How to identify functional groups in organic chemistry

Organic chemistry becomes much easier once you can spot the functional group, because the functional group decides how a compound reacts and what it is called. In SPM you only need a handful of them, and each has a shape you can recognise in a formula and a chemical test you can quote. This guide teaches both, using the carbon compounds syllabus.

What a functional group is

A functional group is the reactive part of an organic molecule, the atom or bond that gives the whole homologous series its characteristic reactions. Members of a homologous series share the same functional group and the same general formula, differing only by a CH₂ unit each step. So if you learn the group, you learn the whole family. Our organic functional groups table lists the ones SPM expects.

The groups you need for SPM

• Alkanes, only C–C and C–H single bonds, general formula CₙH₂ₙ₊₂. They are saturated and have no reactive functional group, so they are fairly unreactive. Example: ethane, C₂H₆.
• Alkenes, contain a carbon–carbon double bond, C=C, general formula CₙH₂ₙ. The C=C is the functional group. Example: ethene, C₂H₄.
• Alcohols, contain the hydroxyl group, –OH. Example: ethanol, C₂H₅OH.
• Carboxylic acids, contain the carboxyl group, –COOH. Example: ethanoic acid, CH₃COOH.
• Esters, contain the ester linkage, –COO–, made when an alcohol reacts with a carboxylic acid. Example: ethyl ethanoate.

Identifying the group from a formula or structure

The fastest route is to read the structure and look for the group:

• A C=C anywhere means an alkene.
• An –OH attached to a carbon chain means an alcohol.
• A –COOH at the end of the chain means a carboxylic acid.
• A –COO– joining two carbon fragments means an ester.
• Only single C–C and C–H bonds, nothing else, means an alkane.

Write the structural formula out fully if you are unsure, the group jumps out once every bond is drawn.

Identifying the group by chemical test

When you are given an unknown liquid, use the tests. These come up constantly in Paper 2 and Paper 3.

• Alkene (C=C): it decolourises brown bromine water quickly at room temperature, without sunlight, because the C=C undergoes an addition reaction. It also decolourises purple acidified potassium manganate(VII). An alkane, by contrast, only reacts with bromine slowly and needs ultraviolet light (substitution).
• Alcohol (–OH): it is oxidised by acidified potassium dichromate(VI), turning it from orange to green, producing a carboxylic acid. See oxidation of alcohols for the equation.
• Carboxylic acid (–COOH): it is acidic, it turns blue litmus red, reacts with a reactive metal to give hydrogen gas, and reacts with a carbonate to give carbon dioxide (which turns limewater cloudy).
• Ester (–COO–): it has a sweet, fruity smell and is made by warming an alcohol with a carboxylic acid and concentrated sulfuric acid as catalyst, see esterification.

The clue in the name

SPM names carry two pieces of information: the prefix tells you the number of carbon atoms (meth- = 1, eth- = 2, prop- = 3, but- = 4), and the suffix tells you the functional group:

• -ane → alkane (ethane)
• -ene → alkene (ethene)
• -ol → alcohol (ethanol)
• -oic acid → carboxylic acid (ethanoic acid)

So “propan-1-ol” is a three-carbon alcohol, and “butanoic acid” is a four-carbon carboxylic acid. Reading the suffix is often the quickest way to name the group.

Worked example

You are given a colourless liquid that decolourises bromine water at room temperature in the dark, and its formula is C₃H₆.

• The general formula CₙH₂ₙ fits an alkene (n = 3).
• Rapid decolourising of bromine water without UV light confirms a C=C double bond.
• So the compound is propene, and the functional group is the carbon–carbon double bond.

Now take a liquid that turns blue litmus red and fizzes with sodium carbonate. That acidic behaviour points straight to a carboxylic acid with the –COOH group, for example ethanoic acid.

Common mistakes to avoid

• Confusing the alkane and alkene bromine test. An alkene decolourises bromine water fast at room temperature; an alkane needs UV light and is slow. The speed and condition are the whole point.
• Forgetting the colour change for alcohols. Acidified dichromate goes orange to green, not the other way.
• Mixing up –COOH and –OH. An alcohol is neutral; a carboxylic acid is acidic. The test tells them apart.
• Reading the prefix as the group. The suffix names the functional group; the prefix only counts carbons.

Practise on mixed sets

Take a mixed list of formulae and names and, for each, state the functional group, the homologous series and one chemical test. Then do it the other way: given a test result, name the group. That two-way drill is what makes organic identification quick and secure. If the tests or the naming keep slipping, that is a common thing we sort out in lessons, our online one-to-one classes run in English from RM50 an hour, with a paid one-hour trial. See how it works if you would like organic chemistry drilled against real Paper 2 questions.