Same formula, different structure
Isomers are compounds that have the same molecular formula but different structural formulae. This phenomenon is called structural isomerism. Because the atoms are joined in different orders, isomers usually have different physical properties (such as boiling point) even though they contain the same numbers of each atom.
Worked example
The molecular formula C4H10 has two isomers:
- Butane — a straight chain: CH3CH2CH2CH3
- 2-methylpropane — a branched chain with the same formula
Both are C4H10, but the branched isomer has a lower boiling point because its molecules pack less closely, giving weaker intermolecular forces.
Types and rules
Isomerism can arise from a different carbon skeleton (chain isomerism), a different position of a functional group (positional isomerism), or a different functional group (functional group isomerism). The number of isomers increases as the number of carbon atoms rises. To count isomers, draw all distinct carbon skeletons without repeating the same structure.
When drawing isomers, start with the longest continuous carbon chain, then move a carbon atom to make progressively shorter, branched chains, checking each new structure is genuinely different rather than the same chain drawn another way. Isomers are given different names to reflect their structures, for example butane and 2-methylpropane. Although a set of isomers shares one molecular formula, differences in shape and packing lead to different boiling points, melting points and densities, so each is treated as a separate compound.
Remember
- Isomers: same molecular formula, different structure.
- C4H10 has 2 isomers; C5H12 has 3 isomers.
- Branched isomers have lower boiling points than straight chains.
- Rotating or flipping the same structure is NOT a new isomer.