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 $\left(HN{O}_3\right)$ partcipates as an acid is not a correct statement, as $HN{O}_3$ in nitrating mixture (with $H_{2}S{O}_4$ ) 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:



$[\because C{H}_{3}C{H}_{2}C{H}_{2}Cl+AlC{l}_3\underset{AlC{l}_4^{-}}{⟶}\underset{1^{\circ }(\text{ less stable })}{C{H}_{3}C{H}_{2}C{H}_2^{+}}$

$\stackrel{\text{ Rearrangement }}{⟶}\underset{2^{\circ }(\text{ More stable })}{C{H}_{3}CHC{H}_3}]$

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