Lesson 4: Chains and names

Naming branches

Section 4 of 4

Common names like isobutane run out fast: octane alone has 18 isomers. Chemists name branched molecules with a few simple rules.

  1. Find the longest chain of carbons. It gives the main name: a chain of 5 is pentane.
  2. Name the branch by its carbons, with the ending -⁠yl: one carbon is methyl, two is ethyl.
  3. Number the chain from the end nearest the branch, and put the branch's number in front: 2-⁠methylpentane.
2-⁠methylpentane with its chain numbered A skeletal formula: a zigzag chain of five carbon atoms numbered 1 to 5 from the left, inside a dashed box marking the longest chain. A short line for a one-carbon methyl branch sticks up from carbon 2. 1 2 3 4 5 methyl branch longest chain: 5 carbons
2-⁠methylpentane: the longest chain has 5 carbons (pentane), numbered from the end nearest the branch. The methyl branch is on carbon 2.

Why carbon 2 and not carbon 4? Counting from the other end would put the branch on carbon 4. The rule is to start at the end nearest the branch, so the number is as small as possible. And a "branch" on an end carbon is not a branch at all: it just makes the chain longer.

The same rules name isobutane: its longest chain has 3 carbons, with a methyl branch on carbon 2, so it is 2-⁠methylpropane, often written just methylpropane, because the branch can only go in one place.

Isobutane (2-methylpropane), C₄H₁₀Three carbon atoms in a chain with a fourth carbon branching off the middle one, and ten hydrogen atoms around them.
Isobutane is 2-⁠methylpropane: a chain of 3 with a methyl branch on carbon 2.

Your turn

Find the longest chain, then the branch and its number. The quick check below asks for this molecule's name.

The mystery molecule, C₅H₁₂A zigzag of four carbon atoms with one more carbon attached to the second carbon from the left.
Your turn: what is this molecule called?

Quick check

What is the name of the molecule in the "Your turn" drawing above?

That's the end of lesson 4

In this lesson you found out:

  • The first eight alkanes, and what -⁠ane, -⁠ene and -⁠yne mean
  • How to read skeletal formulas
  • What isomers are
  • How to name an alkane with one branch
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