Organic functional groups

Carbon chemistry becomes far easier once you group compounds by their functional group, the reactive part that gives a whole family its characteristic reactions. Each family, called a homologous series, shares a general formula and a naming pattern, and successive members differ by CH₂. This reference lists the functional groups the syllabus expects in SPM Chemistry, arranged the way the DSKP presents them, so you can name a compound, write its formula and predict its reactions from the group it belongs to.

Scope note

The table below holds the standard KSSM homologous series for Form 5 Carbon Compounds: alkanes, alkenes, alcohols, carboxylic acids and esters. Learn the functional group and the general formula together, because the group decides both the name ending and the chemistry, and the general formula lets you write any member of the series.

Functional groups and homologous series

Homologous series	Functional group	General formula	Example	Name ending
Alkane	Single C-C bonds only (saturated)	CnH2n+2	Ethane, C2H6	-ane
Alkene	Carbon-carbon double bond, C=C	CnH2n	Ethene, C2H4	-ene
Alcohol	Hydroxyl group, -OH	CnH2n+1OH	Ethanol, C2H5OH	-ol
Carboxylic acid	Carboxyl group, -COOH	CnH2n+1COOH	Ethanoic acid, CH3COOH	-oic acid
Ester	Ester linkage, -COO-	,	Ethyl ethanoate, CH3COOC2H5	-yl -oate

How to use the table

Start from the functional group to name a compound and predict its reactions. An alkane is saturated, so it burns and undergoes substitution with halogens but does not add across a bond. An alkene has a C=C double bond, so it is unsaturated and undergoes addition reactions, decolourising bromine water, a common test. An alcohol has an -OH group, so it burns, is oxidised to a carboxylic acid, and undergoes esterification. A carboxylic acid has a -COOH group, so it behaves as a weak acid and forms esters with alcohols. An ester has the -COO⁻ linkage and has a sweet, fruity smell. Recognising the group is the fastest route to the answer.

Worked example

Name the alcohol with three carbon atoms and give its formula. The alcohol series ends in -ol and has the general formula CnH2n+1OH. With three carbons, the name is propanol and the formula is C₃H₇OH. Because it has an -OH group, you can predict at once that it burns in oxygen, can be oxidised to propanoic acid, and reacts with a carboxylic acid to form an ester. All of this follows from the functional group alone.

Trends within a homologous series

Within any one series the members change gradually and predictably as the carbon chain lengthens. Melting and boiling points rise from one member to the next because the molecules are larger and the forces between them are stronger, so the first few members of a series are gases or volatile liquids while later members are less volatile. Density and viscosity also increase along the series, and solubility in water tends to fall for the larger members. Because every member shares the same functional group, they all give the same kind of chemical reaction, differing only in how vigorously. Knowing these trends lets you answer a comparison question, such as why ethanol boils at a higher temperature than methanol, without memorising individual data.

Common mistakes to avoid

• Confusing the alkene general formula CnH2n with the alkane CnH2n+2.
• Writing an alcohol name without the -ol ending, or misplacing the -OH.
• Mixing up carboxylic acid (-COOH) and ester (-COO⁻) structures.
• Forgetting that an alkene decolourises bromine water while an alkane does not.

How we help

Our online one-to-one teachers teach carbon chemistry group by group, so you reason from the functional group instead of memorising each compound separately. Lessons are in English, one to one, from RM50 per hour, with a paid one-hour trial. Because naming, formulae and reactions all flow from the functional group, learning this table well makes the whole Carbon Compounds chapter and its Paper 2 questions far more manageable.