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.
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.