Question 1

Why is the benzene sigma complex non-aromatic?

Accepted Answer

During electrophilic attack, one ring carbon becomes bonded to the incoming electrophile and temporarily loses its original planar pi contribution. The attacked carbon is approximately sp3 hybridised, so the continuous delocalised six-electron pi system is interrupted until H+ is removed.

Question 2

What is the correct order of steps in electrophilic substitution of benzene?

Accepted Answer

The electrophile attacks the ring first, producing a sigma complex or arenium ion. A base then removes H+ from the attacked carbon, restoring aromaticity and producing the substituted benzene.

Question 3

Why does a nitro group direct a new electrophile to the meta position?

Accepted Answer

The –NO2 group strongly withdraws electron density by resonance and induction. It deactivates the ring and makes attack at the ortho and para positions especially unfavorable because those sigma complexes place positive charge near the electron-withdrawing group, so meta attack is preferred.

Question 4

Why can chlorine be ortho/para directing even though it is deactivating?

Accepted Answer

Chlorine can donate electron density into the ring by resonance, which stabilizes the ortho and para sigma complexes and gives the o/p direction. However, its strong inductive electron withdrawal reduces the overall reactivity, so chlorobenzene is less reactive than benzene.

Question 5

How do Friedel-Crafts alkylation and acylation differ?

Accepted Answer

Alkylation uses an alkyl chloride with anhydrous AlCl3 and can involve a carbocation-like intermediate, so rearrangement may occur. Acylation uses an acyl chloride with anhydrous AlCl3, forms a resonance-stabilised acylium species, and gives an aryl ketone without carbocation rearrangement.

Benzene Electrophilic Substitution 3D Lab

About this lab

Practice