AS-Level Organic Chemistry (Introduction)

  • Molecules that have the same molecular formula but different structures are called structural isomers.

  • Chain isomers have different carbon chains.

  • Positional isomers have a functional group in different positions on a carbon chain.


Structural Isomerism


Organic molecules can have the same number of atoms as one another, but they can be arranged differently. This means molecules can share the same molecular formula, but have different structural formulas. When this happens, they are described as being structural isomers.


Chain Isomerism           

If we take C H  . There are two possibilities of a molecular structure that has the molecular formula  C H  .



Here, it is the carbon chain of the molecule that has been changed between isomers, so it is an example of chain isomerism.



Positional Isomerism


Now if we look at propanol, C H O. We have a three carbon chain, but there are two possible structures that we can have as the –OH alcohol group can bond to two different carbons.




In this case, the difference between molecules is the position of the alcohol group, so it is an example of positional isomerism.