Meta directors activating groups

An ortho—para director takes precedence over a meta director in determining the position of substitution because all ortho—para-directing groups are more activating than meta directors. 

Any substituent whose atom attached to the benzene contains a lone pair of electrons is ortho-para directing (but not necessarily a ring activator). Substituents without a lone pair on the atom attached to the ring are likely meta directors (with the exception of alkyl groups and aromatic rings, which turn out to be ortho-para directors). 

Oxidation of an alkylated benzene (making an aryl carboxylic acid) is a method of converting an ortho-para activator into a meta director. The reduction of a nitro group to make an aryl amine is a way of changing a meta director into an ortho-para activator. 

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. 

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. 

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. 

Label each of the following aromatic rings as activated or deactivated based on the substituent attached, and state whether the group is an ortho-para or meta director. 

Listed in Table 22.2 are the directing and activating-deactivating effects for the major functional groups with which we are concerned in this text. If we compare these ortho-para and meta directors for stmctural similarities and differences, we can make the following generalizations. 

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. 

---