Nomenclature (naming compounds): study guide

Key ideas

IUPAC names are built from a fixed recipe: find the longest chain, name it from the number of carbons, add the ending for the main functional group, then number the chain and list the substituents.

• The stem comes from the longest continuous chain: meth, eth, prop, but, pent, hex, hept, oct, non, dec for 1 to 10 carbons.
• The ending shows the main group: -ane for single bonds, -ene for a double bond, -yne for a triple bond, -ol for an alcohol, -al, -one, -oic acid, and so on.
• Number the chain from the end that gives the lowest numbers to the main group first, then to double bonds, then to substituents.
• List the substituents alphabetically, ignoring di-, tri- and tetra-, and give each its number.

Key equations

• meth- 1, eth- 2, prop- 3, but- 4, pent- 5
• hex- 6, hept- 7, oct- 8, non- 9, dec- 10
• -ane, -ene, -yne
• alkane CₙH₂ₙ₊₂

Worked example

Take CH₃CH(CH₃)CH₂CH₂CH₃. The longest chain has five carbons, so it is pentane. The methyl branch is on carbon 2 when numbered from the left (and on carbon 4 from the right), so the name is 2-methylpentane. For CH₃CH=CHCH₂CH₃ the double bond starts at carbon 2, so the name is pent-2-ene.

Common mistakes

• Choosing a straight-looking chain instead of the longest chain, which may bend around corners of the drawing.
• Numbering from the wrong end so that the substituents get higher numbers than necessary.
• Alphabetizing by the prefix di- or tri-. The word dimethyl is placed under m, so ethyl comes before dimethyl.

Practice

Test yourself with practice questions on this topic: Nomenclature (naming compounds)