1.8.4Unit 1. Structure, Bonding and Introduction to Organic Chemistry
Isomers – Structural — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 30 of 36 Unit quiz
Part 1 of 6What Are Structural Isomers
Isomerism is one reason why organic chemistry has such a vast number of compounds.
Definition
- Structural isomers are compounds with the same molecular formula but different structural formulae.
Different properties
- Isomers have different arrangements of atoms and so can differ in boiling point and in chemical behaviour.
Scope
- This lesson covers structural isomerism; other types of isomerism are met later in the course.
Checking
- Always confirm that each proposed isomer has the same molecular formula and that no two drawings are the same molecule rotated.
Part 2 of 6Types of Structural Isomerism
Types of Structural Isomerism
Chain isomers
- These differ in how the carbon skeleton is branched, for example butane and 2-methylpropane, both C₄H₁₀.
Position isomers
- The functional group is attached at different places on the same skeleton, such as propan-1-ol and propan-2-ol.
Functional group isomers
- Different functional groups appear, for example propan-1-ol and methoxyethane, both C₃H₈O.
Halogenoalkanes
- C₄H₉Br has four structural isomers because the chain may be straight or branched and the bromine may be at different carbons.
Part 3 of 6Finding All the Isomers of C₅H₁₂
1
Start with the straight chain of five carbons, which gives pentane.
2
Shorten the main chain to four carbons and place one methyl group on carbon 2, giving 2-methylbutane.
3
Moving the methyl to carbon 3 gives the same molecule as before, so it is not a new isomer.
4
Shorten the main chain to three carbons and place two methyl groups on carbon 2, giving 2,2-dimethylpropane.
5
No other arrangements are possible, so C₅H₁₂ has exactly three structural isomers.
Part 4 of 6Branching and Boiling Point
Branching and Boiling Point
Straight chains
- Long molecules lie side by side and have more surface contact, so stronger London forces act between them.
Branched chains
- Branched molecules are more compact and touch over less surface area, so the London forces are weaker.
Data
- Pentane boils at about 36 °C, 2-methylbutane at about 28 °C and 2,2-dimethylpropane at about 9.5 °C.
Conclusion
- The more branched the isomer, the lower its boiling point for the same molecular formula.
| Isomer of C₅H₁₂ | Boiling point / °C |
|---|---|
| Pentane | 36 |
| 2-Methylbutane | 28 |
| 2,2-Dimethylpropane | 9.5 |
Part 5 of 6Worked Example
Question Name the two alcohol isomers of C₃H₈O and one non-alcohol isomer.
- Place –OH on an end carbon of the three-carbon chain to get propan-1-ol.
- Place –OH on the middle carbon to get propan-2-ol.
- An ether CH₃OCH₂CH₃ has the same formula C₃H₈O but a different functional group.
Answer Propan-1-ol and propan-2-ol, with methoxyethane as the functional group isomer.
Part 6 of 6Exam Checklist & Tips
1.8.4 – Key Points for the Exam
- Structural isomers Compounds with the same molecular formula but different arrangements of atoms.
- Chain isomerism Isomerism caused by different branching of the carbon skeleton.
- Position isomerism Isomerism where the functional group is in different positions on the same skeleton.
- Functional group isomerism Isomerism where the molecules contain different functional groups.
- London forces Weak attractions between molecules arising from temporary dipoles.
Common Mistakes
- Counting a rotated or flipped drawing as a new isomer.
- Changing the molecular formula while drawing isomers.
- Saying branched isomers have a higher boiling point because they have more bonds.
- Confusing position isomers with functional group isomers.
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