Question #386fb

Jun 13, 2015

A para directing group directs incoming electrophiles to the para position of a benzene ring. A meta directing group directs incoming electrophiles to the meta position of a benzene ring.

Explanation:

Ortho-para directors

Groups with unshared pairs of electrons, such as the methoxy group of anisole and the amino group of aniline, are ortho-para directing.

Through resonance, they donate those unshared electrons to the π system.

This puts extra electron density on the ortho and para carbons and makes them more attractive to an attacking electrophile.

When the electrophile attacks the para position of aniline, the nitrogen atom can donate electron density to the π system.

This gives four resonance structures, as opposed to three if the ${\text{NH}}_{2}$ group were absent.

This further stabilizes the cationic intermediate, lowers the activation energy, and increases the reaction rate.

So groups like $\text{CH"_3"O}$ and ${\text{NH}}_{2}$ are ortho-para directing groups.

Meta directors

Groups with double and triple bonds, such as nitro groups, are meta directing.

Through resonance, they remove electrons from the π system.

This removes electron density from the ortho and para carbons.

The meta carbons have higher electron density, so they are more attractive to an attacking electrophile.

So groups like ${\text{NO}}_{2}$ are meta directing groups.