Halogenoalkanes: Nucleophilic Substitution and Elimination

This guide covers Topic 15, Halogen compounds — subtopic 15.1 Halogenoalkanes in full — from Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is AS Level content.

Before studying this

Halogenoalkanes as a class are not covered at IGCSE or O Level — this is new material at AS Level. What IGCSE / O Level does give you is the reaction that makes some of them: substitution of an alkane by chlorine (subtopic 11.4), which reappears here as one of three production routes. You should also have worked through Hydrocarbons: Alkanes and Alkenes first, since two of the three ways to make a halogenoalkane are reactions of alkanes and alkenes covered there, and through Organic Mechanisms: An Introduction for nucleophile, electrophile and curly-arrow notation.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — AS Level, Topic 15

15.1 Halogenoalkanes — the reactions by which halogenoalkanes are produced: free-radical substitution of an alkane by Cl₂ or Br₂ under ultraviolet light (exemplified by ethane); electrophilic addition of an alkene with a halogen, X₂, or a hydrogen halide, HX(g), at room temperature; substitution of an alcohol (by HX(g); by KCl and concentrated H₂SO₄ or concentrated H₃PO₄; by PCl₃ and heat; by PCl₅; or by SOCl₂); classifying halogenoalkanes as primary, secondary or tertiary; the nucleophilic substitution reactions with NaOH(aq) and heat (to an alcohol), KCN in ethanol and heat (to a nitrile), NH₃ in ethanol heated under pressure (to an amine), and aqueous silver nitrate in ethanol as a method of identifying the halogen present, exemplified by bromoethane; the elimination reaction with NaOH in ethanol and heat to produce an alkene, exemplified by bromoethane; the SN1 and SN2 mechanisms of nucleophilic substitution, including the inductive effects of alkyl groups; that primary halogenoalkanes tend to react by SN2, tertiary by SN1, and secondary by a mixture of the two depending on structure; and the different reactivities of halogenoalkanes, with particular reference to the relative strengths of the C–X bonds, as exemplified by their reactions with aqueous silver nitrate.

Three routes to a halogenoalkane

Halogenoalkanes are made from each of the other AS organic classes you’ve already met, which is worth noticing as a pattern rather than three disconnected facts: free-radical substitution of an alkane with Cl₂ or Br₂ under UV (from Alkanes and Alkenes), electrophilic addition of an alkene with X₂ or HX at room temperature (also from that resource), or substitution of an alcohol, using HX(g), KCl with concentrated H₂SO₄ or H₃PO₄, PCl₃ and heat, PCl₅, or SOCl₂.

Classifying halogenoalkanes

A halogenoalkane is primary if the halogen-bearing carbon is attached to one other carbon, secondary if attached to two, and tertiary if attached to three. This classification matters directly for which mechanism dominates.

Nucleophilic substitution reactions

Reagent and conditions	Product
NaOH(aq), heat	Alcohol
KCN in ethanol, heat	Nitrile
NH₃ in ethanol, heated under pressure	Amine
AgNO₃(aq) in ethanol	Silver halide precipitate — identifies the halogen

Elimination, by contrast, uses NaOH dissolved in ethanol (not water) with heat, and produces an alkene instead of an alcohol — exemplified by bromoethane going to ethene. The solvent is the signal: aqueous NaOH favours substitution, ethanolic NaOH favours elimination, because the ethoxide-rich ethanolic conditions favour the hydroxide ion acting as a base (removing a proton) rather than as a nucleophile (attacking the carbon).

SN1 and SN2 mechanisms

SN2 (bimolecular substitution) — the nucleophile attacks the halogen-bearing carbon from the side opposite the halogen, in a single step: as the new bond forms, the C–X bond breaks simultaneously. (A Level extension, Topic 26, not required at AS: the rate depends on the concentration of both the halogenoalkane and the nucleophile.) Primary halogenoalkanes react this way because the back side of the carbon is sterically unhindered — only a hydrogen and one alkyl group are in the way.

SN1 (unimolecular substitution) — the C–X bond breaks first, in a slow step, forming a carbocation intermediate; the nucleophile then attacks the carbocation in a fast second step. (A Level extension, Topic 26, not required at AS: the rate depends only on the concentration of the halogenoalkane.) Tertiary halogenoalkanes react this way because three alkyl groups donate electron density inductively, stabilising the carbocation intermediate enough for it to form at a reasonable rate — the same stabilisation logic as Markovnikov addition in alkenes.

Secondary halogenoalkanes sit in between and can react by either mechanism depending on the exact structure and conditions, because a secondary carbocation is moderately stabilised (more than primary, less than tertiary) while the carbon is only moderately hindered (less than tertiary, more than primary).

Reactivity and the aqueous silver nitrate test

Reactivity in nucleophilic substitution depends on C–X bond strength, not just the mechanism: C–I is the weakest of the halogen–carbon bonds and breaks fastest, C–Cl is the strongest of the three and breaks slowest. This shows up directly in the aqueous silver nitrate test, where the identity of the halogen affects both the precipitate colour and how quickly it forms.

Worked example. Equal amounts of 1-chlorobutane, 1-bromobutane and 1-iodobutane are each warmed with aqueous silver nitrate. Predict the order in which a precipitate appears, and explain why.

All three are primary halogenoalkanes, so all three react by the SN2 mechanism with water acting as the nucleophile, releasing the halide ion which then precipitates with Ag⁺. The single SN2 step includes breaking the C–X bond, so the weaker that bond, the faster the substitution. Bond strength decreases C–Cl > C–Br > C–I, so reactivity — and precipitate formation — increases in the reverse order: iodobutane reacts fastest (yellow precipitate of AgI appears first), then bromobutane (cream, AgBr), then chlorobutane slowest (white, AgCl).

Common mistakes

• Assuming “primary reacts by SN2” and “tertiary reacts by SN1” is an arbitrary rule to memorise. It follows directly from sterics (SN2 needs an unhindered back side) and carbocation stability (SN1 needs a stabilised intermediate) — reasoning it through is more reliable than recalling it.
• Mixing up which solvent gives substitution and which gives elimination. NaOH(aq) → substitution (alcohol); NaOH in ethanol → mostly elimination (alkene). The nucleophile is the same hydroxide ion in both cases — it’s the solvent and conditions that shift the balance.
• Getting the C–X bond strength trend backwards. Reactivity in nucleophilic substitution increases down the halogens (Cl < Br < I) because bond strength decreases — a weaker bond is easier to break, not harder.
• Forgetting the AgNO₃ test needs ethanol as a co-solvent. The halogenoalkane is not soluble in water alone.

Quick revision checklist

• Three production routes: alkane + Cl₂/Br₂/UV; alkene + X₂/HX; alcohol + HX(g)/PCl₃/PCl₅/SOCl₂/KCl+conc. acid
• Primary, secondary, tertiary classification
• Nucleophilic substitution products: NaOH(aq) → alcohol; KCN/ethanol → nitrile; NH₃/ethanol → amine; AgNO₃(aq)/ethanol → halide identification
• Elimination: NaOH in ethanol + heat → alkene
• SN1 vs SN2: mechanism, the inductive effect of alkyl groups, and which halogenoalkane class favours which (rate dependence is A Level extension)
• C–X bond strength trend and its effect on reactivity and the AgNO₃ test

Related resources

• Hydrocarbons: Alkanes and Alkenes — two of the three routes to a halogenoalkane
• Alcohols: Reactions and Oxidation — the alcohol you get back from NaOH(aq) substitution
• Nitrogen Compounds: Amines and Nitriles — the amine and nitrile you get from the other two nucleophiles
• Organic Mechanisms: An Introduction — nucleophile, electrophile and curly-arrow notation
• Arenes and Halogenoarenes: Electrophilic Substitution and Reactivity — the A Level comparison with halogenoarene reactivity
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.