Haloalkanes and Haloarenes
Haloalkanes and Haloarenes - Organic compounds with halogen atoms. This chapter covers classification, nomenclature, preparation, reactions, SN1/SN2 mechanisms, and polyhalogen compounds.
Board Exam Weightage: 12-15 marks | High importance for JEE/NEET
1. Classification
1.1 Based on Number of Halogen Atoms
| Type | Halogens | Example |
|---|---|---|
| Monohalo | One | CH₃Cl |
| Dihalo | Two | CH₂Cl₂ |
| Trihalo | Three | CHCl₃ |
| Tetrahalo | Four | CCl₄ |
1.2 Based on Carbon Hybridization
A. Haloalkanes (sp³ C-X)
Halogen attached to sp³ hybridized carbon (tetrahedral)
| Class | Structure | Example |
|---|---|---|
| Primary (1°) | -CH₂X | CH₃CH₂Br |
| Secondary (2°) | >CHX | (CH₃)₂CHCl |
| Tertiary (3°) | >CX- | (CH₃)₃CI |
B. Haloarenes (sp² C-X)
Halogen directly attached to aromatic ring
Examples: C₆H₅Cl, C₆H₅Br, C₆H₅I
2. Nomenclature
2.1 IUPAC Rules
- Select longest carbon chain
- Number from end nearest to halogen
- Name halogens as: fluoro-, chloro-, bromo-, iodo-
- Use di-, tri-, tetra- for multiple same halogens
- List alphabetically if different halogens
Example: CH₃CHClCH₂CH₃
IUPAC Name: 2-Chlorobutane
2.2 Common Names
| IUPAC | Common |
|---|---|
| Chloromethane | Methyl chloride |
| Dichloromethane | Methylene chloride |
| Trichloromethane | Chloroform |
| Tetrachloromethane | Carbon tetrachloride |
3. Nature of C-X Bond
C-X bond is polar covalent: Cδ+—Xδ−
| Bond | Length (pm) | Strength (kJ/mol) | Reactivity |
|---|---|---|---|
| C-F | 135 | 484 | Least |
| C-Cl | 177 | 338 | Low |
| C-Br | 193 | 276 | Moderate |
| C-I | 214 | 240 | Most |
Reactivity order: R-I > R-Br > R-Cl > R-F
Reason: C-I weakest bond → easiest to break
📝 Board Tip
Question: Why C-I most reactive despite least polar?
Answer: Bond strength more important than polarity. C-I weakest (240 kJ/mol) → breaks easily → most reactive
4. Methods of Preparation
4.1 From Alcohols
With HX: R-OH + HX → R-X + H₂O
Reactivity: HI > HBr > HCl
With PCl₅: R-OH + PCl₅ → R-Cl + POCl₃ + HCl
With SOCl₂ (Best): R-OH + SOCl₂ → R-Cl + SO₂ + HCl
Advantage: Gaseous by-products
4.2 From Hydrocarbons
Halogenation: R-H + X₂ → R-X + HX (UV light)
Example: CH₄ + Cl₂ → CH₃Cl + HCl
Addition to alkenes: R-CH=CH₂ + HX → R-CHX-CH₃
(Markovnikov's rule)
4.3 Halogen Exchange
Finkelstein: R-X + NaI → R-I + NaX (acetone)
Example: CH₃Br + NaI → CH₃I + NaBr
4.4 Preparation of Haloarenes
Direct halogenation: C₆H₆ + Cl₂ → C₆H₅Cl + HCl (FeCl₃)
Sandmeyer: C₆H₅NH₂ → C₆H₅N₂⁺Cl⁻ → C₆H₅Cl (CuCl)
📊 Exam Trend
Common: "Why SOCl₂ preferred over PCl₅?" (2 marks)
Answer: Gaseous by-products (SO₂, HCl) - easy separation
5. Physical Properties
- State: Lower members gases, higher liquids/solids
- BP: Increases with molecular mass
- Solubility: Insoluble in water, soluble in organic solvents
- Density: Bromo/iodo/polychloro denser than water
| Compound | BP (°C) | Density |
|---|---|---|
| CH₃Cl | -24 | Gas |
| CH₃Br | 3.6 | Gas |
| CH₃I | 42 | 1.7 |
| CHCl₃ | 61 | 1.48 |
| CCl₄ | 77 | 1.59 |
6. Chemical Reactions of Haloalkanes
6.1 Nucleophilic Substitution
With OH⁻: R-X + OH⁻ → R-OH + X⁻
(Alcohol formation)
With RO⁻: R-X + R'O⁻ → R-O-R' + X⁻
(Williamson ether synthesis)
With CN⁻: R-X + CN⁻ → R-CN + X⁻
(Nitrile, chain increases)
With NH₃: R-X + NH₃ → R-NH₂ + HX
(Amine formation)
6.2 Elimination (Dehydrohalogenation)
β-Elimination: R-CH₂-CH₂X + KOH(alc) → R-CH=CH₂ + KX + H₂O
Saytzeff rule: More substituted alkene major
6.3 With Metals
Wurtz: 2R-X + 2Na → R-R + 2NaX (dry ether)
Grignard: R-X + Mg → R-MgX (dry ether)
⚠️ Common Mistake
WRONG: Aqueous KOH gives alkene
CORRECT:
- Aqueous KOH → Substitution → Alcohol
- Alcoholic KOH → Elimination → Alkene
7. Nucleophilic Substitution Mechanisms
7.1 SN2 Mechanism (Bimolecular)
Characteristics:
- One-step concerted mechanism
- Backside attack by nucleophile
- Transition state with partial bonds
- Inversion of configuration (Walden inversion)
- Rate = k[R-X][Nu⁻] (second order)
Nu⁻ + R-X → [Nu---R---X]‡ → Nu-R + X⁻
Favoring factors:
- Primary > Secondary >> Tertiary
- Strong nucleophile
- Polar aprotic solvent
7.2 SN1 Mechanism (Unimolecular)
Characteristics:
- Two-step mechanism
- Carbocation intermediate formation
- Rate = k[R-X] (first order)
- Racemization (loss of optical activity)
Step 1: R-X → R⁺ + X⁻ (slow, rate determining) Step 2: R⁺ + Nu⁻ → R-Nu (fast)
Favoring factors:
- Tertiary > Secondary >> Primary
- Weak nucleophile
- Polar protic solvent
- Stable carbocation
7.3 Optical Rotation
SN2: Inversion of configuration → Optical activity retained (inverted)
SN1: Racemization → 50% inversion + 50% retention → Loss of optical activity
7.4 Comparison Table
| Factor | SN1 | SN2 |
|---|---|---|
| Steps | Two | One |
| Rate | k[R-X] | k[R-X][Nu⁻] |
| Order | 3° > 2° > 1° | 1° > 2° > 3° |
| Intermediate | Carbocation | Transition state |
| Stereochemistry | Racemization | Inversion |
| Solvent | Polar protic | Polar aprotic |
📝 Board Exam Tip: SN1 vs SN2
Common 3-mark question: "Differentiate SN1 and SN2 with examples"
Perfect answer includes:
- Steps (two vs one)
- Rate equation
- Substrate preference
- Stereochemistry
- One example each
8. Reactions of Haloarenes
8.1 Why Haloarenes Less Reactive?
Reasons:
- C-X bond has partial double bond character (resonance)
- sp² carbon more electronegative than sp³
- Aromatic stability (benzene ring stable)
8.2 Nucleophilic Substitution
Difficult under normal conditions. Requires:
- High temperature (300-350°C)
- High pressure
- Strong base (NaOH)
C₆H₅Cl + NaOH → C₆H₅OH + NaCl
(350°C, 300 atm)
Activated by: -NO₂, -CN (electron withdrawing groups)
8.3 Electrophilic Substitution
Halogen = ortho/para directing, ring deactivating
Nitration: C₆H₅Cl + HNO₃ → o-ClC₆H₄NO₂ + p-ClC₆H₄NO₂
(H₂SO₄ catalyst)
9. Polyhalogen Compounds
9.1 Dichloromethane (CH₂Cl₂) - Methylene Chloride
Preparation: CH₄ + Cl₂ → CH₃Cl + Cl₂ → CH₂Cl₂
Properties:
- Colorless liquid, BP 40°C
- Sweet smell
- Immiscible with water
Uses:
- Solvent for fats, oils
- Paint remover
- Propellant in aerosols
9.2 Trichloromethane (CHCl₃) - Chloroform
Preparation: CH₃OH + Cl₂ → CHCl₃ (bleaching powder)
Properties:
- Colorless liquid, BP 61°C
- Sweet smell and taste
- Denser than water (1.48 g/mL)
- Decomposes by light: 2CHCl₃ + O₂ → 2COCl₂ + 2HCl
Storage: In dark bottles with 0.6-1% ethanol (stabilizer)
Uses:
- Formerly anesthetic (now banned - toxic)
- Solvent
- Production of freons
9.3 Triiodomethane (CHI₃) - Iodoform
Iodoform Test:
Compounds with CH₃CO- or CH₃CH(OH)- give yellow precipitate
Reaction: CH₃COCH₃ + I₂ + NaOH → CHI₃↓ + CH₃COONa
Properties:
- Yellow solid
- Characteristic smell
- Antiseptic
Uses: Antiseptic for wounds
9.4 Tetrachloromethane (CCl₄) - Carbon Tetrachloride
Preparation: CS₂ + 3Cl₂ → CCl₄ + S₂Cl₂
Properties:
- Colorless liquid, BP 77°C
- Denser than water (1.59 g/mL)
- Non-flammable
- Toxic (affects liver, kidneys)
Uses:
- Solvent (fats, oils)
- Fire extinguisher (pyrene - now banned)
- Manufacturing CFCs
9.5 Freons (Chlorofluorocarbons - CFCs)
Examples: CCl₂F₂ (Freon-12), CClF₃ (Freon-13)
Properties:
- Chemically inert
- Non-toxic
- Non-corrosive
- Easily liquefiable
Uses: Refrigerants, aerosol propellants
Problem: Deplete ozone layer → Banned (Montreal Protocol)
Reaction: CCl₂F₂ + UV → Cl· → Destroys O₃
9.6 DDT (Dichlorodiphenyltrichloroethane)
Full name: p,p'-Dichlorodiphenyltrichloroethane
Structure: (ClC₆H₄)₂CHCCl₃
Preparation: Chlorobenzene + Chloral (CCl₃CHO) + H₂SO₄
Properties:
- White crystalline solid
- Insoluble in water
- Soluble in organic solvents
- Persistent in environment
Uses: Powerful insecticide
Problem: Toxic to animals, accumulates in food chain → Banned in many countries
📊 Exam Trend: Polyhalogen
High-frequency topics:
- Iodoform test (2-3 marks) - Almost every year
- Why chloroform stored in dark bottles? (1-2 marks)
- Why CFCs banned? (2 marks)
- DDT structure and uses (2-3 marks)
10. Practice MCQs
Q1. Which is most reactive towards SN2?
(a) (CH₃)₃CCl
(b) (CH₃)₂CHCl
(c) CH₃CH₂Cl
(d) CH₃Cl
Primary haloalkanes most reactive in SN2 (less steric hindrance)
Q2. Grignard reagent is prepared in:
(a) Water
(b) Ethanol
(c) Dry ether
(d) Acetone
Grignard reagents react with water/alcohols, must use dry ether
Q3. Iodoform test is given by:
(a) CH₃CHO
(b) HCHO
(c) C₆H₅CHO
(d) CH₃COOH
Compounds with CH₃CO- group give iodoform test
Q4. Chloroform is stored in dark bottles because:
(a) It evaporates
(b) It oxidizes to phosgene
(c) It decomposes
(d) It polymerizes
2CHCl₃ + O₂ → 2COCl₂ (phosgene, toxic) + 2HCl (in light)
Q5. Best method to prepare chlorobenzene:
(a) From phenol
(b) From benzene direct
(c) Sandmeyer reaction
(d) Wurtz reaction
Aniline → Diazonium salt → Chlorobenzene (CuCl)
Q6. SN1 mechanism favored by:
(a) Primary haloalkane
(b) Tertiary haloalkane
(c) Strong nucleophile
(d) Aprotic solvent
Tertiary carbocation most stable → SN1 favored
Q7. Freons are banned because:
(a) Toxic
(b) Deplete ozone layer
(c) Flammable
(d) Expensive
CFCs release Cl radicals that destroy O₃ (Montreal Protocol)
Q8. Alcoholic KOH gives:
(a) Alcohol
(b) Alkene
(c) Ether
(d) Ester
Alcoholic KOH → Elimination → Alkene formation
Q9. C-I bond most reactive because:
(a) Most polar
(b) Longest and weakest
(c) Shortest
(d) Most electronegative
C-I: 214 pm, 240 kJ/mol → Easiest to break
Q10. DDT banned because:
(a) Ineffective
(b) Accumulates in food chain
(c) Water soluble
(d) Degrades quickly
DDT persistent → Bioaccumulation → Toxic to animals
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