Structural isomerism in the alkanes
5 marks · HardThis question is about structural isomerism in the alkanes.
Complete the following table for alkanes, giving the number of possible structural isomers for each formula.
| Molecular formula | Number of structural isomers |
|---|---|
| C3H8 | |
| C4H10 | |
| C5H12 |
Explain why C5H12 has more structural isomers than C3H8.
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(a) — · must show C3H8: 1, C4H10: 2, C5H12: 3
(b) C5H12 contains more carbon atoms than C3H8, so there are more possible ways to arrange the carbon atoms to form different structures, resulting in more structural isomers. · must show More carbon atoms allow more possible different arrangements/branching, Explains that this leads to more isomers
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Marking points
B1 (a) · 1 mark · C3H8: 1 isomer
B2 (a) · 1 mark · C4H10: 2 isomers
B3 (a) · 1 mark · C5H12: 3 isomers
B4 (b) · 1 mark · States that more carbon atoms allow for more possible arrangements/branching.
B5 (b) · 1 mark · Explains that this results in more isomers for larger alkanes.
Worked solution
a) Complete the table:
| Molecular formula | Number of structural isomers |
|---|---|
| C3H8 | 1 |
| C4H10 | 2 |
| C5H12 | 3 |
b) C5H12 has more carbon atoms than C3H8. More carbon atoms allow different ways to arrange or branch the carbon chain, so C5H12 can form more different structural isomers.