Lesson 4: Chains and names

Same formula, different molecule

Section 3 of 4

At the end of lesson 2 you met two molecules with the same formula, C₄H₁₀. Now they get a name.

Butane is a straight chain of 4 carbons. Isobutane is a chain of 3 with one carbon branching off the middle. Same atoms, joined in a different order. Molecules like these are isomers: same formula, different structure.

Butane, C₄H₁₀Four carbon atoms in a straight chain, joined by single bonds, with ten hydrogen atoms around them.
Butane, C₄H₁₀: a straight chain. Boils at about −1 °C.
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, C₄H₁₀: a branched chain. Boils at about −12 °C.

Isomers are different substances with different properties. Branched molecules are more compact, so they cling to each other less and boil at lower temperatures. Isobutane is the coolant inside many modern fridges.

Bending a chain does not make an isomer. Single bonds let a chain twist and bend, but which atoms are joined to which stays the same, so it is still the same molecule. A drawing is only a map: open the 3D model and turn it to see one molecule from many angles.

More carbons means more ways to join them. C₅H₁₂ has 3 isomers:

The three isomers of C₅H₁₂
Common nameShape of the chainBoils at
pentanea straight chain of 536 °C
isopentanea chain of 4 with one branch28 °C
neopentaneone carbon joined to 4 others10 °C, a gas on a warm day

Octane, C₈H₁₈, has 18 isomers.

Quick check

Which pair are isomers?