How to explain homologous series and general formulae

Organic chemistry looks intimidating until you spot the pattern hiding inside it: the millions of carbon compounds are organised into tidy families called homologous series. Once you understand what a homologous series is and how a general formula works, you can predict the formula of any member and describe how properties change down the family. This guide explains both ideas for SPM, using the families in carbon compounds.

What a homologous series is

A homologous series is a family of organic compounds that share four things:

1. The same general formula.
2. The same functional group (the reactive part of the molecule).
3. Similar chemical properties, because they share that functional group.
4. A gradual change in physical properties (like boiling point) as the molecules get bigger.

Consecutive members differ by one CH₂ unit. That single, repeating difference is why the whole family can be captured by one general formula.

What a general formula means

A general formula uses n to stand for the number of carbon atoms, so one short expression generates every member. Put in n = 1, 2, 3… and you get the individual molecular formulae.

Here are the SPM families you must know:

• Alkanes: general formula CₙH₂ₙ₊₂. Functional group: only single C–C bonds (saturated). Example: for n = 3, C₃H₈ (propane).
• Alkenes: general formula CₙH₂ₙ. Functional group: a carbon–carbon double bond, C=C (unsaturated). Example: for n = 2, C₂H₄ (ethene).
• Alcohols: general formula CₙH₂ₙ₊₁OH. Functional group: the hydroxyl group, –OH. Example: for n = 2, C₂H₅OH (ethanol).
• Carboxylic acids: general formula CₙH₂ₙ₊₁COOH. Functional group: the carboxyl group, –COOH. Example: for n = 1, CH₃COOH (ethanoic acid).

If you learn the four general formulae, you can write any member on demand. You can cross-check the functional groups against the organic functional groups reference.

Worked example: building the alkanes

Use CₙH₂ₙ₊₂ and just substitute:

• n = 1: CH₄ (methane)
• n = 2: C₂H₆ (ethane)
• n = 3: C₃H₈ (propane)
• n = 4: C₄H₁₀ (butane)

Notice each step adds exactly CH₂: CH₄ → C₂H₆ adds one C and two H. That is the homologous “step” in action. The same substitution trick works for every family, this is why the general formula is so powerful.

Why chemical properties stay similar

Members of a series react in the same way because they carry the same functional group. All alkenes decolourise bromine water because they all have a C=C bond that undergoes addition reactions. All alcohols can be oxidised and can burn, because they all carry –OH. So if you know how ethene behaves, you can predict propene; if you know ethanol, you can predict propanol. Learn the reaction of the functional group once, and it applies to the whole family.

Why physical properties change gradually

As you go down a series, molecules get larger and heavier, so:

• Boiling and melting points rise, because bigger molecules have stronger forces between them and need more energy to separate.
• Density generally increases and volatility decreases.
• The state at room temperature shifts: small alkanes (methane, ethane) are gases, middle ones are liquids, and long-chain ones are waxy solids.

This is why the change is described as gradual, not sudden, each extra CH₂ nudges the properties a little further along.

A quick way to name the position

The first part of each name gives the carbon number: meth- (1), eth- (2), prop- (3), but- (4). The ending gives the family: -ane (alkane), -ene (alkene), -anol (alcohol), -anoic acid (carboxylic acid). So “propene” instantly tells you three carbons and a C=C double bond, n = 3 in CₙH₂ₙ, which is C₃H₆.

Common mistakes to avoid

• Mixing up alkane and alkene formulae. Alkanes are CₙH₂ₙ₊₂ (saturated); alkenes are CₙH₂ₙ (one double bond).
• Forgetting that members differ by CH₂, not CH or CH₃.
• Writing an alcohol as CₙH₂ₙ₊₂, remember the –OH replaces one H, giving CₙH₂ₙ₊₁OH.
• Saying properties are identical down a series. Chemical properties are similar; physical properties change gradually.

Turn the pattern into marks

Homologous series reward understanding over memorising. If you know the four general formulae, the meaning of a functional group, and the two trends (similar chemistry, gradual physical change), you can answer a large slice of the organic section confidently. Practise by writing out the first four members of each family and naming them.

If organic families still feel like a jumble, a teacher can make the pattern click quickly. Our online one-to-one lessons run in English from RM50 an hour, with a paid one-hour trial; see how it works if you would like organic chemistry taught around these patterns.