Formulae and Homologous Series

Organic compounds are based on carbon chains. A homologous series is a family with the same functional group and general formula, similar chemical properties and a trend in physical properties; each member differs by $\mathrm{CH_2}$ . Names come from the chain length (meth 1, eth 2, prop 3, but 4) and the series ending: -ane (alkanes, $\mathrm{C_nH_{2n+2}}$ ), -ene (alkenes, $\mathrm{C_nH_{2n}}$ ), -ol (alcohols), -oic acid (carboxylic acids). Structural isomers have the same molecular formula but different structures.

Cambridge 5070 Paper 1Cambridge 5070 Paper 2Cambridge 0620 Papers 2, 4Cambridge 0620 Papers 1, 3Edexcel 4CH1 Paper 1CEdexcel 4CH1 Paper 2C≈ 40 minRevision 1 · updated 2026-10-30

Section 1 of 14

Learning objectives

Write molecular, structural, displayed and general formulae.
Name and identify alkanes, alkenes, alcohols and carboxylic acids.
Describe homologous series and identify structural isomers.

Pause and think

Carbon makes more compounds than all the other elements put together. Why?

Section 2 of 14

Core idea

Start hereTen million compounds

Chemists have catalogued well over ten million carbon compounds. Without a naming system and families of similar compounds, organic chemistry would be impossible to learn.

The big idea

Organic compounds are grouped into homologous series by functional group. Members share a general formula and similar reactions, and their names tell you the chain length and the series.

1. Four kinds of formula

Worked examplePropanoic acid

Write the molecular, structural and general formula information for propanoic acid.

Goal: Each type of formula.
Strategy: 3 carbons including the COOH carbon.

$\mathrm{C_3H_6O_2}$
Molecular: total atoms.
$\mathrm{CH_3CH_2COOH}$
Structural: atom groupings in order.
$\mathrm{C_nH_{2n+1}COOH},\ n=2$
General formula of carboxylic acids.

Answer

\mathrm{C_3H_6O_2}

;

\mathrm{CH_3CH_2COOH}

✓ CheckH count:

3+2+1=6

✓.

2. Naming

Building a name

Drawing diagram…

Stem from chain length, ending from functional group.

3. Isomers

$\mathrm{C_4H_{10}}$ can be a straight chain (butane) or branched (methylpropane). Both have the same molecular formula but different structures, so slightly different properties: the branched isomer boils lower because its molecules pack less closely.

Common misconceptionRotated drawings Bending a chain on paper doesn't make an isomer. Check the bonding: which atom is attached to which.

Pause and think

Why do all alcohols react with sodium in a similar way?

Summary

Homologous series: same functional group, general formula.
Meth/eth/prop/but + ending.
Isomers: same molecular formula, different structure.

Functional group: The atom or group that gives a series its characteristic reactions.
Homologous series: Family of compounds with the same functional group and general formula.
Structural isomers: Same molecular formula, different structural formula.

HistoryWöhler's urea. People once believed organic compounds could only be made by living things. In 1828 Friedrich Wöhler made urea from inorganic chemicals, overturning that idea.

Fun factIsomer surprises. Ethanol (in drinks) and methoxymethane (a spray-can propellant) are isomers, both

\mathrm{C_2H_6O}

. One is a liquid at room temperature, the other a gas.

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