Radical reactions curved arrows

Table 6.1 summarizes the many kinds of arrows used in describing organic reactions. Curved arrows are especially important because they explicitly show what electrons are involved in a reaction, how these electrons move in forming and breaking bonds, and if a reaction proceeds via a radical or polar pathway. 

A full description of how a reaction occurs is called its mechanism. There are two general kinds of mechanisms by which reactions take place radical mechanisms and polar mechanisms. Polar reactions, the more common type, occur because of an attractive interaction between a nucleophilic (electron-rich) site in one molecule and an electrophilic (electron-poor) site in another molecule. A bond is formed in a polar reaction when the nucleophile donates an electron pair to the electrophile. This movement of electrons is indicated by a curved arrow showing the direction of electron travel from the nucleophile to... 

Historically, ethylene potymerization was carried out at high pressure (1000-3000 atm) and high temperature (100-250 °C) in the presence of a catalyst such as benzoyl peroxide, although other catalysts and reaction conditions are now more often used. The key step is the addition of a radical to the ethylene double bond, a reaction similar in many respects to what takes place in the addition of an electrophile. In writing the mechanism, recall that a curved half-arrow, or "fishhook" A, is used to show the movement of a single electron, as opposed to the full curved arrow used to show the movement of an electron pair in a polar reaction. 

These arrows (called fishhook arrows) are the hallmark of radical reactions. We use fishhook arrows in radical mechanisms, because they indicate the movement of only one electron, rather than two electrons (by contrast, two-headed curved arrows are used in ionic mechanisms to show the movement of two electrons). 

We begin by bringing you up to speed on mechanisms and reminding you how to push electrons around with those curved arrows. We jog your memory with a discussion of substitution and elimination reactions and their mechanisms, in addition to free radical reactions. Next you review the structure, nomenclature, synthesis, and reactions of alcohols and ethers, and then you get to tackle conjugated unsaturated systems. Finally, we remind you of spectroscopic techniques, from the IR fingerprints to NMR shifts. The review in this part moves at a pretty fast pace, but we re sure you can keep up. 

Allylic halogenation is a substitution reaction involving a free-radical mechanism. The general mechanism is in Figure 4-7. The final X cycles back to the beginning (shown with the Icirge curved arrow). 

A covalent bond consists of a shared pair of electrons. Nonbonded electrons important to the reaction mechanism are designated by dots (— OH). Curved arrows (<- ) represent the movement of electron pairs. For movement of a single electron (as in a free radical reaction), a single-headed (fishhook-type) arrow is used ( ). Most reaction steps involve an unshared electron pair (as in the chymotrypsin mechanism). 

The movement of unpaired electrons in free radical reactions is shown with single-headed curved arrows. 

Hall-headed arrows (fish-hooks, are used to show the movement of individual electrons in free radical reactions, i-contrast to the curved arrows used in polar and molecular reactions to show the movement of electron pairs. 

SCHEME 8.R.5 Representations of some reactions involving bond breaking and bond making using curved arrows (a) a radical s addition to a double bond, (b) an ionic displacement... 

In Section 6.1, we mentioned that a bond can be broken in two different ways heterolytic bond cleavage forms ions, while homolytic bond cleavage forms radicals (Figure 11.1). Until now, we have focused mostly on ionic reactions—that is, we have been exploring mechanisms that involve ions. This chapter wiU focus exclusively on radicals. Look carefully at the curved arrows... 

The mechanisms of these reactions are complex, involve multiple radicals, and are beyond the scope of this discussion. (In some instances they are poorly understood.) We will not use curved arrows to make sense of these reactions. Instead, we will focus on C-H bonds broken, and C-0 and C=0 bonds formed. 

---