8.5 Isomers
Learning Objectives
- Identify and draw the constitutional isomers.
Constitutional Isomers
Imagine you were asked to draw a structure for a compound with the molecular formula [latex]\ce{C_{4}H_{10}}[/latex]. This would not be difficult – you could simply draw as shown Figure 8.5.1:
But when you compared your answer with that of a classmate, they may have drawn this structure as shown below:
Who is correct? The answer, of course, is that both of you are. A molecular formula only tells you how many atoms of each element are present in the compound, not what the actual atom-to-atom connectivity is. There are often many different possible structures for one molecular formula.
Compounds with this relationship, the same molecular formula but different connectivity, are called constitutional isomers (sometimes the term ‘structural isomer’ is also used). The Greek term ‘iso’ means ‘same.’
The word isomer is a relational word that describes the relationship between two things. It is similar in that sense to the word ‘cousin’ or ‘uncle.’ Being a cousin is not an identity essential to you but describes your relationship to someone else. A substance can be an isomer to something else, but being an isomer is not an essential internal characteristic.
Examples
Fructose and glucose, two kinds of sugar molecules, are related as constitutional isomers of one another. Can you figure out the molecular formula for each?
Answer
[latex]\ce{C_{6}H_{12}O_{6}}[/latex]
Key Takeaways
- Constitutional isomers, also known as structural isomers, are compounds that have the same molecular formula but differ in the way the atoms are connected or arranged.
Exercises
