Haloalkanes and Haloarenes – Complete CBSE Class 12 Chemistry Notes
Unit 6 | 6 marks | CBSE 2026–27
1. What are Haloalkanes and Haloarenes?
Haloalkanes contain a halogen attached to an sp³-hybridised carbon of an alkyl group. Haloarenes contain a halogen attached directly to an aromatic ring.
2. Nature of the C–X Bond
The carbon–halogen bond is polar because halogens are more electronegative than carbon. Bond strength and leaving-group behaviour influence substitution reactions.
3. Nucleophilic Substitution
A nucleophile attacks an electron-deficient carbon. In an SN2 process, substitution occurs in one concerted step and favours less hindered substrates. In an SN1 process, ionisation gives a carbocation intermediate and reaction rate depends primarily on substrate concentration.
SN2: one-step backside attack → inversion at a chiral centre
SN1: carbocation formation → nucleophile attack
4. Elimination
Under suitable basic conditions, a haloalkane can lose HX to form an alkene. Substitution and elimination compete depending on substrate, reagent, solvent and conditions.
5. Haloarenes
The C–X bond in chlorobenzene has partial double-bond character due to resonance, making it less reactive toward ordinary nucleophilic substitution than a typical haloalkane.
6. Environmental Applications and Concerns
Important compounds discussed in the syllabus include dichloromethane, chloroform, carbon tetrachloride, iodoform, freons and DDT. Their uses and environmental effects must be distinguished carefully rather than memorised as one generic category.
🧠 Reaction Map
Haloalkane → elimination → alkene
Haloarene → resonance-stabilised C–X bond → lower ordinary SN reactivity
📝 Competency Questions
- Why does an SN2 reaction favour less sterically hindered substrates?
- Why is chlorobenzene less reactive than chloroethane toward nucleophilic substitution?
- Predict whether substitution or elimination is favoured when the reaction conditions are changed.
- Explain why environmental impact should be considered when discussing older halogenated compounds.
🎯 Board Practice
- Distinguish SN1 and SN2 mechanisms.
- Write suitable conversions involving haloalkanes.
- Explain the special behaviour of haloarenes.
- Identify the product of substitution/elimination reactions from given reagents.
MCQs
- SN2 is a concerted mechanism. True.
- SN1 involves a carbocation intermediate. True.
- Chlorobenzene generally undergoes ordinary SN reactions more readily than chloroethane. False.
- Elimination can produce an alkene. True.
Reaction Pathway & Mechanism Guide
Haloalkanes contain a carbon–halogen bond whose reactivity depends on the substrate, leaving group, solvent and reaction conditions. In nucleophilic substitution, the halide is replaced by a nucleophile.
Substitution: replace the leaving group
SN1 vs SN2
| Feature | SN1 | SN2 |
|---|---|---|
| Steps | Two-step via carbocation | One concerted step |
| Favoured substrate | Often tertiary | Often primary |
| Stereochemical feature | Can lead to racemisation | Backside attack/inversion |
Elimination
Alcoholic KOH and heat can favour elimination, producing an alkene. Compare substitution and elimination conditions before choosing the product.
Important Reactions
- Hydrolysis of haloalkanes to alcohols.
- Conversion to amines using suitable nucleophiles.
- Elimination to alkenes.
- Wurtz reaction for coupling suitable haloalkanes.
- Important reactions of haloarenes and the influence of the aromatic ring.
Common Mistakes
- Confusing alcoholic KOH with aqueous KOH.
- Ignoring substrate structure while predicting substitution.
- Writing an impossible carbon skeleton.
Practice
- Predict products for substitution reactions.
- Distinguish SN1 and SN2 conditions.
- Predict elimination products.
- Complete a multi-step conversion involving a haloalkane.
Worked Application: Nucleophilic Substitution
In a simple substitution reaction, an alkyl halide can react with an incoming nucleophile to replace the halogen-containing group. The outcome depends on the substrate, nucleophile, solvent and reaction conditions. Always identify the carbon attached to the leaving group before selecting the expected pathway.
Reaction-Reasoning Practice
- Predict the major product for a specified haloalkane and nucleophile.
- Explain why changing the substrate or conditions can change the reaction pathway.
Haloalkanes & Haloarenes — Reaction Map
Substitution competes with elimination depending on substrate and conditions.
Nucleophilic Substitution
In an SN2 reaction, substitution occurs in a single concerted step and is favoured by less hindered substrates. SN1 proceeds through carbocation formation and is favoured under conditions that stabilise the intermediate.
Elimination
Alcoholic KOH and suitable conditions can promote β-elimination, producing an alkene. Identify the carbon bearing the leaving group and the adjacent β-hydrogen before predicting the product.
Aryl Halides
C–X bonding in haloarenes differs from simple haloalkanes because of resonance and partial double-bond character. This helps explain their lower tendency toward ordinary nucleophilic substitution.
Worked Reaction Reasoning
For a primary haloalkane reacting with a strong nucleophile under SN2-favouring conditions, predict substitution at the carbon attached to halogen and show the leaving group departure in the same step.
CBSE Practice
- Predict products of substitution and elimination reactions.
- Distinguish SN1 and SN2 using substrate structure.
- Explain why haloarenes behave differently from haloalkanes.
- Complete organic conversion sequences.
Worked Organic Conversion
Consider a primary haloalkane under SN2-favouring conditions. The nucleophile attacks the carbon bearing the halogen from the opposite side while the C–X bond breaks. In an exam conversion, identify the substrate, leaving group and nucleophile before writing the product.