Structural isomers

Structural isomers are compounds with the same molecular formula but a different structural formula. There are 3 types of structural isomers:

This page will focus on position and functional group isomers, for examples of chain isomers then click here.

Position isomers

Position isomers have the same carbon skeleton and the same functional group but differ from each other in the location of the functional group in the carbon chain. Since these isomers have the same functional groups present they will have similar chemical properties but are very likely to have slightly different physical properties e.g consider chlorobutane as an example. The image below shows the two position isomers of chlorobutane. The only difference between the two molecules below is in the position of the chlorine atom on the butane chain of carbon atoms.

As a second example consider the alkene, butene. Butene being an alkene has the C=C functional group, however this functional group can be placed in two different positions within a butene molecule to give two structural isomers (position isomers), as shown below:

As with the first example, both butene molecules have the same molecular formula and carbon skeleton, the only difference between but-1-ene and but-2-ene is in the position of the C=C functional group. The numbers in the names of the two position isomers simply indicate where in the carbon chain the functional group is to be found. This means that but-1-ene and but-2-ene are position isomers.

Functional group isomers

Functional group isomers are another type of structural isomer, here the molecules have the same molecular formula but have different functional groups. This means that the isomers will have different chemical and physical properties, since the chemical properties will be dependant on the functional group present and the physical properties will also be largely dependant on the type of functional group present since this can influence properties such as the type of intermolecular bonding present.

Two examples of this type of structural isomer are shown below:

Example 1 - Aldehydes and ketones

Aldehydes and ketones have the same molecular formula but they contain different functional groups. The images below show the aldehyde propanal and the ketone propanone. Both the ketone and the aldehyde have the same molecular formula; C3H6 but they contain different functional groups.

Example 2 - alcohols and ethers

Key Points

Practice questions

Check your understanding- Questions on position and functional group isomers