Haloalkanes and Haloarenes
'Halogen atoms are the HANDLES of organic chemistry — they make molecules REACTIVE and enable transformations that would otherwise be impossible.'
1. Chapter Overview
Haloalkanes (alkyl halides) and haloarenes (aryl halides) are organic compounds containing one or more HALOGEN ATOMS. Topics include: CLASSIFICATION (primary, secondary, tertiary; mono, di, tri, polyhalogen), NOMENCLATURE (common and IUPAC), METHODS OF PREPARATION (from alcohols, alkenes, hydrocarbons), PHYSICAL PROPERTIES (boiling points, density, solubility), CHEMICAL REACTIONS (NUCLEOPHILIC SUBSTITUTION — SN1 and SN2 mechanisms, ELIMINATION reactions, reactions with metals including GRIGNARD REAGENTS), and POLYHALOGEN COMPOUNDS (DDT, chloroform, carbon tetrachloride, freons).
2. Classification and Nomenclature
Based on Number of Halogen Atoms
- Monohalo: R−X. Dihalo: R−X₂. Trihalo: R−X₃. Polyhalo: Multiple X atoms.
Based on Type of Carbon (for Haloalkanes)
- Primary (1°): Halogen on carbon attached to 1 alkyl group. Example: CH₃CH₂Br.
- Secondary (2°): Halogen on carbon attached to 2 alkyl groups. Example: CH₃CHBrCH₃.
- Tertiary (3°): Halogen on carbon attached to 3 alkyl groups. Example: (CH₃)₃CBr.
IUPAC Naming
- Halogens are PREFIXES: fluoro, chloro, bromo, iodo.
- Number the chain to give the LOWEST number to the halogen.
- Example: CH₃CHBrCH₂Cl → 1-Bromo-2-chloropropane.
3. Methods of Preparation
From Alcohols
- R−OH + HX → R−X + H₂O. Reagents: HCl (ZnCl₂ catalyst), HBr, HI.
- Reactivity: HI > HBr > HCl. 3° alcohol > 2° > 1°.
From Alkenes (Electrophilic Addition)
- R−CH=CH₂ + HX → R−CHX−CH₃ (Markovnikov addition — H goes to MORE substituted carbon).
- R−CH=CH₂ + HBr (peroxide) → R−CH₂−CH₂Br (ANTI-Markovnikov — peroxide effect).
From Hydrocarbons (Free Radical Halogenation)
- R−H + X₂ → R−X + HX (UV light / heat required). 'Reactivity: F₂ > Cl₂ > Br₂ > I₂. Selectivity: Br is more SELECTIVE than Cl.'
4. Physical Properties
| Property | Trend | Reason |
|---|---|---|
| Boiling point | R−I > R−Br > R−Cl > R−F | Larger size → stronger van der Waals forces |
| Boiling point (isomers) | 1° > 2° > 3° (same molecular formula) | More branching → LESS surface area → LOWER bp |
| Density | R−F, R−Cl: < water. R−Br, R−I: > water | Atomic mass of halogen determines density |
| Solubility | INSOLUBLE in water | Cannot form H-bonds. Soluble in organic solvents |
5. Chemical Reactions
5.1 Nucleophilic Substitution (SN1 and SN2)
| Feature | SN1 (Unimolecular) | SN2 (Bimolecular) |
|---|---|---|
| Rate law | Rate = k[R−X] | Rate = k[R−X][Nu⁻] |
| Steps | TWO (carbocation intermediate) | ONE (concerted — transition state) |
| Kinetics | FIRST order | SECOND order |
| Stereochemistry | RACEMISATION (50:50 mixture) | INVERSION (Walden inversion) |
| Carbocation stability | 3° > 2° > 1° | Not applicable |
| Best for | Tertiary haloalkanes | Primary haloalkanes |
| Solvent | POLAR PROTONIC (H₂O, ROH) — stabilise carbocation | POLAR APROTIC (acetone, DMF) — increase nucleophile reactivity |
| Leaving group | Good leaving group required | Same |
| Nucleophile | WEAK nucleophile OK | STRONG nucleophile required |
5.2 Elimination Reactions (Dehydrohalogenation)
- R−CH₂−CH₂X + alc. KOH → R−CH=CH₂ + KX + H₂O.
- Saytzeff rule: 'The MORE SUBSTITUTED alkene (with more alkyl groups on the C=C) is the MAJOR product.'
- Example: CH₃CH₂CHBrCH₃ + alc. KOH → CH₃CH=CHCH₃ (major) + CH₃CH₂CH=CH₂ (minor).
5.3 Reactions with Metals
- Grignard reagent: R−X + Mg (dry ether) → RMgX. 'Grignard reagents are CARBANION equivalents — extremely useful for C−C bond formation.'
- Wurtz reaction: 2R−X + 2Na → R−R + 2NaX. Used for symmetrical alkanes.
- Fittig reaction: 2Ar−X + 2Na → Ar−Ar + 2NaX (symmetrical arenes).
- Wurtz-Fittig reaction: R−X + Ar−X + 2Na → R−Ar + 2NaX (mixed product).
6. Polyhalogen Compounds
| Compound | Formula | Common Name | Uses / Concerns |
|---|---|---|---|
| Chloroform | CHCl₃ | Chloroform | Anaesthetic (historical), solvent. Forms phosgene (COCl₂) on exposure to light. |
| Carbon tetrachloride | CCl₄ | Carbon tet | Solvent, fire extinguisher. HAZARDOUS — liver damage. |
| DDT | C₁₄H₉Cl₅ | Dichlorodiphenyltrichloroethane | Pesticide. BANNED in most countries — persistent environmental pollutant. |
| Freons (CFCs) | CCl₂F₂, etc. | Freon | Refrigerants, propellants. OZONE DEPLETION — Montréal Protocol banned them. |
7. Comparison Table: Haloalkanes vs Haloarenes
| Property | Haloalkanes (R−X) | Haloarenes (Ar−X) |
|---|---|---|
| Reactivity towards SN | HIGH (C−X bond is POLAR, easily broken) | LOW (C−X bond has partial DOUBLE BOND character due to resonance) |
| Bond length C−X | Longer | SHORTER |
| Formation of Grignard | EASY (in dry ether) | DIFFICULT (requires special conditions) |
| Boiling point | Lower | HIGHER |
| Dipole moment | Moderate | LOWER (due to resonance) |
8. Common Mistakes
- SN1 vs SN2: SN1 occurs with TERTIARY haloalkanes via a carbocation. SN2 occurs with PRIMARY haloalkanes in ONE step. SECONDARY can go either way depending on conditions.
- Saytzeff rule: The MORE substituted alkene is the major product — NOT necessarily the most stable one (that is Zaitsev's rule too, but they are the same).
- Grignard reagents must be prepared in DRY ETHER: ANY trace of water will DESTROY the Grignard reagent (RMgX + H₂O → R−H + Mg(OH)X).
- CHCl₃ should be stored in DARK bottles: Chloroform forms PHOSGENE (COCl₂) — a deadly gas — when exposed to light and air.
9. CBSE Exam Focus
- Nomenclature — IUPAC names of haloalkanes and haloarenes
- Preparation — from alcohols, alkenes (Markovnikov and anti-Markovnikov)
- SN1 and SN2 reactions — mechanisms, stereochemistry, factors affecting reactivity
- Elimination — dehydrohalogenation, Saytzeff rule
- Grignard reagents — preparation and reactions (qualitative)
- Polyhalogen compounds — chloroform, DDT, freons (environmental concerns)
10. Self-Test
Q1: Identify the product: CH₃CH=CH₂ + HBr (in presence of peroxide). A1: CH₃CH₂CH₂Br (Anti-Markovnikov addition — peroxide effect). Without peroxide: CH₃CHBrCH₃.
Q2: Which compound reacts faster with NaOH — CH₃CH₂Br or (CH₃)₃CBr? Explain. A2: CH₃CH₂Br reacts faster via SN2 (no steric hindrance). (CH₃)₃CBr reacts via SN1 (forms stable 3° carbocation). In NaOH (strong Nu), SN2 is generally faster for 1° substrates.
Q3: Write the product when CHCl₃ is exposed to light and air. A3: 2CHCl₃ + O₂ → 2COCl₂ + 2HCl (phosgene — toxic gas). 'Chloroform stored in clear bottles can KILL you.'
Q4: What is the major product of dehydrohalogenation of 2-bromobutane with alc. KOH? A4: But-2-ene (CH₃CH=CHCH₃) — more substituted alkene (Saytzeff product).
Q5: Give the IUPAC name of CH₃CH₂CHBrCH₃. A5: 2-Bromobutane.
11. Conclusion
Haloalkanes and haloarenes are the BUILDING BLOCKS of organic synthesis:
- REACTIVITY: 'The C−X bond is POLAR — making the carbon ELECTROPHILIC and susceptible to attack by NUCLEOPHILES.'
- SUBSTITUTION: 'SN1 (via carbocation) vs SN2 (via transition state) — two fundamentally different ways to replace a halogen.'
- ELIMINATION: 'Remove HX to make alkenes — Saytzeff's rule predicts the major product.'
- GRIGNARD: 'RMgX — the most versatile organometallic reagent in organic chemistry.'
- 'Haloalkanes are the HANDLES that enable organic chemists to BUILD almost any molecule they can imagine.'
