Part 1 of 6What Structural Isomers Are
Definition
- Structural isomers share the same molecular formula but have different structural formulae, meaning their atoms are connected differently.
Same formula
- Because the molecular formula is identical, isomers have the same relative molecular mass.
Different properties
- Each isomer has its own physical properties, and those with different functional groups also have different reactions.
Drawing
- Check that every carbon has four bonds and the formula matches before accepting a new structure as an isomer.
Part 2 of 6Three Types
Three Types
Chain isomerism
- The carbon skeleton is arranged differently, as in butane and 2-methylpropane, both C₄H₁₀.
Position isomerism
- The functional group is attached at a different position on the same skeleton, as in propan-1-ol and propan-2-ol.
Functional group isomerism
- The atoms form different functional groups, as in propanal and propanone, both C₃H₆O.
Another example
- Propan-1-ol, CH₃CH₂CH₂OH, and methoxyethane, CH₃OCH₂CH₃, both have the formula C₃H₈O.
| Pair | Formula | Type |
|---|---|---|
| Butane / 2-methylpropane | C₄H₁₀ | Chain |
| Propan-1-ol / propan-2-ol | C₃H₈O | Position |
| Propanal / propanone | C₃H₆O | Functional group |
Part 3 of 6Finding All Isomers
1
Start by drawing the longest straight chain that fits the molecular formula.
2
Shorten the main chain by one carbon and place that carbon as a branch in each different position.
3
Shorten again and repeat, taking care not to draw the same molecule twice in a different orientation.
4
Change the position of any functional group on each carbon skeleton to find position isomers.
5
Check the hydrogen count of each structure against the molecular formula.
Part 4 of 6Branching and Boiling Point
Straight chains
- Long straight molecules pack closely and have a large surface area for London forces, so they have higher boiling points.
Branched chains
- Branching reduces the surface contact between molecules, so London forces are weaker and boiling points are lower.
Example
- Pentane boils at about 36 °C but 2,2-dimethylpropane boils at about 10 °C, even though both are C₅H₁₂.
Explanation use
- In answers, name the intermolecular force and relate it to surface area and contact between molecules.
Part 5 of 6Worked Example
Question List all the structural isomers of C₅H₁₂ and say which has the lowest boiling point.
- Straight chain of 5 carbons: pentane, CH₃CH₂CH₂CH₂CH₃.
- Chain of 4 with one methyl branch on carbon 2: 2-methylbutane, CH₃CH(CH₃)CH₂CH₃.
- A branch on carbon 3 gives the same molecule.
- Chain of 3 with two methyl groups on carbon 2: 2,2-dimethylpropane, C(CH₃)₄.
- The most branched isomer has the smallest surface area and so the weakest London forces.
Answer
- Three isomers;
- 2,2-dimethylpropane has the lowest boiling point.
Part 6 of 6Exam Checklist & Tips
4.1.3 – 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 a functional group being in a different position.
- Functional group isomerism Isomerism in which the molecules have different functional groups.
Common Mistakes
- Counting two drawings of the same molecule, rotated or bent, as separate isomers.
- Thinking isomers must have the same properties because they share a formula.
- Forgetting that a structure with an incorrect hydrogen count is not an isomer.
- Explaining lower boiling points of branched isomers by weaker covalent bonds instead of weaker London forces.