What is a meta-director?

1 Answer
Jun 2, 2016

I assume this question was asked on the basis of aromatic chemistry.

Explanation:

When benzene is substituted by a meta-director, X, subsequent aromatic substitutions on the ring are directed to the meta position with respect to X.

The classic meta-director is the nitro group, which contains a quaternized nitrogen centre, -N^(+)(=O)O^-. When the nitro group is substituted on a benzene ring, it deactivates ALL the positions on the ring to some extent. Nitrobenzene is much less reactive towards electrophilic substitution than say benzene. The ortho and para positions (2 and 5 with respect to NO_2) are ESPECIALLY deactivated because of the unfavourable resonance structure, which you will have to draw and establish for yourself.

On the other hand, the meta positions (3 with respect to the nitro group), when substituted, give rise to mesomers that are more stable, and more capable of electron delocalization.

All of what I have written here accounts for the experimental observation that meta substitution of substituted arenes is more favourable, when the initial substituent is a deactivating group (and a meta director). In other words we try to account for the experimental result.