Meta directors deactivating groups

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. 

Groups that stabilize the cation intermediate are orthopara directors groups that destabilize it are deactivators and meta directors. 

A nitro group deactivates an aromatic ring and is a meta director. Most deactivators are meta 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. 

This makes the nitro group a powerful electron withdrawing deactivating substituent and a meta director... 

Croups, which are considered to be deactivating groups cause substitution to occur in the position s meta (m) to the deactivating group (exceptions are the halogens which arc strongly o and p directors). 

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. 

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. 

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. 

Prelab Exercise Draw the complete mechanism for the nitration of methyl benzoate. Show the resonance forms that make the methyl ester group a meta director and deactivator of the aromatic ring. 

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 electrophile is the acylium ion, R-C +, generated by Lewis acid-catalyzed ionization of a leaving group (path Dn) from acyl halides or acid anhydrides (shown in the previous section). The proton that is lost comes from the same carbon that the electrophile attacked. The reaction fails for deactivated rings (Ai wg, meta directors). After the electrophile adds it deactivates the ring toward further attack. No rearrangement of the electrophile occurs. 

Donor groups therefore direct attack of the electrophile to the ortho and para positions and are known as ortholpara directors. Conversely, aromatic compounds containing electron acceptor groups are attacked at the meta position since this is the least electron-deficient site. Such groups are called meta directors. Not all substituents fit exactly into this picture halogens are deactivating but direct attack to the ortho and para positions. 

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