Chapter 18 · Organic Chemistry
18.6Isomers: Same Formula, Different Structure
10 min · two checks
Predict
Butane and 2-methylpropane both are C₄H₁₀. Are they the same substance?
The idea
- Define constitutional isomers.
- Draw two different structures for C₄H₁₀.
Isomers share a molecular formula and differ in structure. Constitutional isomers differ in which atoms are bonded to which. C₄H₁₀ is either a straight chain (butane) or a three-carbon chain with a one-carbon branch (2-methylpropane). They are different substances with different boiling points. As the carbon count rises, the number of possible isomers rises very fast. A formula is not a structure.
Drawing both as flat chains and claiming they differ because one is bent is not isomerism. Single bonds rotate. You have a new isomer only when you would have to break bonds to turn one drawing into the other. Later courses add stereoisomers, which have the same connectivity and differ in spatial arrangement that cannot be rotated away. This section’s job is connectivity.
Keep these
- Same formula, different connectivity: constitutional isomers.
- Rotation around a single bond is not a new compound.
- Branches change properties, including boiling point.