15. Halogenoalkanes
After completing this chapter, you should be able to:
- Recall how halogenoalkanes are made: free-radical substitution of alkanes (Cl₂/Br₂ + UV light), electrophilic addition of alkenes (X₂ or HX), and substitution of alcohols (HX, PCl₃, PCl₅, or SOCl₂).
- Classify halogenoalkanes as primary, secondary or tertiary.
- Describe the nucleophilic substitution reactions with NaOH(aq), ethanolic KCN, ethanolic NH₃ (under pressure), and ethanolic AgNO₃ (for halogen identification).
- Describe the elimination reaction with ethanolic NaOH to form an alkene.
- Describe the SN1 and SN2 mechanisms, including the inductive effects of alkyl groups.
- Recall that primary halogenoalkanes tend to react via SN2, tertiary via SN1, and secondary via a mixture of both.
- Explain the different reactivities of halogenoalkanes, with reference to C–X bond strength, using the reaction with aqueous silver nitrate as evidence.
1 🧬 Introduction & Classification ▾
A halogenoalkane (haloalkane) is an alkane in which one or more hydrogen atoms have been replaced by a halogen atom (F, Cl, Br or I). The general formula is written R–X, where X is the halogen.
Halogenoalkanes are classified by how many carbon groups are attached to the carbon bearing the halogen — not by how many carbons are in the whole molecule.
Search for 1°, 2° and 3° haloalkanes on the MolView website (molview.org) and rotate the 3D structures — it makes the difference in steric bulk around the C–X carbon much easier to picture, which matters a lot in Section 4.
2 ⚗️ Preparation of Halogenoalkanes ▾
There are three main routes onto the syllabus. The first two are recaps from earlier topics; the third — substitution of an alcohol — is the one with the most detail to learn.
(a) Free-radical substitution of an alkane
Alkanes react with Cl₂ or Br₂ in the presence of UV light via a free-radical chain mechanism (initiation, propagation, termination). Ethane, for example, is converted to chloroethane:
(b) Electrophilic addition to an alkene
An alkene reacts with a halogen, X₂, or a hydrogen halide, HX(g), at room temperature — the π bond is the site of attack by the electrophile.
(c) Substitution of an alcohol
This is the route with several named reagents — each with its own condition and by-products. All five are built from ethanol here, but the same patterns apply to any alcohol.
- Tertiary alcohols react readily with concentrated HCl alone, on shaking at room temperature (SN1 mechanism — see Section 4).
- Secondary and primary alcohols react more slowly with concentrated HCl; anhydrous ZnCl₂ is added as a catalyst and the mixture must be heated.
- With PCl₅, hydrogen chloride gas is evolved. The fizzing and misty white fumes are used as a test for an –OH group (provided the sample is dry, since PCl₅ also reacts with water).
- Iodoalkanes aren’t usually made directly from HI, because concentrated HI is a strong reducing agent and is oxidised by concentrated H₂SO₄. Instead, PI₃ is generated in situ from red phosphorus and iodine, and this reacts with the alcohol.
3 🔄 Nucleophilic Substitution Reactions ▾
Halogenoalkanes undergo nucleophilic substitution because the C–X bond is polarised (X is more electronegative than C), leaving the carbon electron-deficient and open to attack by a lone pair from a nucleophile. Four versions of this reaction are on the syllabus.
(a) Hydrolysis — with aqueous NaOH
The hydroxide ion, OH⁻, acts as the nucleophile, producing an alcohol.
(b) With ethanolic KCN
The cyanide ion, CN⁻, attacks through its carbon atom, producing a nitrile. This is a valuable synthetic step because it extends the carbon chain by one carbon — the nitrile can later be hydrolysed to a carboxylic acid or reduced to a primary amine.
(c) With ethanolic ammonia (under pressure)
Excess ammonia is used, heated under pressure in a sealed tube, to form a primary amine. The excess NH₃ helps limit further substitution to secondary or tertiary amines.
(d) With aqueous silver nitrate (in ethanol) — identifying the halogen
Because halogenoalkanes don’t mix with water, a mixed ethanol–water solvent is used (ethanol is miscible with both). As the C–X bond slowly hydrolyses, the halide ion released is trapped by Ag⁺ as an insoluble silver halide precipitate — the colour of which identifies the halogen, and the time taken reflects how reactive that halogenoalkane is.
| Compound | Observation (AgNO₃(aq) in ethanol) | Colour of precipitate |
|---|---|---|
| (CH₃)₂CH–Cl | Faint cloudiness, only after about an hour | White |
| (CH₃)₂CH–Br | Cloudiness appears within a few minutes | Pale cream |
| (CH₃)₂CH–I | Dense precipitate forms within a minute | Pale yellow |
This test is explored fully in Section 6, where the trend is linked to C–X bond strength.
4 🔬 Mechanisms: SN1 and SN2 ▾
Nucleophilic substitution can happen by two distinct pathways. Which one dominates depends on how crowded the carbon bearing the halogen is.
SN2 — favoured by primary halogenoalkanes
- One step; rate depends on both [halogenoalkane] and [nucleophile] (second order).
- Favoured when the back of the carbon is unhindered — i.e. in primary halogenoalkanes.
SN1 — favoured by tertiary halogenoalkanes
- Two steps; rate depends only on [halogenoalkane] (first order) — the nucleophile isn’t involved in the slow step.
- Favoured in tertiary halogenoalkanes: the three alkyl groups push electron density towards the carbon (a positive inductive effect), stabilising the intermediate carbocation and lowering the activation energy of the ionisation step.
Secondary halogenoalkanes react by a mixture of SN1 and SN2 — the exact balance depends on the precise structure and conditions. Don’t force a secondary case into one mechanism only.
5 ⚡ Elimination Reaction ▾
Under different conditions, the very same reagent — sodium hydroxide — gives a completely different product.
Here, OH⁻ acts as a base rather than a nucleophile: it removes a hydrogen atom from the carbon adjacent to the C–X carbon, and the electron pair from that C–H bond forms the new C=C π bond as the halide ion leaves.
Same reagent, different solvent, different outcome:
- NaOH(aq), heat → substitution (alcohol) — OH⁻ acts as a nucleophile.
- NaOH in ethanol, heat → elimination (alkene) — OH⁻ acts as a base.
Always state the solvent in your answer — “NaOH” alone is not enough to specify the product.
6 📊 Reactivity Trends & Bond Strength ▾
Across the halogenoalkanes, reactivity in nucleophilic substitution increases down the group:
It is not the electronegativity of the halogen that decides reactivity — it’s the strength of the C–X bond. As you go down the group, the halogen atom gets larger, so its orbitals overlap less effectively with carbon’s orbital, and the C–X bond gets weaker (lower bond enthalpy). A weaker bond breaks more easily, whether that bond-breaking happens in one step (SN2) or as the rate-determining ionisation (SN1).
This is exactly what the aqueous silver nitrate experiment from Section 3 demonstrates: the C–I bond is weakest, so iodoalkanes hydrolyse fastest and give a precipitate almost immediately, while the stronger C–Cl bond makes chloroalkanes react noticeably more slowly.
Q: Equal volumes of ethanolic AgNO₃ are added to separate samples of 1-chlorobutane, 1-bromobutane and 1-iodobutane. Predict the order in which precipitates appear, and explain your answer.
A: 1-Iodobutane first, then 1-bromobutane, then 1-chlorobutane. The C–I bond is weakest (largest halogen atom, poorest orbital overlap with carbon), so it hydrolyses fastest and releases I⁻ first; the C–Cl bond is strongest, so hydrolysis — and precipitate formation — is slowest.
- Always state the solvent for NaOH — aqueous vs ethanolic changes the product completely.
- Link mechanism to structure: 1° → SN2, 3° → SN1, 2° → mixture.
- Explain reactivity trends using C–X bond strength, not halogen electronegativity.
- Draw curly arrows starting from a lone pair or bond, ending where the new bond forms.
- Don’t forget PI₃ (and PBr₃) must be made in situ from P + halogen — HI/HBr alone can be too unstable or too weak.
- Don’t muddle SN1 (2 steps, carbocation, tertiary) with SN2 (1 step, concerted, primary).
- Don’t write a bare “NaOH” when the mark scheme wants aqueous or ethanolic specified.
- Don’t say iodine is “more reactive” than chlorine — it’s the C–I bond that’s weaker.