ChemistryMediumClass 12

Electrophilic Aromatic Substitution — Mechanism, o/p, m-directors

GOC (General Organic Chemistry)

JEE Qs

Hard

min

Always analyze the stability of the possible sigma complex intermediates (o, p, m) using resonance and inductive effects to correctly predict both the directing effect and relative reactivity.

🧮 Key Formulas

Ar-H + E+ ⇌ [Sigma Complex / Arenium Ion] → Ar-E + H+

Sigma complex: A resonance-stabilized carbocation intermediate with a positive charge delocalized over the benzene ring, often shown with a delocalized pi system and a sp3 hybridized carbon where the electrophile attacked.

✅ Key Points for JEE

1EAS proceeds via a two-step mechanism: 1) electrophilic attack forming a resonance-stabilized sigma complex (arenium ion), which is the rate-determining step. 2) Deprotonation to restore aromaticity.
2Activating groups (+M, +I) donate electron density to the ring, stabilizing the sigma complex and increasing reaction rate. They are generally ortho/para directors because they can directly stabilize the positive charge at ortho and para positions through resonance.
3Deactivating groups (-M, -I) withdraw electron density from the ring, destabilizing the sigma complex and decreasing reaction rate. They are generally meta directors because they make ortho/para positions even more electron deficient (and thus unstable for positive charge development) than the meta position.
4Halogens are a crucial exception: they are deactivating (due to strong -I effect) but ortho/para directing (due to +M effect, which is weaker than -I for reactivity but stronger for regioselectivity).
5The directing ability and activating/deactivating nature of a substituent are determined by its overall electronic effects (inductive and mesomeric), with mesomeric effects usually dominating for regioselectivity.

⚠️ Common Mistakes

✕Confusing activating/deactivating effects with o/p, m-directing effects, especially for halogens.
✕Failing to draw correct resonance structures for the sigma complex to determine the most stable intermediate and thus the directing effect.
✕Incorrectly identifying the strongest activating or deactivating group in a polysubstituted benzene ring, leading to wrong major product prediction.
✕Ignoring steric hindrance, which can sometimes reduce the proportion of ortho products, even for o/p directors.

NCERT Chapters

Class 11 Chemistry Part 2 Ch 12: Organic Chemistry – Some Basic Principles and Techniques
Class 11 Chemistry Part 2 Ch 13: Hydrocarbons
Class 12 Chemistry Part 2 Ch 10: Haloalkanes and Haloarenes
Class 12 Chemistry Part 2 Ch 11: Alcohols, Phenols and Ethers
Class 12 Chemistry Part 2 Ch 12: Aldehydes, Ketones and Carboxylic Acids
Class 12 Chemistry Part 2 Ch 13: Amines