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Beginning with acid-base reactions and continuing though the chapters on nucleophilic substitution, carbonyl addition, acyl transfer, carbanion intermediate, and electrophilic mechanisms, we have studied reaction mechanisms in which both electrons in a covalent bond or lone pair move in the same direction.
In this chapter, we will learn about some reactions in which the key steps involve the movement of single electrons. You may recall from way back in section 6.1A that single electron movement is depicted by a single-barbed'fish-hook' arrow (as opposed to the familiar double-barbed arrows that we have been using throughout the book to show two-electron movement).
Single-electron mechanisms involve the formation and subsequent reaction of free radical species, highly unstable intermediates that contain an unpaired electron. We will learn in this chapter how free radicals are often formed from homolytic cleavage, an event where the two electrons in a breaking covalent bond move in opposite directions.
(In contrast, essentially all of the reactions we have studied up to now involve bond-breaking events in which both electrons move in the same direction: this is called heterolytic cleavage).
We will also learn that many single-electron mechanisms take the form of a radical chain reaction, in which one radical causes the formation of a second radical, which in turn causes the formation of a third radical, and so on.
The high reactivity of free radical species and their ability to initiate chain reactions is often beneficial - we will learn in this chapter about radical polymerization reactions that form useful materials such as plexiglass and polyproylene fabric. We will also learn about radical reactions that are harmful, such as the degradation of atmospheric ozone by freon, and the oxidative damage done to lipids and DNA in our bodies by free radicals species. Finally, we will see how some enzymes use bound metals to catalyze high energy-barrier reactions - such as alkane hydroxylation or alcohol dehydroxylation - through single-electron steps with radical intermediates.