Nomenclature — AQA A-Level Chemistry
Test yourself on Nomenclature with AQA A-Level practice questions.
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Nomenclature explained
Six ways of writing the same molecule, each showing different information.
Read the full explanation
Take butane. Its empirical formula, the simplest whole number ratio, is C₂H₅. Its molecular formula, the actual numbers of atoms, is C₄H₁₀. The general formula for the alkane homologous series is CₙH₂ₙ₊₂. Its structural formula, CH₃CH₂CH₂CH₃, shows the arrangement without drawing every bond. Its displayed formula shows every atom and every bond, although bond angles need not be correct. Its skeletal formula is the plain zigzag, with carbons at the angles and ends and hydrogens on carbon left out. AQA insists that structures are unambiguous: C₃H₇Br will not do for 1-bromopropane, since it could equally be 2-bromopropane.
The characteristics of a homologous series, a series of compounds containing the same functional group.
A homologous series is a family of compounds that share one functional group, so its members share chemical properties. Four characteristics are worth learning. They fit one general formula, such as CₙH₂ₙ₊₂ for alkanes or CₙH₂ₙ₊₁OH for alcohols. Successive members differ by CH₂, so relative molecular masses rise in steps of 14. They react in the same way because the functional group is the same, so propan-1-ol behaves like butan-1-ol. And their physical properties change gradually along the series: boiling point rises as the chain lengthens, because a longer chain has more electrons and a greater surface contact, giving stronger London forces that need more energy to overcome.
IUPAC rules for nomenclature.
Naming is a fixed algorithm. Find the longest continuous carbon chain that contains the principal functional group and name it from the stem: meth, eth, prop, but, pent, hex. Add the suffix for that group, such as -ane, -ene, -ol, -al, -one or -oic acid. Number the chain from whichever end gives the lower locant to the functional group first and then to the substituents. List substituents as prefixes in alphabetical order, using di, tri and tetra for repeats but ignoring those multipliers when alphabetising, so ethyl comes before dimethyl. Punctuate with hyphens between numbers and letters and commas between numbers, as in 2,3-dimethylpentane. In naming questions AQA will not accept a phonetic misspelling.
Your focus
- Convert between the empirical, molecular, structural, displayed and skeletal representations of a given compound.
- Draw a displayed formula showing every bond and a skeletal formula showing every non-carbon atom.
- Explain why a molecular formula can be ambiguous where a structural formula is not.
Show all 9 objectives
- List the characteristics of a homologous series and illustrate each with the alkanes or the alcohols.
- Write the general formula of a given homologous series and use it to deduce a member's molecular formula.
- Explain the rise in boiling point along a homologous series in terms of London forces.
- Name a branched alkane, alkene, alcohol, aldehyde, ketone or carboxylic acid using IUPAC rules.
- Number a carbon chain so that the principal functional group receives the lowest possible locant.
- Deduce a structure from an IUPAC name and check it by renaming the structure you have drawn.
Nomenclature exam tips
Marking Points
- one mark for the empirical formula as the simplest whole number ratio of atoms
- one mark for a displayed formula showing every atom and every bond in the molecule
- one mark for a skeletal formula with carbon atoms shown by angles or intersections and all non-carbon atoms drawn in
- one mark for an unambiguous structural formula, such as CH₃CH₂CH₂Br rather than C₃H₇Br
- one mark for the same functional group in every member of the series
- one mark for the same general formula, such as CₙH₂ₙ₊₂ for alkanes
- one mark for successive members differing by CH₂, or by 14 in relative molecular mass
- one mark for a gradual trend in a physical property explained by increasing London forces
- one mark for the correct stem taken from the longest chain containing the functional group
- one mark for the correct suffix and locant for the principal functional group
- one mark for substituent prefixes in alphabetical order with correct locants
- one mark for correct punctuation, with hyphens between numbers and letters and commas between numbers
Examiner Tips
- 💡Read which type of formula is asked for, because a skeletal drawing scores nothing where displayed was specified.
- 💡In a displayed formula draw every C–H bond, even though the angles need not be accurate.
- 💡Show the carbon to carbon bond clearly whenever the functional group itself contains a carbon atom.
- 💡Give the general formula in terms of n and check it against a member you already know.
- 💡Name the intermolecular force explicitly when explaining a boiling point trend.
- 💡Keep chemical similarity and physical gradation as two separate points.
- 💡Count every possible chain before choosing the longest, because branched drawings hide it.
- 💡Write the name out fully and spell it correctly, since naming questions do not accept phonetic spelling.
- 💡Check the locant set is the lowest possible by numbering from both ends.
Common Mistakes
- giving a molecular formula such as C₃H₇Br where a structural formula was asked for, since it does not distinguish the isomers
- omitting the hydrogen atoms of a functional group in a skeletal formula, when only hydrogens on carbon may be left out
- drawing a displayed formula with an OH group shown as a single unit rather than as O bonded to H
- drawing the bond from the carbon chain to the wrong atom of a nitrile, carboxylic acid or aldehyde group
- saying members differ by CH₃ rather than by CH₂
- claiming all members have identical physical properties instead of a gradual trend
- explaining the boiling point trend by bigger molecules without naming London forces
- giving an example series whose members do not actually share a single functional group
- numbering from the wrong end, so writing pent-4-ene rather than pent-1-ene
- missing a longer chain that turns a corner in a skeletal structure
- alphabetising by the multiplier, so placing dimethyl under d instead of methyl under m
- writing 2 methylpentane or 2,3 dimethylpentane without the hyphens the convention requires