Question

Decreasing order of reactivity towards electrophilic substitution for the following compounds is:

• A. C > B > A > D > E
• B. E > D > A > B > C
• C. A > D > E > B > C
• D. D > A > E > C > B

Hint:

Remember: "Donors activate, Withdrawers deactivate." The more easily a group can push electrons into the ring, the more "tasty" that ring becomes for an electron-seeking electrophile!

Answer 1

The Correct Option is B

To determine the reactivity of the given aromatic compounds towards electrophilic substitution, we should consider the nature of the substituents attached to the benzene ring. The substituents can influence the reactivity by either donating electrons (+R or +I effect) or withdrawing electrons (-R or -I effect).

Here's a breakdown of the substituents:

• (A) CH₃ group: A methyl group is an electron-donating group through the inductive (+I) effect, making the ring more reactive towards electrophilic substitution.
• (B) H: A hydrogen atom on the benzene ring does not have any electron-donating or withdrawing effect.
• (C) CF₃ group: A trifluoromethyl group is strongly electron-withdrawing due to the high electronegativity of fluorine atoms (-I effect), decreasing reactivity.
• (D) OCl₃ group: This group can have a slightly withdrawing effect but less impactful than CF₃.
• (E) N(CH₃)₂ group: This dimethylamino group is a strong electron-donating group due to its lone pair, which increases the electron density on the ring (+R effect), significantly enhancing reactivity.

Now, let's arrange these in order of decreasing reactivity towards electrophilic substitution:

1. (E) N(CH₃)₂: Strongest electron donor.
2. (D) OCl₃: Less withdrawing compared to CF₃.
3. (A) CH₃: Mild electron donor.
4. (B) H: Neutral reference point.
5. (C) CF₃: Strongest electron withdrawer.

The correct order of reactivity towards electrophilic substitution is: E > D > A > B > C.