Answer to Question #102461 in Organic Chemistry for Megha

Question

What do you understand by ortho directing activator and deactivator and meta
directing deactivators?

Answer 1

Ortho, para directing groups are electron donating groups(EDG) in electrofilic aromatic substitution reactions(EASR). They direct electrofilic aromatic substitution in ortho and para positions regarding themselves, because the most negative charge will be in this positions.

Meta directing groups are electron withdrawing groups. The most negative charge in this case will be in meta- position.

Because in EASR group that attack aromatic ring has positive charge, it will come to the atom, which has the most negative charge.

Activators are groups, that increase rate of the EASR in substitued benzene in comparison with non-subsitued benzene. 

Desactivators decrease rate of EASR.

Explanation of it is that almost all ortho para directing groups donate electron density to the aromatic system. Halogens are desactivators because they donate electron density by mesomeric effect, but take it away through the inductive effect. Meta directing groups always take away electron density from the aromatic system.

Ortho-, para- directing activators are hydroxyl groups, ethers, amines, alkyl groups, thiols. 

Ortho-, para- directing desactivators are halogens.

Examples of meta– directing desactivators are nitriles, carbonyl compounds (such as aldehydes, ketones, and esters), sulfones, and nitro groups.