Haloalkanes and Haloarenes

part 1

Classification and Preparation

• Classification of haloalkanes and haloarenes
• Nomenclature
• Nature of C-X bond
• Preparation of haloalkanes from alkanes, alkenes, alcohols
• Preparation of haloarenes

part 2

Physical and Chemical Properties

• Physical properties of haloalkanes
• Nucleophilic substitution reactions — SN1 and SN2
• Stereochemistry of SN1 and SN2 reactions
• Elimination reactions — E1 and E2

part 3

Reactions and Uses

• Reactions of haloarenes — electrophilic substitution
• Nucleophilic substitution in haloarenes
• Polyhalogen compounds — DDT, freons
• Uses and environmental effects of haloalkanes

SN1 mechanism (unimolecular)

Step 1: ionisation → carbocation; Step 2: Nu attack; Rate = k[RX]

★ board favourite

SN2 mechanism (bimolecular)

One step; backside attack; Walden inversion; Rate = k[RX][Nu]

★ board favourite

Grignard reagent formation

R−X + Mg → R−Mg−X (in dry ether)

★ board favourite

Optical isomers: R and S configuration (CIP)

Priorities by atomic number; R = clockwise; S = anticlockwise

★ frequently asked

Resonance of haloarenes (−I but +R effect)

Lone pair on X delocalises into ring → less reactive to Nu

★ frequently asked

Finkelstein reaction

R−Cl + NaI → R−I + NaCl (in dry acetone)

★ frequently asked

Swarts reaction

R−Cl + AgF → R−F + AgCl

★ frequently asked

Wurtz reaction

2R−X + 2Na → R−R + 2NaX

★ frequently asked

Explain why: (i) haloarenes are less reactive than haloalkanes towards nucleophilic substitution, (ii) the C–X bond in haloarenes has partial double-bond character, (iii) chlorobenzene is ortho- and para-directing in electrophilic substitution.

State Saytzeff's rule. Write the major product formed when 2-bromobutane is treated with alcoholic KOH.

Distinguish between SN1 and SN2 mechanisms with examples. Why does tert-butyl bromide prefer SN1 while methyl bromide prefers SN2?

(a) Explain the mechanism of the SN2 reaction with the help of an energy profile diagram, showing the stereochemistry of the product. (b) Arrange the following in increasing order of reactivity towards SN2: 1-bromobutane, 2-bromobutane, 2-bromo-2-methylpropane.

What is optical isomerism? Give an example of a molecule that shows optical isomerism and identify its chiral centre.

How will you convert: (i) chlorobenzene to biphenyl, (ii) propene to 1-iodopropane, (iii) ethyl chloride to ethyl alcohol?

What is the Finkelstein reaction? Write the reaction and mention the reagent and solvent used.

Explain the terms: (i) enantiomers, (ii) racemic mixture, (iii) retention of configuration, with respect to optically active haloalkanes.

Which one of the following undergoes SN1 reaction faster and why: 2-bromopentane or 2-bromo-2-methylbutane?

Write the reactions and name them: (i) Wurtz reaction, (ii) Sandmeyer reaction.

How many marks is Haloalkanes and Haloarenes worth in the Class 12 board exam?

Around 6 marks of the 70-mark CBSE Chemistry theory paper, based on the official unit-wise weightage. SN1 vs SN2 mechanism and optical activity questions.

Is Haloalkanes and Haloarenes easy or hard?

It's rated hard — one of the tougher chapters in Class 12 Chemistry. Most students need about 4 hours to cover it well.

What are the important topics in Haloalkanes and Haloarenes?

The chapter covers 12 NCERT topics in 3 areas: Classification and Preparation; Physical and Chemical Properties; Reactions and Uses.

What questions come from Haloalkanes and Haloarenes in board exams?

Between 2018–2024, CBSE board papers asked questions from this chapter on Reactivity of Haloarenes, Elimination Reactions, SN1 and SN2, SN1 and SN2 Mechanism — 1- to 5-mark questions.