MCQEasyJEE 2023Electrophilic Substitution in Benzene

JEE Chemistry 2023 Question with Solution

Arrange the following compounds in increasing order of rate of aromatic electrophilic substitution reaction:

  • A

    (a) < (b) < (c)

  • B

    (b) < (c) < (a)

  • C

    (c) < (a) < (b)

  • D

    (c) < (b) < (a)

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: The compounds differ by substituents that either donate or withdraw electron density from the aromatic ring during electrophilic aromatic substitution.

Find: The increasing order of rate of aromatic electrophilic substitution reaction.

Groups like OH-\text{OH} and NH2-\text{NH}_2 activate the aromatic ring by donating electron density, whereas NO2-\text{NO}_2 and COOH-\text{COOH} deactivate it by withdrawing electron density.

Four aromatic fused-ring compounds labeled a, b, c and d, showing substituents such as OH and carbonyl with plus and minus resonance effects marked near ring positions.

Benzene becomes more reactive towards electrophilic aromatic substitution when an electron-donating group is attached to the benzene ring. Benzene with electron-withdrawing groups decreases the reactivity, as it deactivates the ring towards electrophilic attack.

Thus, the order is:

(a) < (b) < (c)\text{(a) < (b) < (c)}

Therefore, the correct option is A.

Common mistakes

  • Assuming all oxygen-containing groups activate the ring. This is wrong because groups such as carbonyl-containing substituents withdraw electron density, whereas OH-\text{OH} donates by resonance. Check the actual electronic effect before ranking reactivity.

  • Ignoring resonance donation and considering only inductive effect. This gives an incorrect order because electrophilic aromatic substitution rate is strongly influenced by resonance interaction with the ring. Use activating versus deactivating behavior to compare the compounds.

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