Chemical Education Today edited by
Book & Media Reviews
Jeffrey Kovac University of Tennessee Knoxville, TN 37996-1600
Radical Reactions in Organic Synthesis (Oxford Chemistry Masters) by Samir Z. Zard Oxford University Press: Oxford, U.K., 2003. 256 pp. ISBN 0198502400 (paperback), $64.50; ISBN 0198502419 (cloth), $129.50. reviewed by R. David Crouch
Like many synthetic organic chemists, I’m a bit biased in my opinion of radical reactions. Yes, I teach a handful of radical reactions as part of my sophomore-level organic course. But in the lab they scare me; when I think of them, I instinctively reach for aqua regia or some other strong cleaner to rid my glassware of the polymer I imagine will form. Radical reactions seem to be difficult to control and prone to form multiple products, necessitating tedious separations. The author of this book, the seventh and latest in the Oxford Chemistry Masters series, is specifically trying to convince synthetic chemists like me of the potential of modern radical reactions. After reading his book, I must admit that he has made considerable progress in changing my mind. The book begins with a brief and interesting history of the development of radical reactions in organic chemistry. The reader is led through the earliest radical reactions that were performed long before the technology existed to explain what was happening. A brief discussion of why radical reactions remained in the realm of physical organic chemistry for so long is also provided. The author asserts that the field of organoradical chemistry began to achieve acceptance as a synthetic method following Barton’s description of deoxygenation reactions using xanthate esters and tributyltin hydride, a topic that receives an entire chapter later in the book. Chapter 1 also includes discussions of the differences between ionic and radical reactions, the significance of reaction rate in radical chemistry, and the concept of chain reactions. Chapter 2 uses the prominence of organotin hydride as the starting point in describing a series of reactions such as dehalogenation, deamination, denitration, and deoxygenation reactions. It is unlikely that any of these reactions will ring a bell with the typical undergraduate student. The most alert of our students might be vaguely familiar with the addition of radicals to alkenes, although the use of trialkyltin radicals instead of bromine radicals might render the reaction unrecognizable. With the exception of brief sections in Chapter 4 where oxymercuration is considered and Chapter 6 in which halogen transfer reactions are discussed, the rest of the book covers material that is well beyond the scope of a typical sophomore-level organic course. But this book has a potentially important place in the chemistry curriculum. The author is very clear in the preface
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that this monograph is derived from his graduate-level course in radical reactions and it is at this level—or an advanced course for junior- or senior-level undergraduates—that this book is most appropriately aimed. With a good mix of physical organic, synthetic methodology, and total synthesis, the author provides a clear example of how these three fields fit together. In his preface, he states that “a feel of the factors governing reactivity of radicals and an idea for the scope and potential of radical-based transformations” will allow synthetic organic chemists to see the potential for these reactions. For example, instead of simply listing Baldwin’s rules, the kinetic bases for these preferences are explained and the rate differences (and subsequent preference changes) caused by subtle structural changes are discussed. Chapter 3 extends the discussion of tin-mediated radical reactions to rearrangements including the formation, opening, and expansion of rings and hydrogen transfers. Fragmentations of trialkyltin containing compounds, or  scission, reactions are also covered. Many of the concepts of Chapters 2 and 3 are extended in Chapter 4, which compares and contrasts radical reactions using organo-silicon, -germanium, and -mercury hydrides with organotin-mediated reactions. Chapter 6 opens with a discussion on Kharach’s discovery of the effect of peroxides on HBr addition to alkenes and leads into a consideration of atom and group transfers via radical processes. The bulk of the discussion involves hydrogen transfers from C⫺H bonds and transfers of xanthate and related groups. In Chapter 7, the reader is taken through some radical processes that function via a non-chain mechanism. From my perspective, the highlight of the chapter comes at the beginning with a very clear and concise discussion of the Fischer–Ingold effect in non-mathematical terms. Examples from synthetic chemistry using nitrites and hypoiodites illustrate the importance of these persistent radicals and their role in polymer and vitamin B12 chemistry are presented. The role of metals in radical-mediated organic chemistry is included in Chapter 8. Metal-mediated oxidation reactions using Mn(III) and Cu(II), as well as the less frequently used Fe(III), Ce(IV), and Pb(IV). The inclusion of Ag(II) in this section caught my eye: Ag(I) ions are converted in situ by peroxydisulfate to Ag(II) ions which are, as one might imagine, very strong oxidizing agents. Reductions with Cr(III) and Ti(III) are presented. Examples of the increasingly popular reducing agent, SmI2, are also presented. Other processes, including dissolving metal reduction and electrochemical and photochemical processes, round out the discussion. The monograph closes with a consideration of the advantages and disadvantages of radical reactions in various aspects of chemistry. The toxicity of tin clearly limits these methods in industrial settings, but Kharasch-like atom transfers seem to transfer well. The author also provides a suggestion to those of us who are inexperienced with radical
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Chemical Education Today
Book & Media Reviews chemistry: keep trying! And, a list of seven “points for consideration” is provided as a very general and helpful troubleshooting guide. This book is well-referenced and many complex topics are thoroughly, but concisely, explained. Although it is above the heads of the typical sophomore-level organic student, it would serve as a good text for a special topics course on radical
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chemistry at the advanced undergraduate or graduate level. And this book is certainly a welcome addition to any college or university chemistry library. R. David Crouch is in the Department of Chemistry, Dickinson College, Carlisle, PA 17013-2896; crouch@ dickinson.edu.
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