How do alkenes react by addition across the double bond, and what products form?
Describe the addition reactions of alkenes including hydrogenation, halogenation, hydration and hydrogen halide addition
A focused answer to the WACE Year 12 Chemistry dot point on addition reactions of alkenes, covering hydrogenation, halogenation, hydration to alcohols, and hydrogen halide addition, with the products and conditions, a worked example and common exam mistakes.
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What this dot point is asking
Alkenes are unsaturated: the carbon-carbon double bond contains a reactive region of electron density. In an addition reaction, that double bond is converted to a single bond as a molecule adds across it.
The four key additions
Hydrogenation (adding H2). With a nickel or platinum catalyst, hydrogen adds across the double bond to give the saturated alkane:
This is how unsaturated vegetable oils are hardened into margarine.
Halogenation (adding Br2 or Cl2). A halogen adds across the double bond, with no catalyst or light needed. The rapid decolourising of orange bromine water is the standard test for unsaturation:
Hydration (adding H2O). With steam and an acid catalyst (such as phosphoric acid), water adds across the double bond to form an alcohol. This is an industrial route to ethanol:
Hydrogen halide addition (adding HBr or HCl). A hydrogen halide adds to give a haloalkane:
Addition to unsymmetrical alkenes
When an unsymmetrical reagent (such as HBr or water) adds to an unsymmetrical alkene (such as propene), two products are possible depending on which carbon gains the hydrogen. The major product is usually the one where the hydrogen adds to the carbon that already has more hydrogens (Markovnikov's rule), giving the more substituted product. At WACE level you should recognise that two products are possible and identify the major one.
Why this matters
Addition reactions are the way unsaturated hydrocarbons are converted into the more useful functional families: alcohols, haloalkanes and saturated compounds. The same chemistry, repeated, gives addition polymers. These reactions are central to the synthesis pathways and atom economy themes of Unit 4.