Keep the functional group and just keep adding carbons. What you get is a family whose chemistry stays the same while its physical properties change in a smooth, predictable way — which means you can work out the properties of a molecule you have never met.
📚 What you need to know
A homologous series is a family of compounds with the same functional group and the same general formula.
Consecutive members differ by –CH2–, called the homologous increment.
All members show similar chemical properties, because they share the functional group.
Physical properties such as boiling point change gradually along the series.
Boiling points rise as the chain lengthens, because larger molecules have stronger London dispersion forces.
Carbon’s ability to bond to itself in chains and rings is called catenation, and it is why organic chemistry is so vast.
Catenation: why there are so many compounds
Carbon forms four strong covalent bonds, and it will happily bond to itself. That property is called catenation, and it lets carbon build straight chains, branched chains and rings of essentially any length. Add a handful of functional groups to that and the number of possible molecules becomes effectively unlimited.
Rather than treat every one as a special case, we sort them into families.
What makes a homologous series
Members of a homologous series share four things and differ in one.
The same functional group
The same general formula
Similar chemical properties
The same methods of preparation
But a steadily changing set of physical properties
Every step up the series is the same step: one more carbon, two more hydrogens, same functional group.
One extra carbon and two extra hydrogens each step — that is the whole pattern. It’s why the general formula works, and why the boiling points climb so smoothly.
General formulas
Every series can be written as a single formula in terms of n, the number of carbon atoms.
Series
General formula
Example (n = 3)
alkanes
CnH2n+2
propane, C3H8
alkenes
CnH2n
propene, C3H6
alkynes
CnH2n–2
propyne, C3H4
alcohols
CnH2n+1OH
propan-1-ol, C3H7OH
halogenoalkanes
CnH2n+1X
chloropropane, C3H7Cl
aldehydes
CnH2nO, written RCHO
propanal, C3H6O
ketones
CnH2nO, written RCOR
propanone, C3H6O
carboxylic acids
CnH2n+1COOH
propanoic acid, C2H5COOH
ethers
CnH2n+2O, written ROR
methoxyethane, C3H8O
amines
CnH2n+1NH2
propylamine, C3H7NH2
Spot the overlap. Aldehydes and ketones share the general formula CnH2nO, and so do alkenes and cycloalkanes with CnH2n. Two compounds with the same general formula are not necessarily in the same series — that becomes functional group isomerism later.
Why boiling points rise
This is the one physical trend you must be able to explain, and the reasoning has three steps.
Two effects at once. Along each line, chain length raises the boiling point; between the lines, hydrogen bonding does.
🧩 The explanation examiners want
Each extra –CH2– increases the number of electrons and the surface area of the molecule.
More electrons means stronger London (dispersion) forces between molecules.
Stronger intermolecular forces need more energy to overcome, so the boiling point rises.
The graph also shows something the alkanes alone would hide. At every chain length, the alcohols boil far higher — ethanol boils at 78 °C while ethane boils at –89 °C. That gap is nothing to do with chain length. It is hydrogen bonding between the –OH groups, which is much stronger than dispersion forces alone.
Notice too that the two lines slowly converge. As the chain gets longer, the hydrocarbon part dominates and the single hydrogen bond matters proportionally less.
Two variables, two explanations. Along a series, it is chain length and dispersion forces. Between series, it is the type of intermolecular force the functional group allows. Say which one you’re talking about.
The same reasoning explains other trends: as the chain lengthens, melting point and viscosity increase, while solubility in water decreases — the growing non-polar chain outweighs the small polar group.
WORKED EXAMPLE
An alkane has 7 carbon atoms. Deduce its molecular formula and the formula of the alcohol with the same chain length.
Alkanes are CₙH₂ₙ₊₂, with n = 7H = (2 × 7) + 2 = 16C₇H₁₆ — heptaneAlcohols are CₙH₂ₙ₊₁OHH = (2 × 7) + 1 = 15C₇H₁₅OH, i.e. C₇H₁₆O
WORKED EXAMPLE
Explain why butane boils at –1 °C but butan-1-ol boils at 118 °C, even though both have four carbons.
Same chain length, so dispersion forces are similarThe difference cannot be chain length — it must be the functional group.Butane is non-polar: London dispersion forces onlyButan-1-ol has an –OH groupIts molecules form hydrogen bonds with each other, which are much stronger.more energy needed → much higher boiling point
💡 Exam tip
Name the force. Say “London dispersion forces“, not just “intermolecular forces”, and say why they get stronger.
Comparing along a series → talk about chain length and electrons. Comparing between series → talk about hydrogen bonding or dipoles.
Learn the general formulas properly — they let you deduce a formula from a name in one line.
“Similar chemical properties, gradually changing physical properties” is the phrase to reach for.
⚠️ Common mix-up
Boiling points rise because forces BETWEEN molecules get stronger — the covalent bonds inside the molecule are not broken.
Members differ by CH2, not by CH3.
Same general formula does not mean same series — aldehydes and ketones share CnH2nO.
Chemical properties stay similar; physical ones change. Getting these the wrong way round is a common slip.
Methane has no C–C bond, but it is still the first alkane.
Up next: IUPAC Naming — the system that turns any of these structures into a name that describes it exactly, with no ambiguity.
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