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Naming Compounds

Applied Science: BTEC Level 3 National Extended Diploma, topic 4.3.1

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.
  • They can have multiple functional groups or side chains.
  • 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.

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.

Test yourself on Naming Compounds

  • What is the term for the naming conventions of molecules?

  • Which of these suffixes is correct?

  • 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?

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