Special Topic Benzyne

In Section 14.12, we mentioned the lack of reactivity of halobenzenes with nucleophiles. It took the activation of a nitro group on the ring for substitution to occur (see Figs. 14.94-14.96). In truth, if the nucleophile is basic enough, even chlorobenzene will react. In the very strongly basic medium potassium amide (K NH2) in liquid ammonia (NH3), chlorobenzene is converted into aniline (Fig. 14.111). 

The mechanism is surely not an Sn2 displacement, and there is no reason to think SnAt addition—elimination is possible with an unstabilized benzene ring. This difficult mechanistic problem begins to unravel with the obsen tion by John D. Roberts (b. 1918) and his co-workers that labeled chlorobenzene produces aniline labeled equally in two positions (Fig. 14.112). 

FIGURE 14.112 A clue to the mechanism for this reaction is provided hy Roherts observation of isotopic scramhling when labeled chlorobenzene is used. The red dot represents C, an isotopic label. 

What reactions of halides do you know besides displacement The El and E2 reactions compete with displacement reactions (Chapter 7). If HCl were lost from chlorobenzene, a cyclic alkyne, called benzyne (or dehydrobenzene), would be formed. This symmetrical bent alkyne might be reactive enough to undergo an addition reaction with the amide ion. If this were the case, the labeled material must produce two differently labeled products of addition (Fig. 14.113), because the NH2 can attack either carbon of the alkyne. This mechanism is the elimination-addition version of the SnAt reaction. 

FIGURE 14.113 The elimination-addition variation of the SNAr reaction begins with a halobenzene. The symmetrical intermediate benzyne is formed. Addition of the amide ion to benzyne produces the observed labehng results. 

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Almost all magical migration reactions, such as the mysterious movement of the red label in Figure 14.112, involve the formation of a symmetrical intermediate— here benzyne (Fig. 14.113). The Special Topic in Chapter 21 is full of such problems, but they have come up elsewhere as well. Problems 14.67, 20.16, and 20.42b are nice examples. 

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