Organic Chemistry: Formulae and Naming

This guide covers Topic 11, Organic chemistry — subtopics 11.1 Formulae, functional groups and terminology and 11.2 Naming organic compounds — for Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. This is the vocabulary and notation the rest of Topic 11 depends on — read this first, before Petroleum, Alkanes and Alkenes, Alcohols and Carboxylic Acids, or Polymers.

Where this fits in 0620/5070

Every other organic subtopic assumes you can already draw a displayed formula, recognise a functional group, and name a compound from its formula — this subtopic is where those skills are built. It’s also the foundation for AS Level’s organic chemistry, which reuses the same formula and naming conventions at greater depth.

Syllabus coverage

CAMBRIDGE IGCSE CHEMISTRY 0620

Core

• Drawing and interpreting the displayed formula of a molecule, showing every atom and every bond (11.1)
• Writing and interpreting general formulae for alkanes (CₙH₂ₙ₊₂), alkenes (CₙH₂ₙ), alcohols (CₙH₂ₙ₊₁OH) and carboxylic acids (CₙH₂ₙ₊₁COOH) (11.1)
• Identifying a functional group as the atom/group of atoms that determines a homologous series’ chemical properties (11.1)
• Stating that a homologous series is a family of similar compounds with similar chemical properties, sharing the same functional group (11.1)
• Defining saturated (all C–C bonds single) and unsaturated (one or more C–C bonds not single) compounds (11.1)
• Naming and drawing methane, ethane, ethene, ethanol and ethanoic acid, and naming compound types from the ending -ane/-ene/-ol/-oic acid (11.2)

Supplement / Extended

• Defining a structural formula as an unambiguous description of atom arrangement, e.g. CH₂=CH₂, CH₃CH₂OH, CH₃COOCH₃ (11.1)
• Defining structural isomers — same molecular formula, different structural formula — with butane/methylpropane and but-1-ene/but-2-ene as examples (11.1)
• Describing the general characteristics of a homologous series: same functional group, same general formula, differ by –CH₂– between members, trend in physical properties, similar chemical properties (11.1)
• Naming and drawing unbranched alkanes, alkenes (including but-1-ene and but-2-ene), alcohols (including propan-1-ol/2-ol and butan-1-ol/2-ol) and carboxylic acids up to four carbons (11.2)
• Naming and drawing esters made from unbranched alcohols and carboxylic acids up to four carbons each (11.2)

CAMBRIDGE O LEVEL CHEMISTRY 5070

5070 has no Core/Extended split — every outcome above, Core and Supplement alike, is required for every O Level candidate.

Displayed and general formulae

A displayed formula shows every atom and every bond explicitly — this is the most detailed way to represent a molecule, and the one examiners expect when asking you to “draw the structure.”

A general formula describes an entire homologous series in one expression, using n for the number of carbon atoms:

Series	General formula	n=2 example
Alkanes	CₙH₂ₙ₊₂	C₂H₆ (ethane)
Alkenes	CₙH₂ₙ	C₂H₄ (ethene)
Alcohols	CₙH₂ₙ₊₁OH	C₂H₅OH (ethanol)
Carboxylic acids	CₙH₂ₙ₊₁COOH	CH₃COOH (ethanoic acid)

Worked example. Use the general formula to find the molecular formula of the alkane with 5 carbon atoms.

alkane general formula: CnH2n+2, with n = 5
C5H(2x5)+2 = C5H12   (pentane)

Functional groups and homologous series

A functional group is the specific atom or group of atoms responsible for a homologous series’ characteristic chemical reactions — the –OH group in alcohols, the –COOH group in carboxylic acids, the C=C double bond in alkenes.

A homologous series is a family of compounds sharing that functional group, and therefore sharing similar chemical behaviour. The full set of characteristics (Extended/5070) is:

• Same functional group
• Same general formula
• Each member differs from the next by a –CH₂– unit
• A trend in physical properties (e.g. boiling point rising with chain length)
• Similar chemical properties across the series

Saturated and unsaturated

A saturated compound has only single carbon–carbon bonds — alkanes are saturated. An unsaturated compound has at least one carbon–carbon bond that isn’t single — alkenes, with their C=C double bond, are unsaturated. This distinction drives the different reactivity covered in Petroleum, Alkanes and Alkenes: alkanes mostly undergo substitution, alkenes mostly undergo addition.

Structural isomers

Structural isomers share the same molecular formula but have a different structural formula — the atoms are connected differently.

Worked example. C₄H₁₀ has two structural isomers. Draw and name both.

CH3CH2CH2CH3   — butane (straight chain)
CH3CH(CH3)CH3  — methylpropane (branched chain, "2-methylpropane")

Both have exactly 4 carbon and 10 hydrogen atoms — the formula alone doesn’t tell you which arrangement you’re looking at, which is exactly why structural (not just molecular) formulae matter.

Naming organic compounds

Naming follows the compound’s functional group directly from its ending:

Ending	Compound type	Example
-ane	Alkane	ethane
-ene	Alkene	ethene
-ol	Alcohol	ethanol
-oic acid	Carboxylic acid	ethanoic acid

The stem (meth-, eth-, prop-, but-) tells you the number of carbon atoms (1, 2, 3, 4); the ending tells you the functional group. For branched-chain or longer compounds (Extended/5070), a locant number shows where on the chain the feature sits — propan-1-ol has the –OH on the end carbon, propan-2-ol has it on the middle carbon.

Esters are named from the alcohol and acid that formed them: an alcohol ending in -yl, followed by the acid’s -oate — ethanol + ethanoic acid gives ethyl ethanoate.

Common mistakes

• Confusing molecular formula with structural/displayed formula. A molecular formula (like C₄H₁₀) can match more than one real compound — always check whether the question wants the arrangement, not just the atom count.
• Mixing up general formula variables. CₙH₂ₙ (alkene) and CₙH₂ₙ₊₂ (alkane) differ by only “+2” — a very easy typo to make under exam pressure; double-check which series you’re working with.
• Forgetting the locant number changes the compound. Propan-1-ol and propan-2-ol are different compounds with different structures, not two names for the same thing.
• Assuming “isomers” means “different compounds with different formulae.” It’s the opposite — isomers specifically share the same molecular formula.

Quick revision checklist

• Drawing a displayed formula showing every atom and bond
• General formulae for alkanes, alkenes, alcohols and carboxylic acids
• Functional group and homologous series, defined and linked together
• Saturated vs unsaturated, and which series is which
• (0620 Extended, 5070 required) structural isomers, with a worked example
• Naming compounds from the -ane/-ene/-ol/-oic acid endings
• (0620 Extended, 5070 required) naming and drawing unbranched compounds with locant numbers, and naming esters

Related resources

• Petroleum, Alkanes and Alkenes — the hydrocarbon chemistry this vocabulary describes
• Alcohols and Carboxylic Acids — the two oxygen-containing series named here
• Polymers — repeat units built from the monomers named here
• Cambridge IGCSE Chemistry hub · Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.