Question

5. Which of the following is not a meta-directing substituent when presenbt on the benzene ring? Please give explanation, too.

5. Which of the following is not a meta-directing substituent when present on the benzenering? a) -NHCOCH b) -NO2 d) -C N e).CO2H

Answer 1

a)-NHCOCH₃

The amino (-NH₂) and hydroxyl (-OH) groups are strongest activating and ortho/para-directing substituents in electrophilic aromatic substitution reaction. This ability is due the presence of lone pair of electrons present on the heteroatom of these groups. The amine delocalises these electrons on the aromatic ring to produce this effect.

The activating ability of these strongly activating groups can be attenuated by acetylating the heteroatom. e.g. Aniline is converted to acetanilide

C6H5–NH2 + (CH3CO)2O

pyridine (a base)

C6H5–NHCOCH3

The acetyl derivative is still an ortho/para-directing and activating group but with moderate activating effect. The lone pair electrons on the nitrogen atom of the amide are conjugated with the carbonyl group decreasing the activating effect of NH2 group. The acetyl derivative still can delocalize its lone pair electrons on the aromatic ring although less efficiently to make it ortho para directing group.

0 HN HN delocalization HN HN Et нґ'E

The other groups such as NO2, N(CH3)3⁺, CN, CO2H are elctron withdrawing groups & hence they are meta directing.