Structural isomerism in the alkanes
4 marks · NormalThe following table gives the molecular formulae and numbers of structural isomers possible for members of the alkane series:
| Alkane | Molecular formula | Number of isomers |
|---|---|---|
| Methane | CH4 | 1 |
| Ethane | C2H6 | 1 |
| Propane | C3H8 | 1 |
| Butane | C4H10 | 2 |
| Pentane | C5H12 | 3 |
| Hexane | C6H14 | ? |
Compare the number of isomers as the number of carbon atoms increases from C4H10 to C6H14.
Explain why the number of structural isomers increases as you go from C4H10 to C6H14.
Use the data to predict the number of isomers for C6H14.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Number increases from 2 (C4H10) to 3 (C5H12) to more (C6H14); number of isomers increases rapidly with more carbons
(b) As the number of carbon atoms increases, there are more ways to arrange the carbon atoms (more possible branches), leading to more different isomers.
(c) 5
Common mistakes: Mistaking only a one-by-one increase; Not explaining that branching increases with chain length
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Marking points
B1 (a) · 1 mark · States or describes that the number of isomers increases as the carbon chain grows.
M1 (b) · 1 mark · Mentions increased possible structures/arrangements/branching with more carbons.
A1 (b) · 1 mark · Correctly explains that more possible connections between atoms lead to more isomers.
A2 (c) · 1 mark · Gives correct number: 5 for C6H14.
Worked solution
a) The number of isomers increases from 2 (C4H10) to 3 (C5H12) as the number of carbon atoms increases. We expect more again for C6H14.
b) As the carbon chain gets longer, there are more ways to arrange the carbon atoms, so more possible structures can be made.
c) There are 5 isomers for C6H14.