How can I work out whether a hydrocarbon is an alkene or an alkane from its molecular formula?
Step 1:
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Look at the molecular formula of the hydrocarbon you have been given.
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(This is the formula that tells you exactly how many hydrogens and how many carbons are present.) Step 2: * Refer back to the general formulae of alkenes and alkanes.
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For alkenes it is CnH2n and for alkanes its CnH2n+2. * 'n' represents the number of carbons in the molecule. * Take your molecular formula and see which general formula it fits into.
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If it fits into CnH2n then it is an alkene, meaning the name will end in '-ene'. If it fits into CnH2n+2 then the molecule is an alkane.
EG. From the molecular formula show propane is an alkane Molecular formula: C3H8
n=3 as the number by carbon in the molecular formula is 3
2n = 2 X 3 = 6 (so it can’t be an alkene as it doesn’t fit the general formula)
2n+2 = (2 X 3) + 2 = 8 (the general formula tells us there are 8 hydrogens present so it must be an alkane)
This is proved correct when looking back at the name propAne.
