1. Classification, names and the C–X bond
Monohaloalkanes CₙH₂ₙ₊₁X: 1°, 2°, 3° by the carbon bearing X; allylic and benzylic halides; gem (both X on one C) and vic (adjacent C) dihalides; tri- and polyhalides. IUPAC: halogen prefix + locant (2-bromo-2-methylpropane); CH₂Cl₂ dichloromethane, CHCl₃ trichloromethane (chloroform), CHI₃ triiodomethane (iodoform).
C–X is polar (C δ+), attacked by nucleophiles. Bond length increases and bond enthalpy decreases F → I (C–Cl 351, C–Br 293, C–I 234 kJ mol⁻¹), so reactivity RI > RBr > RCl.
Key terms
- Geminal dihalide
Both halogen atoms on the same carbon, e.g. CH₃CHCl₂.
- Vicinal dihalide
Halogen atoms on adjacent carbons, e.g. CH₂BrCH₂Br.
2. Preparation
- Alkanes: X₂, light (free radical; mixtures).
- Alkenes: HX (Markovnikov); HBr + peroxide (anti-Markovnikov); X₂ → vic-dihalide.
- Alcohols: HCl/ZnCl₂ (Lucas), HBr, HI; PCl₃, PCl₅, PBr₃ (red P + Br₂); SOCl₂ best (SO₂, HCl gases).
- Finkelstein: RCl/RBr + NaI, dry acetone → RI (NaCl/NaBr precipitate).
- Swarts: RCl/RBr + AgF, Hg₂F₂, SbF₃ → RF.
- Hunsdiecker: RCOOAg + Br₂ (CCl₄, heat) → RBr + CO₂ + AgBr (one C fewer).
| Method | Reagent |
|---|---|
| ROH → RCl | SOCl₂ |
| RCl → RI | NaI/acetone |
| RBr → RF | AgF |
| RCOOAg → RBr | Br₂/CCl₄ |
Formulas
- Thionyl chloride method
- Hunsdiecker
Key terms
- Finkelstein reaction
Exchange of Cl or Br for I using NaI in dry acetone.
- Hunsdiecker reaction
Silver salt of a carboxylic acid + Br₂ → bromoalkane with one carbon fewer.
3. Physical properties
CH₃Cl, CH₃Br, C₂H₅Cl are gases; higher members liquids/solids. B.p. higher than alkanes of similar size (polar, heavier); rises with chain length and halogen size (RI > RBr > RCl > RF); falls with branching (smaller surface area). Slightly soluble in water (cannot repay water’s H-bonds), soluble in organic solvents. RBr, RI, CH₂Cl₂, CHCl₃, CCl₄ denser than water.
| Compound | b.p. / K |
|---|---|
| CH₃Cl | 249 |
| CH₂Cl₂ | 313 |
| CHCl₃ | 334 |
| CCl₄ | 350 |
Key terms
- Van der Waals forces
Weak intermolecular attractions that increase with molecular size and contact area.
4. Nucleophilic substitution: SN1 and SN2
SN2: one step, backside attack; rate ; inversion (Walden); CH₃X > 1° > 2° > 3°. SN1: slow ionisation to a planar carbocation, then fast attack; rate ; racemisation; 3° > 2° > 1° > CH₃X; favoured by polar protic solvents; benzylic/allylic fast.
Optical activity: a chiral carbon (four different groups) gives enantiomers that rotate plane-polarised light equally in opposite directions (d/+, l/−); racemic mixture inactive.
Formulas
- SN2 rate law
- SN1 rate law
Key terms
- Walden inversion
Inversion of configuration at carbon in an SN2 reaction.
- Racemic mixture
Equal amounts of two enantiomers; optically inactive.
5. Substitution products
| Reagent | Product |
|---|---|
| aq. KOH / moist Ag₂O | ROH |
| NaI (acetone) | RI |
| NH₃ (excess) | RNH₂ |
| R′NH₂ | RNHR′ |
| KCN | RCN (nitrile) |
| AgCN | RNC (isocyanide) |
| KNO₂ | R–O–N=O (alkyl nitrite) |
| AgNO₂ | R–NO₂ (nitroalkane) |
| R′COOAg | R′COOR (ester) |
| LiAlH₄ | RH |
K salts: CN⁻ attacks via C, NO₂⁻ via O (ionic); Ag salts: via N (covalent).
| Salt | Attacking atom |
|---|---|
| KCN | C |
| AgCN | N |
| KNO₂ | O |
| AgNO₂ | N |
Key terms
- Ambident nucleophile
A nucleophile that can attack through two different atoms, such as CN⁻ or NO₂⁻.
6. Elimination and reactions with metals
β-Elimination: alcoholic KOH, heat → alkene; Saytzeff: more substituted alkene is major (2-bromopentane → pent-2-ene). Aqueous KOH → substitution; alcoholic KOH → elimination (especially 3°).
Wurtz: (symmetrical alkanes). Grignard: ; .
| Reaction | Product |
|---|---|
| RX + alc. KOH | Alkene |
| 2RX + 2Na | R–R |
| RX + Mg | RMgX |
Formulas
- Wurtz reaction
Key terms
- Saytzeff’s rule
In dehydrohalogenation the more substituted alkene is the major product.
- Grignard reagent
An alkylmagnesium halide, RMgX, made from RX and Mg in dry ether.
7. Polyhalogen compounds
Chloroform: from ethanol or acetone + bleaching powder; oxidised in light to phosgene () — stored in dark, full bottles with a little ethanol; with conc. HNO₃ → chloropicrin CCl₃NO₂; with KOH → HCOOK; with Ag → C₂H₂; carbylamine test with 1° amines. Iodoform: ethanol/acetone + I₂ + NaOH → yellow CHI₃; test for CH₃CO– and CH₃CH(OH)–; antiseptic. CH₂Cl₂: solvent, paint remover; affects the nervous system. CCl₄: non-polar (tetrahedral); liver damage; ozone depletion. Freons (CFCs, e.g. CCl₂F₂): refrigerants; UV in stratosphere releases Cl• which destroys O₃.
| Compound | Key point |
|---|---|
| CHCl₃ | Phosgene in light |
| CHI₃ | Yellow; antiseptic |
| CCl₂F₂ | Ozone depletion |
Formulas
- Iodoform from acetone
Key terms
- Phosgene
COCl₂, a poisonous gas formed by oxidation of chloroform in light.
- Iodoform test
Yellow CHI₃ with I₂/NaOH, given by CH₃CO– and CH₃CH(OH)– compounds.
Exam tips
Common mistakes
- KCN → isocyanide (wrong: it gives a nitrile; AgCN gives isocyanide).
- SN1 gives inversion (wrong: racemisation).
- Using the peroxide effect with HCl (only HBr).
- Forgetting that the Hunsdiecker product has one carbon fewer.
How marks are usually awarded
- Mechanisms (2–3 marks): steps, rate law, stereochemistry.
- Products (1 mark each): correct structure and name.
- Polyhalogen compounds: equation plus a use or hazard.