3.1.3 Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) has developed systematic rules for naming organic compounds. These rules ensure consistency so that chemists around the world refer to compounds in the same way. Here are the essential steps for naming organic compounds, focusing on chains and rings with up to six carbon atoms.

Identifying the Longest Carbon Chain

The first step in naming an organic compound is identifying the longest continuous carbon chain. The number of carbon atoms in this chain determines the stem of the compound's name:

• 1 carbon: Meth-
• 2 carbons: Eth-
• 3 carbons: Prop-
• 4 carbons: But-
• 5 carbons: Pent-
• 6 carbons: Hex- If the chain forms a ring, the prefix cyclo- is added before the stem. For example, a six-carbon ring would be named cyclohexane.

Identifying Branches and Functional Groups

Next, identify any branches (alkyl groups) or functional groups attached to the main chain. Functional groups are indicated with a prefix or suffix depending on their position and priority.

Common functional groups

• $-OH$ (hydroxyl group) for alcohols
• $-COOH$ (carboxyl group) for carboxylic acids
• $-NH_2$ (amino group) for amines

Numbering the Carbon Chain

The carbon chain must be numbered so that the branches and functional groups are given the lowest possible numbers. Always start numbering from the end closest to the first branch or functional group.

For example, in pentan-2-ol, the hydroxyl group is on the second carbon of a five-carbon chain.

Multiple Functional Groups or Branches

If multiple identical functional groups or branches are present, use prefixes to indicate their number:

• di- for two
• tri- for three
• tetra- for four

For example, 2,2-dimethylpropane indicates two methyl groups on the second carbon of a propane chain.

Ordering Groups Alphabetically

When multiple different branches or functional groups are present, they should be listed alphabetically in the name, regardless of their position. Note that prefixes like di-, tri-, and tetra- are not considered in alphabetical order.

For example, in 3-ethyl-2-methylpentane, ethyl comes before methyl in the name.

Prioritising Functional Groups

If more than one functional group is present, the following order of priority is used to determine the suffix:

• Lowest priority: Halogens (e.g., $-Cl$, $-Br$)
• Alkyl groups (e.g., $-CH_3$, $-CH_2CH_3$)
• Alkenes ($-C=C-$)
• Highest priority: Other functional groups like carboxylic acids ($-COOH$) or alcohols ($-OH$)

The group with the highest priority is assigned the suffix, and other groups are treated as prefixes.