What is a meta-director?

1 Answer
Jun 2, 2016

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


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.