Nomenclature contd.
Chemistry: IB Diploma Higher Level, topic 3.6.4
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.
Test yourself on Nomenclature contd.
What is the term for the naming conventions of molecules?
Which of these suffixes is correct?