The kind of SPM questions asked on carbon compounds

Students often fear Carbon Compounds because it looks like a wall of new names, but the exam actually recycles a small set of predictable question types. Once you can see the pattern behind each one, the chapter becomes one of the more scoring topics in Form 5. This guide sorts the questions into the shapes SPM keeps setting, and shows the move that earns the marks for each.

Type 1: Name and draw the structure

The most basic type gives you a formula and asks for the name, or gives a name and asks you to draw the structural formula. To handle it you must know the general formula and naming of each homologous series: alkanes CnH2n+2, alkenes CnH2n, alcohols CnH2n+1OH and carboxylic acids CnH2n+1COOH, with the prefixes meth-, eth-, prop-, but- for one to four carbons.

Worked example: Draw the structural formula of propene. Propene is C₃H₆, an alkene with the double bond, so you draw CH₂=CH–CH₃ showing every bond. Miss the double bond and it becomes propane, a common and costly slip.

Type 2: Isomerism

A very frequent structured question asks you to draw the structural isomers of a molecule and name them. Isomers share the same molecular formula but have different structural formulae.

Worked example: Draw the isomers of C₄H₁₀. There are two: butane (a straight chain of four carbons) and 2-methylpropane (a three-carbon chain with a –CH₃ branch on the middle carbon). Both are C₄H₁₀; only the arrangement differs. Getting the names right, not just the shapes, is where the marks sit, you can read more in our isomerism glossary entry.

Type 3: Distinguishing an alkane from an alkene

This test appears almost every year, in both Paper 2 and the practical. The reagent is bromine water, and the answer hinges on the words decolourise and unsaturated.

Worked example: Describe a chemical test to distinguish ethane from ethene. Add bromine water to each. Ethene is unsaturated (has a C=C double bond), so it undergoes an addition reaction and decolourises the brown bromine water rapidly. Ethane is saturated and does not decolourise it (except slowly in sunlight). The full experiment is on our testing for unsaturation page.

Type 4: The reactions of the homologous series

The highest-value type asks you to state products, write equations or describe the reactions of a series. The examinable set is small and worth memorising cold:

• Combustion. Complete combustion of any hydrocarbon gives carbon dioxide and water: CH₄ + 2O₂ → CO₂ + 2H₂O.
• Addition (alkenes). Ethene reacts with bromine to give 1,2-dibromoethane, with hydrogen (nickel catalyst) to give ethane, and with steam (phosphoric acid catalyst) to give ethanol. See our addition reactions of alkenes page.
• Substitution (alkanes). Methane reacts with chlorine in ultraviolet light: CH₄ + Cl₂ → CH₃Cl + HCl.
• Oxidation (alcohols). Ethanol is oxidised by acidified potassium manganate(VII) to ethanoic acid; the purple solution turns colourless.
• Esterification. A carboxylic acid and an alcohol, warmed with concentrated sulfuric acid, give an ester and water: ethanoic acid + ethanol → ethyl ethanoate + water.

Worked example: State the product when ethene is bubbled through bromine water and name the reaction. The product is 1,2-dibromoethane and the reaction is addition. Naming the reaction type, addition, substitution, oxidation, esterification, is frequently a separate mark, so never skip it.

Type 5: Trends within a homologous series

An explanation type asks how a physical property changes down a series. As the number of carbon atoms increases, the molecular size and mass rise, so boiling point increases and the substances become less volatile. Members of a series share the same chemical properties because they have the same functional group, an idea you can revise in our homologous series glossary entry.

Type 6: The essay or context question

Paper 2 Section B or C often frames organic chemistry in a real context, the fermentation of glucose to ethanol, the making of soap from a fat, the properties that make esters useful as flavourings, or the environmental cost of burning fossil fuels. Here you must combine an equation, a named condition and a clear explanation. The SPM papers hub sorts these by section so you can drill the longer-answer style.

How to revise across the six

Because the same six shapes come round every year, revise by move, not by re-reading notes: learn the general formulae, drill isomer drawing, lock in the bromine-water wording, memorise the reaction set, learn one property trend and practise one context essay. That is exactly how our online one-to-one lessons work through this chapter, in English from RM50 an hour with a paid one-hour trial to start. Get the reactions exact and the naming clean, and Carbon Compounds turns from a memory test into a reliable source of marks.