Deactivating groups ortho-para directors

So far, groups have been either activating and ortho/para directors or deactivating and meta directors. The halogens are exceptions to this generalization. They are slightly deactivating compared to benzene but still direct to the ortho and para positions. For example, chlorobenzene is nitrated 17 times slower than benzene and produces predominantly ortho- and /Mra-chloronitrobenzene. 

Route A works fine. Toluene is readily nitrated, and the methyl group is an ortho/para director. The only problem is that both the desired compound and its ortho-isomer are produced and must be separated. (This is a common problem, and we usually assume that the separation can be accomplished, although it is not always easy in the laboratory.) Route B is unsatisfactory because the Friedel-Crafts alkylation reaction does not work with deactivated compounds such as nitrobenzene. Furthermore, even if the alkylation could be made to go, the nitro group is a meta director, so the desired product would not be formed. 

The halobenzenes are exceptions to the general rules. Halogens are deactivating groups, yet they are ortho, para-directors. We can explain this unusual combination of properties by considering that... 

A group may release or withdraw electrons by an inductive effect, a resonance effect, or both. These effects oppose each other only for the —NHi and —OH groups (and their derivatives) and for the halogens, —X. For —NH2 and —OH the resonance effect is much the more important for —X the effects are more evenly matched. It is because of this that the halogens occupy the unusual position of being deactivating groups but ortho,para directors. 

Note the differences between electrophilic and nucleophilic aromatic 1 substitutions Electrophilic substitutions are favored by electron-donofi g substituents, which stabilize the carbocation intermediate, while nucleophilic substitutions are favored by d ron-withdrawing substituents, which stabilize a carbanion intermediate. The electron-withdrawing groups that deactivate rings for electrophilic substitution (nitro, carbonyl, cyano, and so on) activate them for nucleophilic substitution. What s more, these groups are meta directors in electrophilic substitution, but are ortho-para directors in nucleophilic substitution. 

The ring attached to the nitrogen is activated, whereas the ring attached to the carbonyl group is deactivated. The N is an ortho-para director. 

Whether a substituent is an activating group or a deactivating group, and whether it is an ortho—para director or a meta director, depends largely on whether the substituent donates electrons to the ring or whether it withdraws electrons. 

Halogen substituents are weakly deactivating groups and are ortho—para directors. 

The chloro, bromo, and iodo groups are ortho—para directors. However, even though they contain unshared electron pairs, they are deactivating toward electrophilic aromatic substitution because of the electronegative effect of the halogens. 

Ortho - para directors are electron repelling groups and the meta directors are electron attracting groups. For example, toluene can be more readily nitrated than benzene. Halogens deactivate the ring but also cause ortho - para orientation. 

The substituent in this case (Cl) is an ortho-para director for the same reason that an OH group is an ortho-para director (the explanation for this directing effect can be found in your textbook). But, unlike OH (which is an activator). Cl is a deactivator. To explain why, we must explore the effect of a halogen (such as Cl) on the electron density of the aromatic ring. As we have seen in the previous section, our analysis must focus on two factors induction and resonance. Let s start with induction. Just like an OH group, a halogen is also electron-withdrawing by induction ... 

Ortho-para directors always beat meta directors. The example we just saw is a perfect illustration of this rule. The methyl group is an activator (an ortho-para director), and the nitro group is a deactivator (a meta director), so the methyl group wins. 

So, there is a competition in the directing effects. Between the two groups, the strong activator beats the strong deactivator because the strong activator is an ortho-para director. So the directing effects are ... 

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