Halogenoalkanes
1. What are Halogenoalkanes?
Definition: organic compounds in which one or more hydrogen atoms are replaced by a halogen (F, Cl, Br, I).
Naming: longest chain; number so the carbon carrying the halogen gets the lowest number; chloro / bromo / iodo and alkyl prefixes go in alphabetical order.
| Class | Definition | Board examples |
|---|---|---|
| Primary (1°) | C bonded to the halogen is attached to only 1 alkyl group (has 2 H) | iodoethane · bromoethane · 1-chloropropane · 1-bromo-2-methylpropane · 1-iodo-2-methylpropane |
| Secondary (2°) | C bonded to the halogen is attached to 2 alkyl groups (has 1 H) | 2-chloropropane · 2-bromopropane · 2-chlorobutane · 2-iodo-3-methylbutane |
| Tertiary (3°) | C bonded to the halogen is attached to 3 alkyl groups (has 0 H) | 2-bromo-2-methylpropane · 2-chloro-2-methylbutane · 3-ethyl-3-iodohexane |
2. Physical Properties
Halogenoalkanes have higher b.p. than the alkanes with the same carbon skeleton because:
- more electrons ⇒ stronger temporary (induced) dipoles;
- C–F, C–Cl, C–Br bonds are polar ⇒ also permanent dipole–dipole forces.
| Trend | Explanation |
|---|---|
| Longer carbon chain | more electrons ⇒ stronger temporary dipoles ⇒ b.p. ↑ |
| F → Cl → Br → I (same alkyl) | b.p. ↑ — the number of electrons grows so much it outweighs the fall in permanent dipoles (electronegativity / bond polarity decrease down the group; C, H and I have almost the same electronegativity) |
| Isomers 1° → 2° → 3° | b.p. ↓ — branched isomers can’t pack as closely: 1-bromobutane > 2-bromobutane > 2-bromo-2-methylpropane |
| Solubility | C–F / C–Cl compounds may be slightly soluble (polar); in general insoluble in water |
3. Bond Energy & Reactivity
Bond energies decrease C–F → C–Cl → C–Br → C–I (board graph, kJ mol⁻¹: C–I ≈ 230, C–Br ≈ 290, C–Cl ≈ 350, C–F ≈ 485, C–H ≈ 415):
- smaller halogens overlap more efficiently with carbon (shorter, stronger bond);
- bond polarity decreases from C–F to C–I.
4. Reactions of Halogenoalkanes
① Elimination
Reagent & conditions: ethanolic NaOH (or KOH), heat under reflux. OH⁻ acts as a base, removes an H from the carbon next to C–X, and a C=C forms.
Several different neighbouring H ⇒ several alkenes: 2-bromobutane → but-1-ene 50 % + cis-but-2-ene 25 % + trans-but-2-ene 25 % (so 3 isomeric alkenes —Q47, Q69).
② Nucleophilic substitution — OH
Reagent & conditions: aqueous NaOH (or KOH), heat under reflux ⇒ alcohol (hydrolysis).
Mechanism: nucleophilic substitution
Nucleophile: a species that donates a pair of electrons (OH⁻, CN⁻, NH₃). It attacks the δ+ carbon of the polar C–X bond; X⁻ leaves (leaving group).
SN2 (bimolecular) — primary halogenoalkanes
- Bimolecular: the slow step involves 2 species (halogenoalkane + OH⁻). One step, no intermediate — only a transition state (in brackets, with partial bonds dotted and the overall negative charge shown outside).
- OH⁻ uses its lone pair to attack the δ+ carbon from opposite the leaving group (repulsion: Br is δ–, OH⁻ is negative ⇒ they push apart).
- Primary halogenoalkanes do not undergo SN1: only 1 alkyl group, which can’t stabilise the carbocation. They have lots of room for attack (H atoms are small).
SN1 (unimolecular) — tertiary halogenoalkanes
- Unimolecular: the slowest (rate-determining) step is the first — the halogenoalkane breaks by itself, no OH⁻ involved. The 2nd step (OH⁻ + C⁺) is fast.
- The tertiary carbocation is stabilised by the positive inductive effect of the 3 alkyl groups (the “fat” alkyl groups push electron density onto C⁺). This cannot happen for primary halogenoalkanes.
- Planar carbocation ⇒ OH⁻ can attack from above or below (no leaving group in the way) — it doesn’t matter which you draw.
- Tertiary halogenoalkanes do not undergo SN2: the large alkyl groups block OH⁻ from reaching the carbon — steric hindrance.
| Mechanism | SN2 | SN1 |
| Which halogenoalkane | primary (secondary if told) | tertiary (secondary if told) |
| Steps | 1 step (transition state) | 2 steps (carbocation intermediate) |
| Rate depends on | [RX] and [OH⁻] — bimolecular | [RX] only — unimolecular |
| Why | little steric hindrance; 1 alkyl group can’t stabilise C⁺ | 3 alkyl groups stabilise C⁺ (+I effect); too crowded for backside attack |
| Profile | one hump | two humps (dip = intermediate) |
③ Checking which halogen is present
Reagents: NaOH(aq), heat (hydrolysis releases X⁻) → add dilute HNO₃ (acidify: removes excess OH⁻ so it can’t precipitate Ag₂O) → add AgNO₃(aq). The corresponding silver halide forms.
| Salt | Precipitate | Confirm with NH₃(aq) | Rate / bond energy |
|---|---|---|---|
| AgCl | white ppt | dissolves in dilute NH₃(aq) | forms slowly (takes too long) — C–Cl bond energy highest |
| AgBr | cream ppt | dissolves in concentrated NH₃(aq) only | forms in some time |
| AgI | yellow ppt | does not dissolve in NH₃ | forms immediately — C–I bond energy lowest |
④ Substitution — NH₂ (ethanolic ammonia)
Reagent & conditions: ethanolic NH₃ (excess) in a sealed tube, under pressure, with heat ⇒ primary amine + ammonium salt.
- Ethanolic, not aqueous: water would let OH⁻ substitute instead of NH₃.
- Sealed tube: NH₃ is less dense than air, so it would rise up the condenser and escape — it wouldn’t react.
- Excess NH₃: stops further substitution; the 2nd NH₃ removes the H⁺ (forms NH₄Br).
⑤ Substitution — CN (making nitriles, lengthening the chain)
Reagent & conditions: NaCN or KCN dissolved in ethanol, heat under reflux (ethanol, otherwise OH⁻ would substitute). Product: a nitrile. The cyanide ion is :C≡N:⁻ — the lone pair on C attacks.
| Hydrolysis | Reagent & conditions | Equation | Product |
|---|---|---|---|
| Acid hydrolysis | dil. HCl or dil. H₂SO₄, heat under reflux | CH₃CN + 2H₂O + HCl → CH₃COOH + NH₄Cl ionic: CH₃CN + 2H₂O + H⁺ → CH₃COOH + NH₄⁺ |
gives the carboxylic acid directly |
| Alkaline hydrolysis | dil. NaOH, heat under reflux | CH₃CN + H₂O + NaOH → CH₃COONa + NH₃ ionic: CH₃CN + H₂O + OH⁻ → CH₃COO⁻ + NH₃ |
gives the carboxylate salt; add dil. HCl afterwards to get the acid |
5. Summary Table of Reagents
| Reaction | Reagent & conditions | Product |
|---|---|---|
| Elimination | ethanolic NaOH / KOH, heat under reflux | alkene |
| Hydrolysis (OH substitution) | aqueous NaOH / KOH, heat under reflux | alcohol |
| Amine formation | excess ethanolic NH₃, sealed tube, heat | primary amine |
| Nitrile formation | NaCN / KCN in ethanol, heat under reflux | nitrile (+1 C) |
| Nitrile → acid | dil. H₂SO₄ / HCl, heat under reflux | carboxylic acid |
| Halide test | NaOH(aq), heat; HNO₃; AgNO₃(aq) | AgCl white / AgBr cream / AgI yellow |
6. Past-Paper Questions (board)
| Q | Question | Working / answer |
|---|---|---|
| 1 | halothane: CCl₂=CHCl → (HF) → CF₃CH₂Cl → (Br₂) → CF₃CHBrCl, stage 2? | Br₂ replaces H on an alkane carbon in UV ⇒ free-radical substitution ⇒ C ✓ |
| 5 | best synthesis of 1,2-dibromo-3-chloropropane | CH₂=CHCH₂Cl + Br₂ ⇒ adds Br on the 2 alkene carbons (electrophilic addition) ⇒ C ✓; A/B would give a mixture by free-radical substitution; D wrong |
| 6 | hydrolysis of 2-chloro-2-methylpropane (slow step then fast step): reaction profile? | SN1 = two steps ⇒ two humps with an intermediate dip ⇒ D ✓ |
| 10 | why CCl₂F₂ is chemically inert | C–F bond energy is large ⇒ A ✓ |
| 20 | which compound undergoes SN1? | tertiary only ⇒ ring compound with CH₃/Cl on the same C ⇒ C ✓ (A and B primary; D is a vinyl chloride — C–Cl bond not substituted) |
| 21 | CH₃CH=CHCH₃ → (HI) → CH₃CH₂CHICH₃ → (NaOH aq) → CH₃CH₂CH(OH)CH₃ | 1st: electrophilic addition; 2nd: nucleophilic substitution ⇒ B ✓ |
| 23 | 1-chloro-, 1-bromo-, 1-iodobutane + AgNO₃(aq): RCl < RBr < RI. Why? | bond energy of R–X decreases ⇒ D ✓ |
| 30 | which compound gives a precipitate quickest with AgNO₃? | the one containing I (weakest C–X) ⇒ D ✓; fluorine compounds give no ppt |
| Q | Question | Working / answer |
|---|---|---|
| 31 | phenacyl chloride + NaOH(aq), SN2 — species formed | transition state: HO···C···Cl with δ– on both and overall ⁻ ⇒ C ✓ (carbocation A is SN1; B has 5 full bonds) |
| 34 | NaOH(aq) + 1-bromopropane, first step | curly arrow from a lone pair on OH⁻ to the Cδ⁺ ⇒ A ✓ |
| 36 | Y: hydrolysed by AgNO₃ to a ppt soluble in dilute NH₃ and gives an alkene by elimination | ppt soluble in dilute NH₃ ⇒ AgCl ⇒ chloroalkane ⇒ C ✓ (Br ⇒ cream, I ⇒ yellow) |
| 41 | CN⁻ + 2-bromopropane faster than I⁻ (same mechanism) | cyanide is a stronger nucleophile than iodide ⇒ C ✓ |
| 47 | alcoholic KOH + 2-chlorobutane — number of isomeric products | but-1-ene + cis + trans-but-2-ene ⇒ 3 ⇒ C ✓ |
| 57 | 2-bromopropane + hot conc. NaOH in ethanol, major product | elimination ⇒ propene ⇒ D ✓ |
| 64 | tertiary bromoalkane + NaOH(aq): points on profile | dip Y = carbocation (CH₃)₃C⁺ ⇒ C ✓ (peaks X are transition states) |
| 69 | J + ethanolic KOH ⇒ 3 isomeric C₄H₈ alkenes | 2-bromobutane (1-ene + cis + trans 2-ene) ⇒ B ✓ |
7. Long CIE-style Questions (board)
(ii) nucleophilic substitution; (iii) SN2 mechanism (OH⁻ lone pair → Cδ⁺, Br leaves with the bond pair, transition state in brackets).
(b) (i) elimination; (ii) from 1-bromobutane: CH₃CH₂CH=CH₂ (I); from 2-bromo-2-methylpropane: CH₂=C(CH₃)₂ (II). (iii) hot conc. MnO₄⁻: I ⇒ CH₃CH₂COOH (+ CO₂ + H₂O); II ⇒ CH₃COCH₃ (+ CO₂ + H₂O).
(c) 2,2-dimethylpropanoic acid from (CH₃)₃CBr: step I NaCN in ethanol, heat under reflux ⇒ (CH₃)₃C–CN; step II dil. H₂SO₄, heat under reflux ⇒ (CH₃)₃C–COOH.
(c) chiral centre = a carbon atom bonded to 4 different atoms or groups of atoms. R–CHBrCN is chiral (H, Br, CN, R) — draw the two displayed mirror images across a dotted mirror line, with wedge (towards you) and dash (away) bonds.
| Box | Compound | Reagent |
|---|---|---|
| A | CH₃CHBrCH₂Br | Br₂ |
| B | CH₃CH(OH)CH₂OH | cold dilute acidified KMnO₄ |
| C | CH₃CHBrCH₃ | HBr (H to C with more H) |
| D | CH₃CH(CN)CH₃ | KCN in aqueous ethanol (from C) |
| E | CH₃CH(COOH)CH₃ | H₂SO₄(aq), heat under reflux (from D) |
| F | CH₃CH(NH₂)CH₃ | excess NH₃ (from C) |
| G | CH₃CH=CH₂ | NaOH in ethanol, heat under reflux (from C) |
Part (b) (E reacting with B ⇒ ester) is crossed out on the board — it belongs to the Carboxylic acids & esters chapter.
(a) reaction 1 (⇒ butan-1-ol): NaOH, water · reaction 2 (⇒ butylamine): NH₃, ethanol · reaction 3 (⇒ but-1-ene): NaOH, ethanol.
(b) 1-iodobutane reacts faster: the C–I bond energy is less than C–Br, so it breaks more easily ⇒ rate faster. (Data Booklet part crossed out on board.)
(b)(i) mechanism of C₂H₅Br + OH⁻ (aq): SN2 — lone pair from :OH⁻ → Cδ⁺, Brδ⁻ leaves, transition state bracket with ⁻ charge, product C₂H₅OH + :Br⁻. (ii) with ethanolic NaOH: ethene (+ NaBr + H₂O — board wrote “ethene + HBr”). (iii) ethanolic NaOH, heat under reflux. (iv) elimination.
(c) 1,4-dichlorobutane: (i) aqueous NaOH ⇒ HOCH₂CH₂CH₂CH₂OH (butane-1,4-diol; displayed). (ii) ethanolic NaOH ⇒ elimination twice ⇒ skeletal buta-1,3-diene (CH₂=CH–CH=CH₂).
- 1° / 2° / 3° = number of alkyl groups on the C–X carbon (2 H / 1 H / 0 H).
- b.p. higher than alkanes (more electrons + permanent dipoles); ↑ F→I (electrons win); ↓ 1°→3° (branching). Bond energy C–F > C–Cl > C–Br > C–I ⇒ reactivity opposite.
- Elimination: ethanolic NaOH/KOH, reflux ⇒ alkene (2-bromobutane ⇒ 3 alkenes). Substitution: aq NaOH ⇒ alcohol; excess ethanolic NH₃, sealed tube ⇒ amine; NaCN in ethanol, reflux ⇒ nitrile (+1 C) ⇒ dil. acid reflux ⇒ COOH.
- SN2: primary, 1 step, TS in brackets, attack from opposite the leaving group. SN1: tertiary, 2 steps, slow step first, carbocation stabilised by +I; tertiary can’t SN2 (steric hindrance); primary can’t SN1.
- Halide test: NaOH(aq), heat; dil. HNO₃; AgNO₃ ⇒ AgCl white (dil NH₃ dissolves), AgBr cream (conc NH₃), AgI yellow (insoluble). Rate: RI > RBr > RCl (bond energy); 3° > 2° > 1°.