Activating 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. 

The amino group is a strong activator and ortho, para-director. The nonbonding electrons on nitrogen stabilize the [Pg.896]

We have already seen that the —NH2, —NHR, and —NR2 groups act as powerful activators and ortho para directors in electrophilic aromatic substitution. These effects were accounted for by assuming that the intermediate carbonium ion is stabilized by structures like I and 11 in which nitrogen bears a positive charge... 

In summary, we have seen that both a methyl group and a methoxy group activate the ring and are ortho-para directors. This is in fact a general rule that will be used extensively throughout the rest of this chapter All activators are ortho-para directors. 

Therefore, we predict that this group will be a moderate activator. All moderate activators are ortho-para directors. 

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. 

Some of the most powerful activating substituents are those m which an oxygen atom IS attached directly to the nng These substituents include the hydroxyl group as well as alkoxy and acyloxy groups All are ortho para directors... 

The second point is somewhat less obvious but is readily illustrated by the synthesis of 1,3,5-tribromobenzene. This particular- substitution pattern cannot be obtained by direct brornination of benzene because bromine is an ortho, para director. Instead, advantage is taken of the powerful activating and ortho, para-directing effects of the fflnino group in aniline. Brornination of aniline yields 2,4,6-tribromoaniline in quantitative yield. Diazotization of the resulting 2,4,6-tribromoaniline and reduction of the diazonium salt gives the desired 1,3,5-tribromobenzene. 

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). 

All alkyl groups, not just methyl, are activating substituents and ortho, para directors. This is because any alkyl group, be it methyl, ethyl, isopropyl, tert-butyl, or any other, stabilizes a carbocation site to which it is directly attached. When R = alkyl,... 

B-3. All the following groups are activating ortho, para directors when attached to a benzene ring... 

From these results it can be seen that the methoxy group is also an ortho/para director and is a much stronger activating group than is the methyl group. Examination of the resonance structure for the para arenium ion that has the positive charge located on the carbon bonded to the methoxy group explains these results. 

The dimethylamino group is a strong activator and an ortho/para director, yet the major product from the reaction is the meta-isomer. This unexpected result is due to the... 

Both groups are ortho-para directors, but direct to different positions. Because -NH2 group is a more powerful activator, substitution occurs ortho and para to it. 

Toluene reacts faster than benzene in all substitution reactions. Thus, its electron-donating CH3 group activates the benzene ring to electrophilic attack. Although three products are possible, eompounds with the new group ortho or para to the CH3 group predominate. The CH3 group is therefore called an ortho, para director. 

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. 

---