Alkanes and cycloalkanes
Naming alkanes and cycloalkanes: Practice exercises
The rules for naming alkanes and cycloalkanes in five steps, two names read step by step, and exercises with instant feedback and worked answers.
Alkanes contain only carbon and hydrogen joined by single bonds; cycloalkanes close the chain into a ring. Their names are the base of every other family: the same chain names (meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-) and the same rules for substituents come back in alkenes, alcohols, acids and the rest.
The rules you need
- Find the longest continuous chain of carbons, wherever it runs in the drawing; it gives the parent name: pentane, hexane, heptane. If two chains are equally long, take the one with more substituents.
- Number the chain from the end that gives the substituents the lowest locants, compared at the first point of difference. If both ends give the same set, the substituent cited first in alphabetical order gets the lower number.
- Name the substituents as alkyl groups (methyl, ethyl, propyl) and give each one its own locant, even on the same carbon: 2,2-dimethyl, not 2-dimethyl.
- Write the prefixes in alphabetical order, ignoring di-, tri- and tetra-, then the parent name, all as one word.
- Rings: add cyclo- to the parent name (cyclohexane). With one substituent no locant is needed (methylcyclohexane); with several, number the ring to give them the lowest locants.
Two names, read step by step
3-Ethyl-2-methylpentane
Several chains have five carbons, of two kinds: some carry a methyl and an ethyl group, one carries a single larger branch. A chain with two substituents is the parent. Numbering from the end nearer the branches gives the locants 2 and 3 (from the other end, 3 and 4). Ethyl is written before methyl because the prefixes are in alphabetical order, whatever their locants.
1,1-Dimethylcyclohexane
A six-carbon ring with two methyl groups on the same carbon. That carbon is C1, and each methyl group gets its own locant: 1,1-dimethyl. With a single methyl group the name would be just methylcyclohexane.
Watch out for
- The chain drawn in a straight line is not always the longest one: follow the chain around corners and into branches.
- A methyl group on the end carbon of a chain is not a branch: it makes the chain one carbon longer.
- Multipliers do not count for alphabetical order: dimethyl is filed under m, so 3-ethyl-2,2-dimethylpentane starts with ethyl.
Draw from the name: 18 exercises
Each name opens its exercise. Draw the structure and check it: any correct drawing is accepted, and hints and a worked answer help when you are stuck.
- 2-methylbutane Beginner
- ethylcyclohexane Beginner
- 3-methylpentane Beginner
- 2,3-dimethylbutane Beginner
- methylcyclohexane Beginner
- 2,2,4-trimethylpentane Intermediate
- 1-ethyl-1-methylcyclohexane Intermediate
- 3-ethylhexane Intermediate
- 4-ethyl-2-methylhexane Intermediate
- 1,2-dimethylcyclohexane Intermediate
- (propan-2-yl)cyclohexane Intermediate
- 3,3-diethylpentane Intermediate
- 2,2,3,3-tetramethylbutane Intermediate
- 3-ethyl-2,2-dimethylpentane Advanced
- 4-(propan-2-yl)heptane Advanced
- 1-ethyl-3-methylcyclohexane Advanced
- 3-ethyl-2,3-dimethylpentane Advanced
- 1,1,3-trimethylcyclohexane Advanced
Name the structure
18 structures of this family to name, each with four names to choose from and an explanation of every wrong choice.
Printable worksheet
For practice on paper: names to draw and structures to name, with an answer key to check your work.
The full guide
The guide Alkanes and cycloalkanes explains these names in more detail, with more worked examples and the most common mistakes.