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; if your looking for examples of chain isomers then click here.
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:
🎯Position isomers are structural isomers that have the same molecular formula and the same functional group, but differ in the position of that functional group on the same carbon chain.
As a simple example of a position isomer consider chlorobutane; the image below shows the two position isomers of chlorobutane. Each isomer differs only in the position of the chlorine atom on the butane chain of four carbon atoms.
As a second example consider the alkene butene. Butene being an alkene contains 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 in the image below:
As with the first example above; both butene molecules have the same molecular formula and carbon skeleton; the only difference between but-1-ene and but-2-ene is 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 are another type of structural isomer.
Here the molecules have the same molecular formula
but have different functional groups.
This means that the two isomers will have different chemical and physical properties;
since the chemical properties depend on the functional group present.
The physical properties will also be largely dependent on the type of functional group
present; since this can influence the type of intermolecular bonding present.
Two examples of this type of structural isomer are shown below:
Similar sized molecules of aldehydes and ketones will have the same molecular formula but they will 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 (C3H6O) but they contain different functional groups.
Alcohols and ethers can also be functional group isomers. For example the alcohol ethanol and the ether dimethyl ether both have the same molecular formula (C2H6O), but they have different functional groups, this is outlined in the image below:
Try the quiz to check your understanding of chain, position and functional group isomers:
Click the correct isomer type. You will get an exam-style reason straight away.
🧠 Exam method: identify the functional group first, then decide whether it moved position or changed to a different group.
It is very easy to mix up the three types of structural isomers, so to ensure you always get it right use the flow chart below before attempting the short activity below activity. Always remember:
🧠 Exam habit: (1) Identify the functional group present in a molecule, (2) Then decide if the functional group is the same or different, (3) Then classify the isomer type.
Before you decide the type of structural isomer present; examiners will expect you to identify what key thing first? Choose the best first step for each pair of molecules below.
🧠 Exam habit: (1) Identify the functional group, (2) decide same or different, (3) then classify the isomer type.