Question

Some meta - directing substituent in aromatic substitution is given. Which one is most deactivating?
(A) CN
(B) ${\text{S}}{{\text{O}}_{\text{3}}}{\text{H}}$
(C) COOH
(D) ${\text{N}}{{\text{O}}_{\text{2}}}$

Answer 1

Meta-directing groups are the chemical compounds which withdraw electrons from the aromatic compounds. They are also known as electron withdrawing groups. Ortho-para directing groups are the compounds which donate or lose electrons from the aromatic compounds. They are also known as the electron donating groups.

Complete step by step solution:
Aromatic compounds are the chemical compounds that consist of delocalized pi-electron clouds in place of the individual alternating double bonds, single bonds and they consist of conjugated planar ring systems. They are also called aromatics or arenes. Aromatic compounds need to satisfy the Huckel’s rule. By Huckel's rule, we estimate whether a planar ring molecule will have aromatic properties or not. The Huckel's rule says that for a compound to be aromatic it should have the following properties,
Planarity
Presence of electrons in the ring where n is an integer (n = 0, 1, 2, ...)
Complete delocalization of the electrons in the ring.
The examples of aromatic compounds are benzene and naphthalene.
Nitro $\left( {{\text{N}}{{\text{O}}_{\text{2}}}} \right)$ ​ group that is nitrogen dioxide is the most deactivating in electrophilic aromatic substitution.
The deactivating effect is due to inductive and resonance effects.
−CN (cyanide) and $- {\text{S}}{{\text{O}}_{\text{3}}}{\text{H}}$ (sulfonic acid) are strongly deactivating groups.
−COOH is a moderately deactivating group.
Therefore, the correct answer is option D.

Note:
The halide ions which are electron withdrawing groups should be meta but they are ortho, para directing groups (they are an exception).