Question

Arrange the following compounds in the increasing order of their reactivity when each of them is reacted with chloroethane / anhydrous AlCl₃.

• A. [Benzene with Br]
• B. [Benzene with CH₃]
• C. [Benzene with NO₂]
• D. [Benzene with NO₂]

Hint:

In Friedel-Crafts reactions, electron-donating groups (such as CH₃) enhance the reactivity of the aromatic ring, while electron-withdrawing groups (such as NO₂) deactivate it.

Answer 1

The Correct Option is C

When compounds are reacted with chloroethane and anhydrous AlCl₃, they undergo Friedel-Crafts alkylation or acylation reactions. The reactivity in these reactions is primarily influenced by the electronic nature of the substituents on the benzene ring. The reactivity of the compounds follows this pattern: 
- Electron-donating groups (such as CH₃) increase the electron density on the benzene ring, making it more reactive in electrophilic aromatic substitution reactions (such as Friedel-Crafts reactions). Hence, Benzene with CH₃ (compound [B]) will be highly reactive. 
- Electron-withdrawing groups (such as NO₂) decrease the electron density on the benzene ring, making it less reactive. Hence, Benzene with NO₂ (compound [D]) will be less reactive. - Br is a halogen, and it has a slightly electron-withdrawing effect through induction but also has a lone pair that can participate in resonance donation, making it slightly less deactivating than NO₂. 

Thus, Benzene with Br (compound [A]) will have medium reactivity. - NO₂ is a strongly electron-withdrawing group, significantly reducing the electron density on the benzene ring, making the compound C (Benzene with NO₂) much less reactive compared to other compounds. Thus, the correct order of increasing reactivity is: C<B<A<D