Isomerism: Structural isomerism.

The idea

Structural isomers have the same molecular formula but their atoms are joined in a different order. There are three kinds and the exam expects you to name the one you have drawn. Chain isomers differ in the carbon skeleton: butane and 2-methylpropane are both C₄H₁₀, one straight and one branched. Position isomers keep the skeleton and move the functional group: propan-1-ol and propan-2-ol are both C₃H₈O. Functional group isomers have a different functional group altogether while keeping the same formula, so propan-1-ol and methoxyethane are again both C₃H₈O, and propanal and propanone are both C₃H₆O. Because of isomerism, a molecular formula often cannot identify a compound, though small molecules like CH₄ or C₂H₆ are unique.

What earns the marks

• The definition, compounds with the same molecular formula but a different structural formula.
• A correctly drawn pair of chain, position or functional group isomers.
• Naming which of the three types the drawn pair illustrates.
• Showing that both structures have the same molecular formula.

Common mistakes and how to put them right

• Drawing the same molecule twice, simply bent or rotated on the page.

• Drawing a structure with the wrong number of hydrogen atoms, so that it is not actually an isomer.

• Calling a pair position isomers when the carbon skeleton has changed as well.

• Offering E and Z forms as structural isomers, when they are stereoisomers.

Exam tips

• Count the atoms in both structures before claiming that they are isomers.
• Name the type of isomerism even when the question only says draw, because a follow-up mark often needs it.
• Redraw a suspicious pair with the longest chain horizontal, since identical molecules then become obvious.