Chemistry · General chemistry II · Concept
Naming organic compounds by functional group
A functional group decides a compound’s class and the ending of its name. The parent chain must contain the group, and it is numbered to give the group the lowest number: 2-pentanol, 2-pentanone. Aldehydes and acids have the group at carbon 1, so they need no number. Esters are named for the alkyl group on the oxygen, then the acid with -oate; ethers and amines are named for the groups on the oxygen or nitrogen. Benzene rings take their substituents as prefixes, with familiar names such as toluene and phenol.
Alcohols: -ol
Choose the longest chain containing the carbon with the –OH group, number it to give that carbon the lowest number, and put the number before the name: 2-propanol, 3-methyl-1-butanol. Today’s IUPAC form puts it inside: propan-2-ol.
Aldehydes and ketones: -al and -one
An aldehyde’s carbonyl group is at the end of the chain, so its carbon is carbon 1 and needs no number: butanal. A ketone’s carbonyl group is inside the chain; number to give it the lowest number: 2-pentanone, 3-hexanone. Propanone and butanone need no number, because the carbonyl group can only be on carbon 2.
Carboxylic acids and esters: -oic acid and -oate
A carboxylic acid is named for its chain with -oic acid: butanoic acid. An ester is named like the salt of an acid: the alkyl group on the single-bonded oxygen comes first, then the acid’s name with -ate: methyl butanoate.
Ethers and amines
Ethers are named by the two groups on the oxygen, in alphabetical order, followed by ether: ethyl methyl ether, diethyl ether. Amines are named by the groups on the nitrogen, followed by -amine: ethylamine, ethylmethylamine, trimethylamine. Today’s IUPAC rules also allow substitutive names such as methoxyethane and ethanamine.
Benzene derivatives
One group on a benzene ring is a prefix to benzene, as in ethylbenzene and chlorobenzene, but several have familiar names: toluene (methyl), phenol (–OH), aniline (–NH₂) and styrene (–CH=CH₂). With two groups, number the ring so the group first in alphabetical order is at carbon 1: 1-chloro-3-ethylbenzene. Two identical groups can also be ortho (1,2), meta (1,3) or para (1,4): p-dichlorobenzene.
Common mistakes
- Numbering an alcohol from the end that gives the –OH the higher number, as in 4-pentanol.
- Giving an aldehyde a number or a ketone the ending -al.
- Writing an ester’s name in the wrong order: methyl butanoate and butyl methanoate are different compounds.
- Numbering a disubstituted benzene without starting at the group first in the alphabet.
Key terms
- Alcohol
- An organic compound with a hydroxyl group attached to a saturated carbon atom.
- Aldehyde
- A compound with a carbonyl carbon bonded to at least one hydrogen, conventionally written R–CHO.
- Ketone
- A compound with a carbonyl carbon bonded to two carbon groups.
- Carboxylic acid
- An organic acid containing a carboxyl group, –C(=O)OH.
- Ester
- A carboxylic-acid derivative containing the unit –C(=O)OR, where R is a carbon chain or ring.
- Ether
- An organic compound with an oxygen atom singly bonded to two carbon groups.
- Amine
- An organic derivative of ammonia in which one or more hydrogens are replaced by carbon groups.
- Functional group
- A characteristic atom or group of atoms that strongly influences an organic compound’s reactions and properties.
Work through an example
Name (CH₃)₂CHCH₂CH₂OH.
Name a branched alcohol →Sources and scope
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Draw it and check the name Check the name 2-methyl-4-butanol See 3-methyl-1-butanol in every representation Open worked example on a board Organic nomenclature in Chemistry ReferenceYour existing work stays on this device. Examples open as editable copies.