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Nomenclature and formulaeAQA A-Level Chemistry: Revision notes

Section 1

Types of formula

Organic compounds can be represented in several ways. Using butan-1-ol as an example:

  • Empirical formula: the simplest whole-number ratio of atoms of each element, C₄H₁₀O
  • Molecular formula: the actual number of atoms of each element, C₄H₁₀O
  • General formula: an algebraic formula for a homologous series, such as CₙH₂ₙ₊₂ for alkanes
  • Structural formula: shows the arrangement of atoms unambiguously without every bond, CH₃CH₂CH₂CH₂OH
  • Displayed formula: shows all the atoms and all the bonds
  • Skeletal formula: shows the carbon skeleton as lines, with the functional group atoms and no C or H labels on carbon
Key termsempirical formulamolecular formulageneral formulastructural formuladisplayed formulaskeletal formula
Exam tip

The empirical formula of hexane, C₆H₁₄, is C₃H₇. Always divide by the highest common factor.

Section 2

Homologous series and functional groups

A homologous series is a series of compounds containing the same functional group. Members have the same general formula, differ by a CH₂ unit, show similar chemical properties and show a gradual change in physical properties. Common series:

  • alkanes (C–C only): -ane
  • alkenes (C=C): -ene
  • halogenoalkanes (C–Cl, C–Br, C–I): chloro-, bromo-, iodo-
  • alcohols (–OH): -ol
  • aldehydes (–CHO): -al
  • ketones (C=O within the chain): -one
  • carboxylic acids (–COOH): -oic acid
Key termshomologous seriesfunctional group

Section 3

IUPAC rules for naming

Follow these steps:

  1. Find the longest carbon chain (containing the functional group) for the stem: meth-, eth-, prop-, but-, pent-, hex- for 1 to 6 carbons. Rings use cyclo-.
  2. Add the suffix for the functional group, such as -ol, -ene, -al, -one or -oic acid.
  3. Number the chain from the end that gives the lowest locant to the functional group, then to any C=C, then to the substituents.
  4. List substituents in alphabetical order before the stem, with a locant for each. Use di-, tri- and tetra- for repeats, which are ignored when ordering alphabetically.
  5. Use commas between numbers and hyphens between numbers and letters.
Key termslocantstemsubstituent
Common mistake

Choosing the first chain you see. The longest chain may bend around the page.

Section 4

Worked examples

CH₃CH(CH₃)CH(CH₃)CH₃: the longest chain is four carbons (butane) with two methyl groups on carbons 2 and 3, so 2,3-dimethylbutane.

CH₃CHBrCH₂Cl: three-carbon chain with Br and Cl. Number from the CH₂Cl end to get locants 1,2. Alphabetical order puts bromo first: 2-bromo-1-chloropropane.

CH₃CH(CH₃)CH(OH)CH₂CH₃: five-carbon chain, OH on carbon 3 from either end, so the methyl takes the lowest number: 2-methylpentan-3-ol.

CH₃CH=CHCH₂CH₃: five carbons, C=C starting on carbon 2: pent-2-ene.

Key termsalphabetical order

Section 5

Drawing structures from names

Work in reverse. For 3-methylhex-1-ene: hex = six carbons in a chain, -1-ene = a double bond between carbons 1 and 2, and a methyl group on carbon 3. The structural formula is CH₂=CHCH(CH₃)CH₂CH₂CH₃. Check that every carbon has four bonds. For rings, cyclohexene has a six-membered ring with one C=C. In a skeletal formula each corner and line end is a carbon atom, and the hydrogen atoms on carbon are not drawn.

Key termsskeletal formula
Exam tip

Count carbons in your structure and compare with the stem in the name.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Nomenclature and formulae

  1. Petrol contains a mixture of alkanes. A chemist analyses one branched alkane with the structural formula CH₃CH(CH₃)CH₂CH₂CH₃ and compares it with the straight-chain alkane that has the same number of carbon atoms.
    State what is meant by a homologous series.2 marks
  2. A solvent supplier labels its bottles with systematic names. The stock list includes an alcohol with the structural formula CH₃CH(OH)CH₂CH₃, a dihalogenoalkane with the structural formula CH₃CHBrCH₂Cl, and a halogenoalkane with the structural formula (CH₃)₃CCl.
    Write the structural formula of 2-chloro-2-methylpropane and give its molecular formula.2 marks
  3. A stock room holds three hydrocarbons. Compound P has the structural formula CH₃CH=CHCH₂CH₃. Compound Q has the structural formula CH₂=C(CH₃)CH₂CH₃. Compound R is a ring of five carbon atoms joined by single bonds, in which one carbon atom carries a methyl group.
    Give the IUPAC names of P, Q and R.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).