Halogens The Exception

13 When 1,3-dinltrobenzene is treated with nitric acid and sulfuric acid at elevated temperature, the product is 1,3,5-trinitrobenzene. Explain the regiochemical outcome of this reaction. In 

In the previous sections, we have seen that activators are ortho-para directors and deactivators are meta directors. 

There is one important exception to these general rules—the halogens (F, Cl, Br, or I). Halogens are ortho-para directors despite the fact that they are deactivators. To rationalize this curious exception, we must explore the electronic effects of a halogen connected to an aromatic ring. As we have seen several times, it is necessary to consider both inductive effects and resonance. Let s begin by exploring the inductive effects. 

Halogens are fairly electronegative (more so than carbon) and are therefore inductively electron withdrawing. When we draw the resonance structures, a different picture emerges. 

A comparison of the sigma complexes formed for each of the possible regiochemical outcomes for the nitration of chlorobenzene. 

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Friedel-Crafts Alkylation Friedel-Crafts Acylation Activating Groups Deactivating Groups Halogens The Exception Determining the Directing Effects of a Substituent... 

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