Naming Conventions

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Formulae

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

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

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

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

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

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.

IUPAC rules

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

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

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

Complex molecules

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

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

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

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

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

Every homologous series has its own general formula:
- E.g. Alkanes: C_nH_2n+2
- E.g. Alkenes: C_nH_2n
- E.g. Alcohols: C_nH_2n+1OH
The general formula gives the molecular formula for each compound in the homologous series.
Different molecules in the series differ mainly in the length of their aliphatic chains.
- Aliphatic chains are carbon chains that do not have a benzene ring.

Functional groups

A functional group is a group of atoms that characterise the chemistry of a molecule. Functional groups cause specific reactions to happen with certain reagents.
Examples of functional groups are:
- Hydroxyl group (-OH).
- Carboxylic acid group (-COOH) (pictured).
- Carbon-carbon double bond (C=C).

Alkyl group

An alkyl group is a saturated hydrocarbon chain attached to a molecule.
- The letter R can be used to represent alkyl groups.

Aliphatic

Aliphatic compounds contain carbon chains that do not involve a benzene ring.
Aliphatic chains can be saturated, unsaturated, straight, branched or circular.

Alicyclic

Alicyclic compounds are aliphatic compounds that have non-aromatic rings but no side chains.
- E.g. Cyclohexane.

Aromatic

Aromatic compounds contain at least one benzene ring.
Benzene rings tend to make these compounds smell.
- This is why these compounds are described as aromatic.

Saturated/unsaturated

Saturated compounds contain single bonds only between carbon atoms (C-C).
Unsaturated compounds contain at least one different carbon-carbon bond such as C=C or an aromatic ring.