Meta directors benzenes

Because substitution m (trifluoromethyl)benzene occurs primarily at positions meta to the substituent we say that a trifluoromethyl group is a meta director... 

Meta (Section 117) Term descnbing a 1 3 relationship be tween substituents on a benzene nng Meta director (Section 12 9) A group that when present on a benzene nng directs an incoming electrophile to a position meta to itself... 

Next, ask yourself what an immediate precursor of the target might be. Since an acetyl group is a meta director while a hydroxyl group is an ortho and para director, acetophenone might be a precursor of m-hydroxyacetophenone. Benzene, in turn, is a precursor of acetophenone. 

Some substituents are meta-directors, encouraging electrophilic attack upon the meta position. The presence of a meta-director means the major (or only) product of the reaction will be a meta-disubstituted benzene. 

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. 

The first examples illustrate that the order of addition of the substituents is important in controlling their orientation. For example, suppose we needed to prepare m-chloronitrobenzene from benzene. Because the chlorine is an ortho/para director and the nitro group is a meta director, it is apparent that the nitro group must be added first if the meta product is desired ... 

Nitrobenzene reacts more slowly than benzene in all substitution reactions. Thus, its electron-withdrawing NO2 group deactivates the benzene ring to electrophilic attack. Although three products are possible, the compound with the new group meta to the NO2 group predominates. The NO2 group is called a meta director. 

All meta directors have a full or partial positive charge on the atom bonded to the benzene ring. 

A benzene ring deactivated by a strong electron-withdrawing group—that is, any of the meta directors—is not electron rich enough to undergo Friedel-Crafts reactions. 

For example, the Br group in p-bromonitrobenzene is an ortho, para director and the NO2 group is a meta director. Because the two substituents are para to each other, the ortho, para director must be introduced first when. synthesizing this compound from benzene. 

Meta director (Section 18.7) A substituent on a benzene ring that directs a new group to the meta position during electrophilic aromatic substitution. 

In this way it has been found that every group can be put into one of two classes ortho,para directors or meta directors. Table 11.1 summarizes the orientation of nitration in a number of substituted benzenes. Of the five positions open to attack, three (60%) are ortho and para to the substituent group, and two (40%) are meta to the group if there were no selectivity in the substitution reaction, we... 

The yield of nitration depends upon the concentrations of the acids, temperature and the length of the nitration process. In the nitration process, a maximum of three nitronium ions (NO -) can be attached to the benzene ring since the nitro group is a meta director. However attaching the third nitro group is very difficult. The existence of the -CH3 or -OH group on a benzene ring makes nitration easier. 

A benzene ring with a meta director cannot undergo a Friedel-Crafts reaction. 

Both substituents of mefa-nitroacetophenone are meta directors. However, the Friedel-Crafts acylation reaction must be carried out first because the benzene ring of nitrobenzene is too deactivated to undergo a Friedel-Crafts reaction (Section 16.7). 

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