Haloalkanes and Haloarenes are organic compounds in which one or more hydrogen atoms of hydrocarbons are replaced by halogens (F, Cl, Br, I).
These compounds are widely used in:
- Solvents
- Medicines
- Refrigerants
- Pesticides
👉 Core Idea: The presence of a C–X bond (carbon–halogen bond) determines the reactivity and properties of these compounds.
1. Classification
(A) Haloalkanes (Alkyl Halides)
Definition
Compounds in which halogen is attached to an alkyl group (sp³ carbon).
Structure
R — X
Where:
- R = alkyl group
- X = halogen
Example
CH₃Cl (Chloromethane)
Types of Haloalkanes
(1) Primary (1°)
R — CH₂ — X
(2) Secondary (2°)
R — CH — X
|
R
(3) Tertiary (3°)
X
|
R — C — R
|
R
(B) Haloarenes (Aryl Halides)
Definition
Compounds where halogen is directly attached to an aromatic ring.
Structure
C6H5 — X
Example
Chlorobenzene (C₆H₅Cl)
Concept Clarity
👉 WHY haloarenes behave differently?
Because halogen is attached to sp² carbon and involved in resonance.
2. Nomenclature
Haloalkanes
Name = alkane + halo prefix
Example:
CH₃Cl → Chloromethane
Haloarenes
Example:
C₆H₅Br → Bromobenzene
3. Nature of C–X Bond
- Polar bond
- Carbon → δ⁺
- Halogen → δ⁻
Bond Strength
C–F > C–Cl > C–Br > C–I
Concept Clarity
👉 WHY C–I bond is weakest?
Because iodine is large → weaker overlap.
4. Physical Properties
- Boiling point increases with molecular mass
- Insoluble in water
- Soluble in organic solvents
5. Preparation of Haloalkanes
(A) From Alcohols
R–OH + HX → R–X
(B) From Alkenes
Addition of HX
(C) Free Radical Halogenation
Alkane + Cl₂ → Haloalkane
6. Preparation of Haloarenes
(A) From Benzene
Electrophilic substitution
(B) From Diazonium Salts
Sandmeyer reaction
7. Chemical Reactions of Haloalkanes
(A) Nucleophilic Substitution (Very Important)
General Reaction
R–X + Nu⁻ → R–Nu + X⁻
Types
SN1 Reaction
- Two-step mechanism
- Carbocation intermediate
- Favoured by 3° halides
SN2 Reaction
- One-step mechanism
- Backside attack
- Favoured by 1° halides
Diagram (SN2 Concept)
Nu⁻ → R — X → R — Nu + X⁻
Concept Clarity
👉 WHY SN1 favoured by tertiary?
Because carbocation is more stable.
👉 WHY SN2 favoured by primary?
Because less steric hindrance.
(B) Elimination Reaction
β-Elimination
R–X → Alkene
Zaitsev’s Rule
More substituted alkene is major product
8. Reactions of Haloarenes
Key Difference
Haloarenes do not undergo SN1/SN2 easily.
Reason
- Resonance stabilizes C–X bond
- Partial double bond character
(A) Nucleophilic Substitution
Occurs only under harsh conditions
(B) Electrophilic Substitution
Occurs easily
Example:
- Nitration
- Sulfonation
Concept Clarity
👉 WHY haloarenes resist substitution?
Because C–X bond is stronger due to resonance.
9. Important Named Reactions
(A) Finkelstein Reaction
R–Cl + NaI → R–I
(B) Swarts Reaction
R–Cl → R–F
(C) Wurtz Reaction
2R–X + Na → R–R
(D) Sandmeyer Reaction
Ar–N₂⁺ → Ar–X
10. Polyhalogen Compounds
Examples
- CHCl₃ (Chloroform)
- CCl₄ (Carbon tetrachloride)
- DDT
Uses
- Solvents
- Insecticides
Environmental Concern
Some are toxic and harmful
11. Important Differences
Haloalkanes vs Haloarenes
| Haloalkanes | Haloarenes |
|---|---|
| sp³ carbon | sp² carbon |
| Undergo SN1/SN2 easily | Do not react easily |
| No resonance | Resonance present |
12. Important Concepts
👉 WHY haloalkanes undergo substitution?
Because C–X bond is polar and reactive.
👉 WHY haloarenes are less reactive?
Because of resonance stabilization.
👉 WHY tertiary halides more reactive in SN1?
Because of stable carbocation.
13. Common Mistakes
- Confusing SN1 and SN2
- Ignoring steric hindrance
- Mixing haloalkanes and haloarenes
Conclusion
Haloalkanes and Haloarenes are fundamental organic compounds important for understanding substitution and elimination reactions.
👉 Focus on:
- Reaction mechanisms (SN1/SN2)
- Differences between haloalkanes and haloarenes
- Named reactions