Organic Compounds Containing Halogens/Oxygen/Nitrogen

Syllabus Unit: Organic Chemistry

Why it matters

Chapter overview

This unit covers the largest functional-group family in JEE organic chemistry: haloalkanes and haloarenes (nucleophilic substitution via SN1/SN2), alcohols/phenols/ethers (acidity and substitution trends built on the C-O bond), aldehydes/ketones/carboxylic acids (nucleophilic addition and acid-base chemistry built on the carbonyl), and amines/diazonium salts (basicity and the synthetic gateway diazonium chemistry provides). The recurring theme is mechanism: knowing WHY a reaction goes the way it does, not just memorising the product.

Chemistry Syllabus Structure Organic Chemistry ~34% weightage

Live Animation1 / 4

Nucleophilic Substitution

1SN13° substrates, carbocation

2SN21° substrates, backside attack

3HaloarenesResonance-resistant

4Forcing ConditionsNaOH, 623K, 300 atm

1Haloalkanes & Haloarenes (SN1, SN2)

Haloalkanes undergo nucleophilic substitution by two competing mechanisms — SN1 via a carbocation (favoured by tertiary halides) and SN2 by concerted backside attack with inversion (favoured by primary halides) — while haloarenes resist substitution almost entirely due to resonance.

Learning Objectives

Compare the SN1 (two-step, carbocation, racemisation) and SN2 (one-step, concerted, Walden inversion) mechanisms.
Predict which mechanism dominates based on substrate class (1°/2°/3°), nucleophile strength, and solvent polarity.
Explain why haloarenes are far less reactive toward nucleophilic substitution than haloalkanes.
State the forcing conditions under which haloarenes CAN undergo nucleophilic substitution.

Key Terms

SN1 (unimolecular nucleophilic substitution)A two-step mechanism via a carbocation intermediate; rate depends only on substrate concentration; gives racemisation.

SN2 (bimolecular nucleophilic substitution)A one-step concerted mechanism with backside nucleophilic attack; rate depends on both substrate and nucleophile; gives Walden inversion.

Walden inversionThe complete inversion of configuration at a stereocentre during an SN2 reaction, as the nucleophile attacks from the side opposite the leaving group.

Resonance in haloarenesA lone pair on the halogen delocalises into the aromatic ring, giving the C-X bond partial double-bond character that shortens and strengthens it against substitution.

Haloalkanes (R-X) undergo nucleophilic substitution, replacing X with an incoming nucleophile, via one of two competing mechanisms.

SN1 mechanism

Two steps — the C-X bond breaks first (rate-determining) to form a planar carbocation, then the nucleophile attacks from either face. Favoured by 3° substrates (stable carbocation), polar protic solvents, and weak nucleophiles; gives racemisation since the nucleophile can attack from either side of the planar carbocation.

SN2 mechanism

One concerted step — the nucleophile attacks the carbon from the side opposite the leaving group exactly as the C-X bond breaks, giving Walden inversion. Favoured by 1° substrates (least steric hindrance), polar aprotic solvents, and strong nucleophiles.

SN1: R-X → R+ (slow) → R-Nu (fast, racemisation) | SN2: Nu- + R-X → Nu-R + X- (one step, inversion)

Exam TipJEE tests substrate reactivity order directly: SN1 favours 3°>2°>1° (carbocation stability); SN2 favours 1°>2°>3° (steric accessibility) — opposite orders.

Haloarenes: resistant to substitution

In chlorobenzene, a lone pair on Cl delocalises into the aromatic ring by resonance, giving the C-Cl bond partial double-bond character — shorter and stronger than a normal C-Cl single bond, resistant to nucleophilic attack.

Common MistakeHaloarenes undergo nucleophilic substitution only under forcing conditions — strong base + high temperature/pressure (e.g. chlorobenzene + NaOH at 623K, 300 atm gives phenol), or when strong electron-withdrawing groups (like -NO2) sit ortho/para to the halogen.