How to name organic compounds for SPM

Naming organic compounds looks like a foreign language until you realise it is built from just a few repeating parts. Once you can read those parts, an unfamiliar name like propan-1-ol tells you exactly what the molecule is, three carbons, an alcohol group, on the first carbon. This guide shows you how the naming system is put together and walks through each family of carbon compounds in the SPM syllabus, so you can both read a name and build one yourself.

Every name has three parts

An IUPAC name for the compounds you meet in SPM is made of three pieces:

• The stem tells you how many carbon atoms are in the longest chain. Learn these six by heart: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6).
• The suffix tells you the functional group, the family the molecule belongs to. Alkanes end in -ane, alkenes in -ene, alcohols in -ol, and carboxylic acids in -oic acid.
• A locant is a number that says where on the chain the important group sits, such as the 1 in propan-1-ol.

Put together, stem + suffix + locant describes the molecule completely. You can revise all of these families in the carbon compounds chapter, and look up any unfamiliar term in the chemistry glossary.

The naming method, step by step

1. Find the longest continuous carbon chain. Count its carbons, that gives the stem.
2. Identify the functional group to choose the suffix. A C=C double bond means an alkene; an –OH group means an alcohol; a –COOH group means a carboxylic acid.
3. Number the chain from whichever end gives the functional group the lowest possible number, and write that number as the locant.

That order, chain, group, number, works for every straight-chain compound in the syllabus.

The families, with examples

Alkanes are saturated, with only single bonds, general formula CₙH₂ₙ₊₂. CH₄ is methane, C₂H₆ is ethane, C₃H₈ is propane. No locant is needed because there is no special group to locate.

Alkenes have a C=C double bond, general formula CₙH₂ₙ. CH₂=CH₂ is ethene. For CH₃CH=CH₂ the double bond starts on the first carbon, so it is prop-1-ene (often just called propene). The double bond is why alkenes decolourise bromine water and undergo the addition reactions you study in this chapter.

Alcohols carry an –OH group, suffix -ol. CH₃OH is methanol and C₂H₅OH is ethanol. For CH₃CH₂CH₂OH, the –OH is on carbon 1 of a three-carbon chain, giving propan-1-ol. If the –OH were on the middle carbon, CH₃CH(OH)CH₃, it would be propan-2-ol. Alcohols burn cleanly, which you can revise at combustion of alcohols.

Carboxylic acids carry the –COOH group, suffix -oic acid. HCOOH is methanoic acid, CH₃COOH is ethanoic acid, and CH₃CH₂COOH is propanoic acid. The carboxyl carbon is always counted as part of the chain and is always carbon 1, so these rarely need a locant.

Naming esters

Esters are named in two words, and the trick is knowing which parent gives which word. An ester made from an alcohol and a carboxylic acid is named alkyl alkanoate: the alkyl part comes from the alcohol, and the alkanoate part comes from the acid. Ethanol plus ethanoic acid gives ethyl ethanoate, “ethyl” from the ethanol, “ethanoate” from the ethanoic acid. Methanol plus ethanoic acid would give methyl ethanoate. You can see how the acid and alcohol join at esterification.

A note on branched chains

SPM also expects you to name simple branched alkanes. A branch is written as a prefix with its own locant: a –CH₃ side group is “methyl”, a –C₂H₅ side group is “ethyl”. So a four-carbon chain with a methyl branch on the second carbon is 2-methylbutane. Number the main chain from the end that gives the branch the lower number, exactly as you did for functional groups.

Practise reading names both ways

The fastest way to master naming is to practise in both directions: given a structure, build the name; given a name, draw the structure. Do a few of each daily across all the families and the pattern of stem, suffix and locant becomes automatic, so a name you have never seen still tells you the whole molecule at a glance. If you would like a teacher to check your structures and names and iron out the tricky locant rules, our online one-to-one lessons with our experienced SPM Chemistry teachers are built around that kind of guided practice, taught in English from RM50 an hour with a paid one-hour trial.