Name simple alkanes and alkenes, interpret formulae and identify structural isomers.
Method and worked examples
Hydrocarbons contain only carbon and hydrogen. Acyclic saturated alkanes follow CnH2n₊₂; an acyclic hydrocarbon with one C=C bond follows CnH2n. The chain-length roots from one to eight carbons are meth−,eth−, prop−,but−, pent−,hex−, hept− and oct−. Use−ane for single bonds only and −ene for an alkene. Choose a longest chain containing the relevant double bonds and number it to give those bonds the lowest locants.
Name methyl and ethyl branches with their positions and use di− for repeated branches. A condensed structure such as CH3CH(CH3)CH3 is 2−methylpropane. Structural isomers have the same molecular formula but different atom connectivity. An empirical formula is the simplest atom ratio, while a molecular formula gives actual atom counts. Neither formula alone specifies how all atoms connect.
Example 1
Name CH3CH2CH=CH2.
There are four carbons and a terminal double bond. Number from that end: but−1−ene.
Example 2
Find the molecular formula of an acyclic alkane with eight carbons.
Use CnH2n₊₂ with n=8:C8H18, octane.
Watch out:Structural isomers share a formula but differ in atom connectivity.
Hydrocarbon formulae, names and isomers | Year 11 Chemistry | Ascent Arcade