The meta directors are a class of atoms or a group of atoms that, when attached to an aromatic ring, render it with the ability to direct an incoming electrophile to its meta (third or fifth) position in an electrophile aromatic substitution reaction.
The meta directors are thought to affect the rate of an electrophilic aromatic substitution reaction by reducing the innate electron density of the Benzene ring to make it less susceptible to an electrophile attack compared to an unsubstituted, electron-dense Benzene ring. Therefore, they have an electron-withdrawing and ring-deactivating effect.
One of the most prominent features of the meta directors is their ability to control the regioselectivity of a reaction. Such an ability stems from the impact the meta directors have on the carbocation intermediate generated during the course of the reaction.
Only the meta position shields the electron-withdrawing groups from the destabilizing impact of having an adjacent carbocation intermediate. Such a reaction with an electron-withdrawing group is therefore suitable for aromatic electrophile substitution (usually replacing a H+ with an incoming electrophile) and proceeds faster than a substitution at the ortho or para positions. But it still occurs slowly than it does with Benzene.
One of the best ways to identity a meta directing group is to check for the atom that is directly bonded to the Benzene ring for a positive or partial positive charge. Such an atom would be electron-withdrawing in nature by resonance or the inductive effect.
Another hint is to look for the lack of lone pairs on the joining atom that reduces its ability to function as an electron donor to the Benzene ring, via the resonance effect.
A list of common meta-directing groups, all of which are deactivating, is:
-NO₂ (nitro), -CN (cyano), -COR (carbonyl, e.g., ketones, aldehydes), -COOH (carboxylic acid), -COOR (ester), -CONH₂ (amide), -SO₃H (sulfonic acid), -NR₃⁺ (quaternary ammonium, e.g., -N(CH₃)₃⁺), -CHO (aldehyde), -CCl₃ (trichloromethyl), -CF₃ (trifluoromethyl)
Related Chapters –
Fundamentals of Organic Reactions
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Meta Directors in Organic Chemistry