charge-transfer salts based upon TTF (tetrathiafulvalene), have played an important role in these developments. These TTF derivatives will not be discussed here but are dealt with elsewhere in this book. Instead we focus on recent developments in the area of group 15/16 free radicals. Up until the latter end of the last century, these radicals posed fundamental questions regarding the structure and bonding in main group chemistry. Now, in many cases, their thermodynamic and kinetic stability allows them to be used in the construction of molecular magnets and conductors. In this overview we will focus on the synthesis and characterisation of these radicals with a particular emphasis on their physical properties. 

We begin this overview of manganese biochemistry with a brief account of its role in the detoxification of free radicals, before considering the function of a dinuclear Mn(II) active site in the important eukaryotic urea cycle enzyme arginase. We then pass in review a few microbial Mn-containing enzymes involved in intermediary metabolism, and conclude with the very exciting recent results on the structure and function of the catalytic manganese cluster involved in the photosynthetic oxidation of water. 

The use of free-radical reactions in organic synthesis started with the reduction of functional groups. The purpose of this chapter is to give an overview of the relevance of silanes as efficient and effective sources for facile hydrogen atom transfer by radical chain processes. A number of reviews [1-7] have described some specific areas in detail. Reaction (4.1) represents the reduction of a functional group by silicon hydride which, in order to be a radical chain process, has to be associated with initiation, propagation and termination steps of the radical species. Scheme 4.1 illustrates the insertion of Reaction (4.1) in a radical chain process. 

It should be evident that radicals play a very important role within the realm of organofluorine chemistry. Fluorine substituents impart unique reactivity characteristics to free radical intermediates, and knowledge of how to generate and utilize such species is very important for those synthetic chemists who wish to incorporate fluorinated alkyl groups into organic substrates. It has been attempted in this review to provide a strategic overview of all aspects of organo-fluorine radical chemistry, with the hope that readers with an interest in the field will able to get their basic questions answered as well as be stimulated to dig deeper into specific aspects of the subject via the detailed references which have been provided. 

For good overview of such radicals, see Fischer H, editor. Magnetic properties of free radicals nitrogen and oxygen centered radicals. New York NY Springer 2004 Vol. 26C. 

Due to some special structural and magnetic peculiarities, in particular, free-radical properties, the quinone ligands and their metal complexes are apart from the other kinds of coordination compounds [125a, 138a], as will be shown below. Here we present an overview of the main methods for the synthesis of complexes containing benzoquinone, semiquinone, and catecholate ligands, and peculiarities of the products. 

This discussion is not intended to be an exhaustive review of the wood-polymer literature, but rather an overview of the processing procedures used today. In general, the free radicals used for the polymerization reaction come from two sources, temperature sensitive catalysts and Cobalt-60 gamma radiation. In each case a free radical is generated by the process, but from that point the vinyl polymerization mechanism is the same. Each... 

Lewis and Singer (17) overview the role and importance of stable free radicals in carbonization processes. Fitzer et al (18) provide a comprehensive description of the chemistry involved in the conversion of specific organic compounds to carbon. Marsh (19-23) and Forrest and Marsh (24) relate the chemistry of mesophase formation to its properties and applications. The journal CARBON published an issue devoted specially to studies of mesophase and its applications (25). The world availability of pitch materials is such that there is an abundance of pitch which produces cokes of little industrial value. 

This review will consist of an overview of the structure and chemistry of FeIV and then outline how this is utilised in biology. The role of free radicals in catalysis by FeIV will be stressed, especially those that are bound to the enzyme. The complementary use of different spectroscopic methods to probe these structures will then be discussed. Finally the problems that can be... 

This is the introductory chapter which sets the stage for those that follow. We start by attempting the definition of the term free radical together with a very brief historical background. This is followed by brief overviews of some of the types of radical that might be encountered in biological systems. 

Moses Gomberg and the triphenylmethyl radical 3 Overview of free radicals in the 20th Century 3... 

The thermal pyrolysis of hydrocarbons proceeds by free radical chain reaction processes. These processes are exceedingly complex and this overview concentrates on the details as it impacts on the technology and economics of olefin production. 

Chapter 1 is used to review the history of polyethylene, to survey quintessential features and nomenclatures for this versatile polymer and to introduce transition metal catalysts (the most important catalysts for industrial polyethylene). Free radical polymerization of ethylene and organic peroxide initiators are discussed in Chapter 2. Also in Chapter 2, hazards of organic peroxides and high pressure processes are briefly addressed. Transition metal catalysts are essential to production of nearly three quarters of all polyethylene manufactured and are described in Chapters 3, 5 and 6. Metal alkyl cocatalysts used with transition metal catalysts and their potentially hazardous reactivity with air and water are reviewed in Chapter 4. Chapter 7 gives an overview of processes used in manufacture of polyethylene and contrasts the wide range of operating conditions characteristic of each process. Chapter 8 surveys downstream aspects of polyethylene (additives, rheology, environmental issues, etc.). However, topics in Chapter 8 are complex and extensive subjects unto themselves and detailed discussions are beyond the scope of an introductory text. 

From the point of view of the biochemist, there are three major classes of samples that might be investigated with ESR spectroscopy (1) samples containing transition or lanthanide elements (2) samples containing free radicals (3) samples that have been extrinsically labeled by the introduction of nitroxide radical spin labels or spin probes. We provide an overview of these three subjects in this chapter. Basic references are given at the beginning of each section. The material described is by no means comprehensive the topics reviewed have become very massive subjects. Rather, we have tried to provide information, often from our own work and experience, that we hope will be of general interest. 

The use of free radical chemistry at the anomeric center to produce carhon-carhon bonds, especially in an intramolecular fashion, remains a popular method for C-glycoside synthesis. A review entitled C-Glycosidation Technology with Free Radical Reactions appeared in early 1998, and the reader is referred to that somce [2] as well as to the more traditional ones for complete overviews on the subject. Again, the coverage here focuses on the most recent developments. 

There is much to learn and admire in the hindered amine story. Chemists can take pride in how effectively they have worked together across national boundaries to make hindered amine stabilizers an important product group for the stabilization of polymers. This introduction is a modest effort to review some of the early history of stable-free radicals including triacetoneamine-N-oxyl. This chapter was intended to serve primarily as an introduction to the hindered amine review which took place at the symposium and intentionally avoids covering material which other participants were expected to present. It is a "light-touch" overview. 

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