Structural isomerism in the alkanes
3 marks · EasyThe table shows the number of isomers for selected alkanes.
| Alkane | Molecular formula | Number of isomers |
|---|---|---|
| Methane | CH4 | 1 |
| Ethane | C2H6 | 1 |
| Propane | C3H8 | 1 |
| Pentane | C5H12 | 3 |
Describe the trend in the number of isomers as the number of carbon atoms increases.
Suggest why methane, ethane, and propane each have only one isomer.
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.
Show answer
(a) The number of isomers increases as the number of carbon atoms increases.
(b) Because these small alkanes do not have enough carbon atoms to allow branching; there is only one way to arrange the atoms for each formula.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Describes trend: number of isomers increases with number of carbons
M1 (b) · 1 mark · States there are not enough carbons for branching
A1 (b) · 1 mark · Explains there is only one possible structure for each small alkane
Worked solution
a) The number of isomers increases with more carbon atoms.
b) Methane, ethane, and propane do not have enough carbons for branching, so only one structure is possible for each.