Naming functional groups (IUPAC)

Key structures for this topic — drawn live.

The idea: one group gets the suffix

An IUPAC name is built around a parent chain, and one functional group — the highest-priority one present — is chosen as the principal characteristic group. That group determines the name's suffix and is given the lowest possible locant (position number). Every other functional group present is demoted to a prefix. So the first job in naming a polyfunctional molecule is to rank the groups and pick the winner.

The priority order

From highest to lowest priority for the suffix, the common groups you'll meet in a first orgo course are:

1. Carboxylic acid — R–COOH
2. Ester — R–COO–R'
3. Amide — R–CONH₂
4. Nitrile — R–C≡N
5. Aldehyde — R–CHO
6. Ketone — R–CO–R'
7. Alcohol — R–OH
8. Amine — R–NH₂

(A fuller list places carboxylic acid derivatives — anhydrides, esters, acyl halides, amides — above nitriles, then the carbonyls, then alcohols, amines, and finally C=C/C≡C and alkyl/halogen substituents, which are always prefixes.) The single most common exam skill is knowing that a carboxylic acid outranks a ketone, which outranks an alcohol, and so on.

The suffixes

Each group has a characteristic suffix used when it's the principal group:

• Carboxylic acid → -oic acid (e.g., butanoic acid)
• Ester → -oate, preceded by the alkyl group name (e.g., methyl butanoate)
• Amide → -amide (e.g., butanamide)
• Nitrile → -nitrile (e.g., butanenitrile)
• Aldehyde → -al (e.g., butanal)
• Ketone → -one (e.g., butan-2-one)
• Alcohol → -ol (e.g., butan-1-ol)
• Amine → -amine (e.g., butan-1-amine)

The C=C double bond and C≡C triple bond are handled with the -ene and -yne endings on the parent chain (e.g., but-2-ene, but-1-yne). Degrees of unsaturation always affect the name but never outrank a true functional group for the suffix.

Lower-priority groups become prefixes

When a group isn't the principal one, it's named as a substituent prefix instead of a suffix. Common prefix forms:

• Alcohol as a substituent → hydroxy-
• Ketone/carbonyl as a substituent → oxo-
• Amine as a substituent → amino-
• Halogens → fluoro-, chloro-, bromo-, iodo-
• Alkyl chains → methyl-, ethyl-, etc.

So a molecule that has both an –OH and a higher-priority –COOH is named as an acid with a "hydroxy" prefix, not as an alcohol.

Numbering and assembling the name

Once you've chosen the principal group, number the parent chain so that group gets the lowest locant — its priority beats the general "lowest set of locants" rule. Then:

1. Identify the longest chain that contains the principal characteristic group; that's the parent.
2. Number from the end that gives the principal group the lowest number.
3. Name substituents as prefixes, alphabetized, each with its locant.
4. Attach the suffix for the principal group with its locant.

A worked example

Take a five-carbon chain with a carboxylic acid on C1, a ketone at C4, and a hydroxyl on C5: HOOC–CH₂–CH₂–CO–CH₂OH.

• The highest-priority group is the carboxylic acid, so the suffix is -oic acid and it takes C1.
• The parent is a five-carbon chain → pentanoic acid.
• The ketone at C4 can't be the suffix (acid outranks it), so it's a prefix: 4-oxo.
• The alcohol at C5 is also outranked, so it's a prefix: 5-hydroxy.
• Alphabetize the prefixes (hydroxy before oxo): 5-hydroxy-4-oxopentanoic acid.

That single name tells a reader the chain length, every functional group, and exactly where each one sits — which is the whole point of the IUPAC system. Master the priority list and the matching suffixes, and even messy polyfunctional molecules become straightforward to name.