Describe the addition reactions of alkenes and the substitution reactions of haloalkanes, writing equations and products.

What this dot point is asking

SACE expects you to distinguish addition from substitution, write balanced equations with structural products, and apply Markovnikov reasoning.

Lead worked calculation

Addition reactions of alkenes

The double bond is a region of high electron density that opens to let atoms add across it. The four key additions:

Markovnikov's rule

When an unsymmetrical reagent (such as $\text{HBr}$ or $\text{H}_2\text{O}$) adds to an unsymmetrical alkene, two products are possible. Markovnikov's rule predicts which dominates.

Substitution reactions of haloalkanes

Haloalkanes are saturated, so they cannot add; instead the halogen, a good leaving group, is replaced by a nucleophile.

• Hydrolysis with aqueous hydroxide gives an alcohol: $\text{R-X} + \text{OH}^- \rightarrow \text{R-OH} + \text{X}^-$.
• The halogen leaves as a halide ion, and the incoming group takes its place on the same carbon.

This is the reverse logic of addition: addition increases the number of groups on the carbons; substitution swaps one group for another.

Comparing the two

Addition is characteristic of unsaturated compounds (alkenes), where there is a multiple bond to open. Substitution is characteristic of saturated compounds (haloalkanes and alkanes), where a group must be replaced because there is no multiple bond to add across. Identifying whether the starting material is saturated or unsaturated tells you which reaction type to expect.

Why it matters

These reactions convert simple hydrocarbons into alcohols, haloalkanes and polymers, the gateway compounds for synthesising almost all other organic materials. Markovnikov reasoning and the addition-versus-substitution distinction are recurring tools in the synthesis pathways assessed later in this topic.