Structural isomers are molecules with the same molecular formula but a different arrangement of atoms … you need to recognise whether we are talking about chain isomerism, position isomerism or functional group isomerism and how the physical and chemical properties of the isomers are affected (a common exam question), so let’s get started!
- chain isomerism (straight chain vs. branched)
Both butane and methylpropane are only capable of forming instantaneous dipole – induce dipole intermolecular bonds (London forces) which are very weak and short range. If molecules cannot pack closely together then an instantaneous dipole in one molecule won’t be able to induce a dipole in a neighbour effectively.
Butane molecules have a more regular structure and so can pack more closely together, hence stronger intermolecular bonds and a higher boiling point.
- position isomerism (as in the position of the functional group)
The position of a functional group may affect both the physical and chemical properties of an isomer.
- functional group isomerism (differing functional groups)
Isomers with different functional groups will behave very differently chemically (alcohol are fairly reactive and useful chemical reagents, ethers are practically inert and used as solvents), as well as having different physical properties due to the types of intermolecular bonds they are capable of forming.