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Naming Conventions

Chemistry: AQA A Level, topic 5.1.1

Formulae

There are multiple kinds of formula that we use to represent molecules. They all give different information about a molecule.

Empirical formulae

Empirical formulae

  • An empirical formula is the simplest whole number ratio of atoms in a molecule.
  • E.g. CH3 is the empirical formula of ethane.
  • E.g. CH is the empirical formula of benzene.
Molecular formulae

Molecular formulae

  • The molecular formula of a compound gives the number of each type of atom in a molecule.
  • E.g. C2H6 is the molecular formula of ethane.
Structural formulae

Structural formulae

  • The structural formula is a written one-line formula showing which groups are connected to which.
  • E.g. CH3CH3 is the structural formula of ethane.
  • E.g. CH3CH2CH2CH3 is the structural formula of butane.
Displayed formulae

Displayed formulae

  • A displayed formula is a diagram which shows every bond in a molecule.
Skeletal formulae

Skeletal formulae

  • A skeletal formula is a simplified displayed formula.
  • Hydrogen atoms are not shown and carbon atoms are at the corners of any lines.
  • A skeletal formula shows the 'skeleton' of the molecule, hence the name.

Homologous Series

A homologous series is a set of compounds with the same functional group. Members of a homologous series have very similar properties because they have the same functional group.

General formulae

General formulae

  • Every homologous series has its own general formula. Examples include:
  • Alkanes: CnH2n+2
  • Alkenes: CnH2n
  • Alcohols: CnH2n+1OH
  • The general formula gives the molecular formula for each compound in the homologous series.
Boiling points of alkanes

Boiling points of alkanes

  • The boiling point of alkanes increases with chain length.
  • This is a good example of a trend in a homologous series.
  • Branched alkanes have lower boiling points as there is reduced contact between the molecules.

IUPAC rules

Molecules are named following the internationally-recognised IUPAC rules. The rules for naming compounds is known as 'nomenclature'.

Name stem

Name stem

  • The stem of a name is the first part of the name. It is decided by the number of carbon atoms:
  • 1 carbon → meth-
  • 2 carbons → eth-
  • 3 carbons → prop-
  • 4 carbons → but-
  • 5 carbons → pent-
Name ending

Name ending

  • The functional group gives the ending to the name.
  • Carboxylic acid: → Propanoic acid.
  • Alcohol: → ethanol.
  • Ketone: → propanone.
  • Aldehyde: → ethanal.
Complex molecules

Complex molecules

  • Most molecules are more complex. This means they have multiple functional groups or side chains.
Naming conventions

Naming conventions

  • The molecule is always named after the longest possible carbon chain that includes the functional group.
  • The carbons are numbered to give the functional group the lowest number possible.
  • Side chains are written as prefixes to the name.
An example

An example

  • The molecule shown above is 4-methyl-hexan-1-ol, NOT 2-methyl-hexan-6-ol or 4-ethyl-pentan-1-ol.
  • First, count the longest chain.
  • This gives a hexanol.
  • Put the alcohol at the lowest number.
  • Now name the position of the side chain.
Rings

Rings

  • For molecules that contain rings, add 'cyclo-' at the start of the name:
  • E.g. Cyclohexane.
  • E.g. Cyclohexanol.
  • E.g. Cyclohexanone.

Test yourself on Naming Conventions

  • What type of formula is shown in this diagram?

  • What is the empirical formula of benzene?

  • Which of the following are accurate descriptions of formulae?

  • Which of these general formulae are correct?

  • Which of the following is true of alkane boiling points?

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