Describe the addition reactions of alkenes including hydrogenation, halogenation, hydration and hydrogen halide addition

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 the double bond reacts: the pi bond

The reactivity of alkenes comes from the structure of the double bond. A C=C double bond is made of one sigma bond (a strong, direct overlap along the bond axis) and one pi bond (a weaker, sideways overlap of p orbitals above and below the axis). The pi bond is a region of exposed, loosely held electron density, so it readily attacks electron-poor reagents. In an addition reaction it is the pi bond that breaks, leaving the sigma bond intact, while two new sigma bonds form to the adding atoms. This explains why addition is fast and requires milder conditions than the substitution of alkanes, which must break a strong, unreactive sigma bond by a high-energy radical mechanism.

A note on Markovnikov and stability

When an unsymmetrical reagent adds, the orientation is set by the stability of the intermediate. Adding $\text{H}^+$ first to the alkene creates a carbocation, and the more stable carbocation (tertiary more stable than secondary, secondary more stable than primary) forms preferentially. The remaining part of the reagent then bonds to that carbon, which is why the major product places the larger group on the more substituted carbon. At WACE level you are expected to identify the major product and link it to this idea of forming the more stable intermediate, rather than to give a full mechanism.

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.