Section 1 of 5What Structural Isomers Are
What Structural Isomers Are
Definition
- Structural isomers have the same molecular formula but different structural formulae, so the atoms are joined in a different order.
Physical properties
- Isomers often have different boiling points and melting points because their shapes and intermolecular forces differ.
Chemical properties
- Isomers with different functional groups react differently, whereas chain isomers usually react in a similar way.
Finding them
- Fix the longest chain first, then move the functional group or shorten the chain and add branches to find every isomer.
Section 2 of 5Three Types of Structural Isomerism
Chain isomerism
- The carbon skeleton is arranged differently, such as pentane and 2-methylbutane, both C₅H₁₂.
Position isomerism
- The same functional group sits on different carbon atoms, such as propan-1-ol and propan-2-ol.
Functional group isomerism
- The atoms form different functional groups, such as ethanol CH₃CH₂OH and methoxymethane CH₃OCH₃.
| Formula | Isomer 1 | Isomer 2 |
|---|---|---|
| C₄H₁₀ | butane | 2-methylpropane |
| C₃H₈O | propan-1-ol | propan-2-ol |
| C₃H₆O | propanal | propanone |
| C₂H₆O | ethanol | methoxymethane |
Section 3 of 5Branching and Boiling Point
1
Straight-chain molecules pack closely together, so their surface contact is large and London dispersion forces between them are strong.
2
Adding branches makes the molecule more compact and spherical, which reduces the area of contact between neighbouring molecules.
3
Weaker London forces need less energy to overcome, so the more highly branched isomer has the lower boiling point.
4
Pentane boils at about 36 °C, 2-methylbutane at about 28 °C and 2,2-dimethylpropane at about 10 °C.
Section 4 of 5Worked Example
Question Draw and name all the chain isomers of C₅H₁₂.
- Start with the unbranched chain of five carbons: CH₃CH₂CH₂CH₂CH₃, which is pentane.
- Shorten the main chain to four carbons and place a methyl group on carbon 2: CH₃CH(CH₃)CH₂CH₃, which is 2-methylbutane.
- Shorten the chain to three carbons and place two methyl groups on carbon 2: C(CH₃)₄, which is 2,2-dimethylpropane.
- A methyl on carbon 3 of a four-carbon chain is the same as carbon 2 when numbered from the other end, so there are no more isomers.
Answer Three isomers: pentane, 2-methylbutane and 2,2-dimethylpropane.
Section 5 of 5Exam Checklist & Tips
3.1.3 – Key Points for the Exam
- Structural isomers Compounds with the same molecular formula but different structural formulae.
- Chain isomerism Isomerism caused by different arrangements of the carbon skeleton.
- Position isomerism Isomerism caused by the functional group being on a different carbon.
- Functional group isomerism Isomerism in which the isomers contain different functional groups.
Common Mistakes
- Counting a molecule twice because it has been drawn rotated or flipped, although the connectivity is identical.
- Naming a chain from the wrong end so the same isomer appears to be different.
- Assuming that isomers must be in the same homologous series, when functional group isomers are not.
- Saying more branching raises the boiling point; it lowers it by reducing surface contact.