Functional Groups - Blog Posts

Structural Isomerism

In one boring history lesson, you and your friend (who both love chemistry) are doodling displayed formulas in the back of your textbook. You both decide to draw C5H12 - however, when you come to name what you’ve drawn, your friend has something completely different. You know what you’ve drawn is pentane and your friend knows what they’ve drawn is 2,3-dimethylpropane. So which one is C5H12?

The answer is both! What you and your friend have hypothetically drawn are structural isomers of C5H12 (another is 2-methylbutane). These are compounds which have the same molecular formula but different structural formulas.

Isomers are two or more compounds with the same formula but a different arrangement of atoms in the molecule and often different properties. 

There are several different kinds of structural isomers: chain, positional and functional group. 

Chain isomerism happens when there is more than one way of arranging carbon atoms in the longest chain. If we continue with the example C5H12, it exists as the three chain isomers shown above. Chain isomers have similar chemical properties but different physical properties because more branched isomers have weaker Van der Waals and therefore lower boiling points.

Positional isomers have the same carbon chain and the same functional group but it is attached at different points along the chain. 

This is a halogenoalkane. The locant “1″ describes where the chlorine is on the chain. For more on naming organic compounds, check out my nomenclature post.

The final type of isomer you need to know is a functional group isomer. This is a compound with the same molecular formula but a different functional group. For example, C2H6O could be ethanol or methoxymethane.

And surprisingly, that is all you need to know for the AS exam. There are also things called stereoisomers but those will be covered next year. Just make sure you know how to name and draw the three different kinds of structural isomers for the exam. Practice makes perfect!

SUMMARY

Structural isomers are compounds which have the same molecular formula but different structural formulas.

Isomers are two or more compounds with the same formula but a different arrangement of atoms in the molecule and often different properties.

There are several different kinds of structural isomers: chain, positional and functional group.

Chain isomerism happens when there is more than one way of arranging carbon atoms in the longest chain. Chain isomers have similar chemical properties but different physical properties because more branched isomers have weaker Van der Waals and therefore lower boiling points.

Positional isomers have the same carbon chain and the same functional group but it is attached at different points along the chain.

A functional group isomer is a compound with the same molecular formula but a different functional group.

Happy studying!