Unsaturated hydrocarbons
4 marks · NormalThe table below shows the number of carbon and hydrogen atoms in three different hydrocarbons.
| Compound | Number of carbon atoms | Number of hydrogen atoms |
|---|---|---|
| W | 4 | 10 |
| X | 5 | 12 |
| Y | 6 | 12 |
Which compound(s) could be an alkene? Explain your answer using the general formula.
For any one compound that is not an alkene, state what structural feature it likely has instead.
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) Compound X could be an alkene because for 5 carbons, the general formula gives 10 hydrogens for an alkene (CnH2n), but X has 12. However, none of the compounds strictly fit the alkene general formula CnH2n, but you could argue Y (6 carbons, 12 H) fits. · must show Checks CnH2n for each; points out the match or mismatch
(b) Compound W is likely an alkane (has only single C–C bonds and is saturated). · must show Links to saturated structure, single bonds, or being an alkane
Common mistakes: Mismatch of formula to general formula; Not explaining reasoning
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Marking points
M1 (a) · 1 mark · Checks the general formula CnH2n for each compound; shows correct calculations/matches.
A1 (a) · 1 mark · Correctly identifies which compound(s), if any, could be alkenes based on the formulae.
B1 (b) · 1 mark · Names a compound that does not match alkene series.
B2 (b) · 1 mark · States the structural feature: likely an alkane or saturated/all single bonds.
Worked solution
a) The general formula for alkenes is CnH2n.
For each compound:
W: 4C, 10H. CnH2n = 4 × 2 = 8. W has 10 H, so it's not an alkene.
X: 5C, 12H. CnH2n = 5 × 2 = 10. X has 12H, so not an alkene.
Y: 6C, 12H. CnH2n = 6 × 2 = 12. Y matches, so Y could be an alkene.
b) For compound W or X, the excess hydrogens suggest the compound is saturated—likely an alkane, which has only single C–C bonds.