Question

Which of the following is most reactive towards electrophilic substitution reaction ?

• A.
• B.
• C.
• D.

Answer 1

The correct option is B

Rate of electrophilic substitution reaction is proportional to the stability of the carbocation intermediate (Arenium ion) formed in the reaction.

$\text{Stability of Arenium ion}\propto \text{+R, +H, +I groups}$

$\text{Stability of Arenium ion}\propto \frac{1}{\text{-R, -H, -I groups}}$


In anilinium chloride (a), the lone pair on nitrogen atom already has a positive charge which decreases the electron density on benzene and hence electrophilic substitution is not favorable as the ring is deactivated.

In aniline (b), the lone pair of electron involves in th delocalisation and increases the electron density in the ring.

In N - acetylaniline(c), the lone pair of electrons on N is also involve in the conjugation with acetyl group. This decreases the electron density on benzene ring relatively.

In nitrobenzene (d), the electron withdrawing nitro group decreases the electron density, which is unfavourable for electrophilic substitution reaction

Hence, the electron density on benzene ring is maximum in (b) aniline. Therefore, aniline is the most reactive compound towards electrophilic aromatic substitution.