Naming and drawing alkanes and cycloalkanes

Alkanes contain only carbon and hydrogen joined by single bonds. Their names are the foundation of all organic nomenclature: almost every other name on this site is built by adding suffixes and prefixes to an alkane parent. If you can turn an alkane name into a structure reliably, the rest is a matter of adding groups in the right places.

Parent names

The last part of the name tells you the length of the main (parent) chain. The ending -ane means all carbon–carbon bonds are single.

Carbons	Parent	Carbons	Parent
1	methane	6	hexane
2	ethane	7	heptane
3	propane	8	octane
4	butane	9	nonane
5	pentane	10	decane

A substituent made from an alkane is an alkyl group: replace -ane with -yl. Methyl is –CH₃, ethyl is –CH₂CH₃, propyl is –CH₂CH₂CH₃. Two branched groups you will meet constantly areisopropyl (propan-2-yl, attached through the middle carbon) and tert-butyl (2-methylpropan-2-yl, attached through a carbon that carries three methyl groups).

From name to structure in four steps

Find the parent at the end of the name and draw that many carbons as a zigzag chain.
Number the chain from one end, C1, C2, C3…
Attach each substituent at the carbon given by its locant (the number in front of it). Di-, tri- and tetra- tell you how many identical groups there are, and each needs its own locant.
Check by counting the carbons and confirming every carbon has four bonds.

Worked example: 2-methylbutane

The parent is butane: four carbons. "2-methyl" puts one –CH₃ on C2. That gives five carbons in total, C₅H₁₂. If you number from the other end, the branch is on C3, but the molecule is the same; the name uses the lower number, 2.

Worked example: 3-ethyl-2-methylhexane

Draw six carbons for hexane and number them. Put a methyl on C2 and an ethyl (two carbons) on C3. The substituents are listed alphabetically (ethyl before methyl), not in numerical order. Total: 6 + 1 + 2 = 9 carbons, C₉H₂₀.

Worked example: 2,2,4-trimethylpentane

"Trimethyl" means three methyl groups, and the three locants say where: two on C2 and one on C4. C2 ends up bonded to four carbons, so it has no hydrogens. This compound is the reference standard for the octane rating of petrol and is often called isooctane.

Why names look the way they do

Knowing the rules used to name a structure helps you avoid drawing something that has a different name. In brief:

The parent is the longest continuous chain, wherever it runs in the drawing. If two chains are equally long, the one with more substituents is chosen.
The chain is numbered to give the substituents the lowest locants, compared at the first point of difference.
Substituents are cited in alphabetical order. Multiplying prefixes (di-, tri-) and the italic prefixes sec- andtert- are ignored when alphabetising; iso is not.
Numbers are separated by commas (2,2,4-) and numbers are separated from words by hyphens (2,2,4-trimethylpentane).

A name that is not quite right: "2-ethylbutane"

If you follow "2-ethylbutane" literally, you draw a four-carbon chain with an ethyl group on C2. Now look for the longest chain: it runs through the ethyl group and has five carbons, with a methyl on the middle carbon. The correct name of that structure is 3-methylpentane. An ethyl group on C2 of a chain always lengthens the chain, which is why you will never see it in a correct name.

Cycloalkanes

A ring of carbons is named by adding cyclo- to the alkane with the same number of carbons: cyclopropane, cyclobutane, cyclopentane, cyclohexane. With one substituent no locant is needed (methylcyclohexane), because every ring carbon is equivalent. With two or more, the ring is numbered to give the lowest locants, starting at a substituted carbon.

Two substituents on different ring carbons, as in 1,2-dimethylcyclohexane, can be on the same face of the ring (cis) or opposite faces (trans). Those are different compounds; the exercises in this topic avoid them, and the stereochemistry topic covers how to draw them.

Conventions differ. When a molecule contains both a ring and a chain, the current IUPAC recommendations (2013) choose the ring as the parent. Many textbooks use an older rule that picks whichever has more carbons. Check which convention your course uses; the exercises here avoid cases where the two rules disagree.

Common mistakes

Miscounting the parent chain, especially when it is drawn with a branch in the middle.
Putting a methyl on C1. That just makes the chain one carbon longer: "1-methylbutane" is pentane.
Using one locant for "dimethyl". 2,2-dimethyl means two methyls on the same carbon; 2,3-dimethyl means one on C2 and one on C3.
Confusing propyl and isopropyl. Propyl attaches through an end carbon; isopropyl attaches through the middle one.