Question

Which of the following statements are correct?
I) In nitrating mixture nitric acid participates as an acid
II) $\sigma$ complex is the intermediate substance in electrophilic substitution of benzene
III) Benzene on Friedel-Crafts alkylation with $n$-propyl chloride gives isopropyl benzene

• A. II, III only
• B. I, II only
• C. I, III only
• D. I, II, III

Answer 1

Answer: (A)

(I) In nitrating mixture, nitric acid $\left( HNO _{3}\right)$ partcipates as an acid is not a correct statement, as $HNO _{3}$ in nitrating mixture (with $H _{2} SO _{4}$ ) behaves as a base.

(II) Sigma complex is the intermediate substance in electrophilic substitution of benzene is a correct statement as it forms arenium ion as intermediate when electrophile $\left(E^{+}\right)$ attacks on benzene ring.

$E^{+}$ attacks on the benzene ring to give sigma complex or arenium ion, in which one of the carbon is $s p^{3}$ -hybridised.



(III) Benzene on Friedel-Craft's alkylation with $n$ -propyl chloride gives isopropyl benzene is the correct statement, as $2^{\circ}$ carbocation is more stable than that of $1^{\circ}$ carbocation.

The reaction occurs as follows:



$\left[\because CH _{3} CH _{2} CH _{2} Cl + AlCl _{3} \underset{ AlCl _{4}^{-}}{\longrightarrow } \underset{1^{\circ}(\text { less stable })}{ CH _{3} CH _{2} CH _{2}^{+}}\right.$

$\left. \stackrel{\text { Rearrangement }}{\longrightarrow } \underset{2^{\circ}(\text { More stable })}{ CH _{3} CH CH _{3}}\right]$

Hence, statements (II) and (III) are correct