How do substitution reactions replace one atom or group with another in alkanes and haloalkanes?
Describe substitution reactions, including the halogenation of alkanes and the conversion of haloalkanes to alcohols
A focused answer to the WACE Year 12 Chemistry dot point on substitution reactions, covering the ultraviolet halogenation of alkanes and the conversion of haloalkanes into alcohols by hydrolysis, with conditions, a worked example and common exam mistakes.
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What this dot point is asking
A substitution reaction replaces one atom or group in a molecule with another. Unlike addition, substitution always produces a small by-product molecule, so it is the characteristic reaction of saturated compounds, which have no multiple bond to add across.
Halogenation of alkanes
Alkanes are unreactive, but in the presence of ultraviolet light a halogen will substitute for a hydrogen atom. For example, methane reacts with chlorine:
The hydrogen halide () is the by-product. The reaction can continue, substituting further hydrogens to give a mixture of products (, and so on), which is why it gives poor selectivity.
Hydrolysis of haloalkanes to alcohols
A haloalkane can be converted to an alcohol by substitution of the halogen with a hydroxide ion. Warming a haloalkane with aqueous sodium hydroxide replaces the halogen:
The product is an alcohol and the halide ion leaves. This is a useful synthetic step: it converts a haloalkane (which itself can be made from an alkene) into an alcohol, opening up the chemistry of alcohols.
Reactivity of different haloalkanes
The carbon-halogen bond strength decreases down the halogen group, so iodoalkanes react fastest and fluoroalkanes slowest. The weaker C-I bond is broken more easily, so the rate of hydrolysis follows C-I > C-Br > C-Cl.
Substitution versus elimination
A haloalkane warmed with hydroxide can react in two competing ways, and the conditions decide which dominates. With aqueous sodium hydroxide, the hydroxide acts as a nucleophile and substitutes for the halogen, giving the alcohol (substitution, as above). With hot alcoholic (ethanolic) sodium hydroxide, the hydroxide instead acts as a base and removes a hydrogen from the carbon next to the halogen, so the halogen and that hydrogen leave together as a double bond forms: this is elimination, producing an alkene and water. For example, gives ethanol by aqueous substitution but ethene by alcoholic elimination. Quoting the solvent (aqueous versus alcoholic) is therefore essential when a question asks you to direct a haloalkane towards an alcohol rather than an alkene.
Why this matters
Substitution reactions are essential interconversions in synthesis: they link alkanes, haloalkanes and alcohols. Recognising substitution versus addition, and remembering that substitution releases a by-product (lowering atom economy), connects this dot point to reaction pathways and green chemistry.
Exam-style practice questions
Practice questions written in the style of SCSA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WACE 20216 marks1-chlorobutane, 1-bromobutane and 1-iodobutane are each warmed separately with aqueous silver nitrate. (a) Write the general equation for the hydrolysis of a haloalkane with water. (b) Predict the order in which a precipitate of silver halide appears, and justify it using bond strength. (c) State the type of reaction and the role of the silver nitrate.Show worked answer →
A 6 mark question rewards the equation, the bond-strength reasoning, and the reaction type.
(a) (the halide ion is released; in practice warm aqueous conditions are used).
(b) The precipitate appears fastest for the iodide, then the bromide, then the chloride (1-iodobutane first, 1-chlorobutane last). This is because the carbon-halogen bond strength decreases down the group (); the weaker bond breaks most easily, so iodide is released fastest and the silver iodide precipitate forms first.
(c) The reaction is nucleophilic substitution (hydrolysis), replacing the halogen with . The silver nitrate supplies ions, which react with the released halide ions to form a coloured silver halide precipitate, signalling that substitution has occurred.
Markers reward the hydrolysis equation, the C-I < C-Br < C-Cl bond-strength order giving the fastest-for-iodide prediction, and substitution with detecting the halide.
WACE 20235 marksCompare the reaction of methane with chlorine and the reaction of ethene with chlorine. (a) State the reaction type, conditions and by-product (if any) for each. (b) Explain why the alkane reaction gives a mixture of products whereas the alkene reaction gives a single product.Show worked answer →
A 5 mark compare answer needs both reactions characterised and the selectivity reason.
(a) Methane with chlorine: substitution, requiring ultraviolet light, with the by-product : . Ethene with chlorine: addition, fast at room temperature with no catalyst or light and no by-product: .
(b) Alkane halogenation proceeds by a free-radical mechanism that, once started, can substitute a second, third and fourth hydrogen, giving a mixture (, , , ) and so is poorly selective. The alkene reaction simply adds one chlorine molecule across the single double bond, giving one product cleanly.
Markers reward substitution-with-UV-and-HCl versus addition-with-no-by-product, and the multiple-substitution versus single-addition explanation of selectivity.