Haloalkanes and Haloarenes – Complete JEE Notes
1. Introduction
Haloalkanes and haloarenes are organic compounds in which one or more hydrogen atoms
of alkanes or aromatic hydrocarbons are replaced by halogen atoms
(F, Cl, Br, I).
2. Classification
(a) Haloalkanes (Alkyl Halides)
General formula: $R-X$ where $R$ is alkyl group and $X$ is halogen.
(b) Haloarenes (Aryl Halides)
Halogen directly attached to aromatic ring.
3. Classification of Haloalkanes
| Type | Example |
|---|---|
| Primary (1°) | $CH_3CH_2Cl$ |
| Secondary (2°) | $CH_3CHClCH_3$ |
| Tertiary (3°) | $(CH_3)_3CCl$ |
4. Nature of C–X Bond
Bond strength: $C-F > C-Cl > C-Br > C-I$
As bond length increases, bond strength decreases.
5. Physical Properties
- Generally insoluble in water
- Soluble in organic solvents
- Boiling point increases with molecular mass
6. Methods of Preparation of Haloalkanes
(a) From Alcohols
$ROH + HX \rightarrow RX + H_2O$
(b) From Alkanes (Free Radical Halogenation)
$RH + Cl_2 \xrightarrow{hv} RCl + HCl$
7. Nucleophilic Substitution Reactions
Replacement of halogen by nucleophile.
SN2 Mechanism
Rate $\propto [RX][Nu^-]$
Occurs in primary haloalkanes, single-step, inversion of configuration.
SN1 Mechanism
Rate $\propto [RX]$
Occurs in tertiary haloalkanes, two-step, racemization.
8. Factors Affecting SN1 and SN2
| Factor | SN1 | SN2 |
|---|---|---|
| Substrate | 3° > 2° > 1° | 1° > 2° > 3° |
| Solvent | Polar protic | Polar aprotic |
9. Elimination Reactions
$RX + KOH_{(alc)} \rightarrow Alkene + KX + H_2O$
Zaitsev’s rule: More substituted alkene is major product.
10. Reaction with Metals
Wurtz Reaction
$2RX + 2Na \rightarrow R-R + 2NaX$
11. Haloarenes – Special Nature
Haloarenes are less reactive towards nucleophilic substitution
due to resonance and partial double bond character.
12. Nucleophilic Substitution in Haloarenes
Occurs only under drastic conditions or when electron withdrawing groups are present.
13. Electrophilic Substitution in Haloarenes
Halogens are deactivating but ortho-para directing.
14. Environmental Effects
- CFCs cause ozone depletion
- DDT is non-biodegradable
15. Common JEE Traps
- SN1 does not cause inversion
- Haloarenes do not follow SN2
- Finkelstein reaction does not work with aryl halides
16. Quick Revision Checklist
You have mastered this chapter if you can:
- Differentiate SN1 and SN2 clearly
- Predict products of substitution and elimination
- Explain why haloarenes are less reactive
- Handle reaction-based MCQs confidently