How to explain addition and substitution reactions

Addition and substitution are the two reaction types the carbon compounds chapter keeps coming back to, and many students lose marks because they describe them in vague words. The examiner wants you to say exactly what happens to the bonds, name the conditions, and give a balanced equation. This guide gives you a way to explain both that is precise enough to score full marks.

The one-line difference

• In an addition reaction, a molecule adds across a carbon–carbon double bond. The double bond opens, atoms join on, and there is only one product. Addition is the reaction of unsaturated compounds such as alkenes.
• In a substitution reaction, one atom in a molecule is replaced by another atom or group. There are two products, the new organic molecule and a small molecule such as hydrogen chloride. Substitution is the reaction of saturated compounds such as alkanes.

If you remember “addition = double bond opens, one product” and “substitution = one atom swapped, two products”, you already have the backbone of every answer.

Addition reactions of alkenes

Alkenes have a C=C double bond, so they are unsaturated and react by addition. The double bond breaks and the two carbon atoms each gain a new atom. Learn these four for SPM.

1. Hydrogenation (with hydrogen). Ethene reacts with hydrogen over a nickel catalyst at about 180 °C to form ethane:

CH₂=CH₂ + H₂ → CH₃CH₃

2. Halogenation (with bromine or chlorine). Ethene reacts with bromine at room temperature to form 1,2-dibromoethane:

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

This is the reaction behind the bromine-water test: the brown colour disappears.

3. Hydration (with steam). Ethene reacts with steam over a phosphoric acid catalyst to form ethanol:

CH₂=CH₂ + H₂O → CH₃CH₂OH

4. Addition with a hydrogen halide. Ethene reacts with hydrogen bromide to form bromoethane:

CH₂=CH₂ + HBr → CH₃CH₂Br

Notice the pattern in all four: the double bond opens, one molecule joins on, and there is a single product with no by-product. That single-product feature is the giveaway of an addition reaction. You can drill more of these on our addition reactions of alkenes page.

Substitution reactions of alkanes

Alkanes have only single bonds, so they are saturated and cannot do addition. Instead they react by substitution, in which a hydrogen atom is replaced by a halogen atom. The classic example is methane reacting with chlorine in the presence of ultraviolet light (sunlight):

CH₄ + Cl₂ → CH₃Cl + HCl

Here one hydrogen atom of methane is replaced by a chlorine atom, giving chloromethane, and the displaced hydrogen leaves as hydrogen chloride. There are two products, which is the signature of substitution. If more chlorine is present, further substitution gives dichloromethane, trichloromethane and tetrachloromethane in turn. See substitution of alkanes for the full sequence.

The condition matters: UV light is essential. Without sunlight, methane and chlorine do not react. This is a favourite examiner detail, many students forget to state the condition and drop the mark.

A side-by-side comparison you can memorise

Feature	Addition	Substitution
Type of compound	Unsaturated (alkene)	Saturated (alkane)
What happens to bonds	C=C double bond opens	One C–H bond broken, atom replaced
Number of products	One	Two
Typical condition	Room temperature / catalyst	Ultraviolet light
Test link	Decolourises bromine water fast	No fast reaction with bromine water in the dark

How to write a full-mark explanation

Suppose a question asks you to explain how ethene reacts with bromine and how it differs from the reaction of ethane. A strong answer names the mechanism type, states the condition, gives the equation, and contrasts the two:

Ethene is unsaturated and undergoes an addition reaction with bromine at room temperature. The C=C double bond opens and a bromine atom joins each carbon, forming 1,2-dibromoethane as the only product, so the brown bromine is decolourised. Ethane is saturated and can only undergo a substitution reaction with bromine, and only in UV light, replacing one hydrogen to give bromoethane and hydrogen bromide, two products.

That paragraph earns marks for the reaction type, the bonding change, the conditions, the equations and the comparison. That is the level of precision the carbon compounds questions reward.

Common mistakes to avoid

• Saying “the double bond breaks” for an alkane. Alkanes have no double bond, they substitute, they do not add.
• Forgetting the UV light condition for substitution.
• Writing one product for substitution. There are always two.
• Mixing up the catalyst. Hydrogenation uses nickel; hydration uses phosphoric acid.

Get these drilled properly

Addition and substitution reward students who state bonds, conditions and products in the right order every time. If your child keeps writing the idea but missing the marking points, this is exactly the kind of thing our online one-to-one teachers fix quickly, lessons run in English from RM50 an hour, with a paid one-hour trial. See how it works if you would like these worked against real Paper 2 questions.