How can I work out whether a hydrocarbon is an alkene or an alkane from its molecular formula?

Step 1:

• Look at the molecular formula of the hydrocarbon you have been given.

• (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.

• For alkenes it is C_nH_2n and for alkanes its C_nH_2n+2. * 'n' represents the number of carbons in the molecule. * Take your molecular formula and see which general formula it fits into.

• If it fits into C_nH_2n then it is an alkene, meaning the name will end in '-ene'. If it fits into C_nH_2n+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.