Naming organic compounds and isomers · Grade 10
Organic compounds are named by IUPAC rules so that every name stands for exactly one structure. The name is built from three parts: a stem for the number of carbons in the longest chain, an ending for the functional group, and prefixes, with numbers, for any branches or halogen atoms. Compounds with the same molecular formula but different structures are structural isomers.
Key facts
- A homologous series is a family with the same functional group and general formula, each member differing by CH₂: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alcohols CₙH₂ₙ₊₁OH, carboxylic acids CₙH₂ₙ₊₁COOH.
- Stems give the number of carbons in the longest chain: meth- 1, eth- 2, prop- 3, but- 4, pent- 5, hex- 6, hept- 7, oct- 8.
- Endings give the functional group: -ane (alkane), -ene (C=C), -ol (–OH), -al (–CHO, always carbon 1), -one (C=O inside the chain), -oic acid (–COOH, carbon 1). Halogens and alkyl branches are prefixes: chloro-, bromo-, methyl-, ethyl-.
- Find the longest carbon chain that holds the functional group; it may bend round corners in a drawing.
- Number the chain from the end that gives the functional group the lowest number, then the substituents the lowest numbers. Write prefixes in alphabetical order, with di-, tri- for repeats (these do not count for the order): 2-bromo-3-methylbutane, 2,2-dimethylpropane.
- Structural isomers have the same molecular formula but a different structural formula: chain isomers (different branching), position isomers (the group on a different carbon) and functional group isomers (a different functional group, such as propanal and propanone).