Structural Isomers of Butane

Given

• Molecular formula: C4H10 (an alkane, CnH2n+2 with n = 4)

• Structural isomers share a molecular formula but differ in how atoms are connected.

Find

All distinct structural isomers of C4H10, with structures and names.

Principle

Structural (constitutional) isomers have the same molecular formula but different connectivity. To find them all, systematically vary the carbon skeleton: start with the longest straight chain, then move carbons into branches. Each unique skeleton is named; if two drawings give the same IUPAC name, they are the same compound drawn differently.

Formula

Solution

1. Start with the longest possible chain: all 4 carbons in a row → CH3CH2CH2CH3, named butane (n-butane). This is isomer 1.

2. Shorten the parent chain to 3 carbons and move the fourth carbon into a branch: CH3CH(CH3)CH3, a propane chain with a methyl group on carbon 2 → 2-methylpropane. This is isomer 2.

3. Try a 2-carbon parent chain: the two remaining carbons would both attach to the same chain carbon, recreating the 3-carbon skeleton of isomer 2 — no new structure.

4. Confirm the count: the only distinct skeletons are the 4-chain and the branched 3+1 chain, so C4H10 has exactly 2 structural isomers.

5. Verify formulas: CH3CH2CH2CH3 has 4 C and 10 H; CH3CH(CH3)CH3 has 4 C and 10 H — both match C4H10.

Unit check

Isomer counting is a combinatorial task with no physical units; the consistency check is atom counting — both proposed structures contain exactly 4 carbons and 10 hydrogens, matching C4H10.

What this means physically

Even with just four carbons, two different molecules exist: straight-chain n-butane and branched 2-methylpropane. They have different boiling points (-0.5 C vs -11.7 C) and shapes despite identical formulas — a first glimpse of how structure controls properties in organic chemistry.