Position Isomerism

Structural isomers have the same molecular formula but a different structural formula. Position isomerism is one type of structural isomerism: the carbon skeleton and the functional group are the same, but the functional group is in a different position on the chain.For alkenes, the C=C can sit at different places along the chain. The locant in the name shows where it is, so position isomers have different numbers in their names.C₄H₈ has two straight-chain position isomers:CH₂=CHCH₂CH₃   but-1-ene
CH₃CH=CHCH₃   but-2-eneWorked example: how many straight-chain position isomers does C₅H₁₀ have? The double bond can start at carbon 1 (pent-1-ene) or carbon 2 (pent-2-ene). A double bond starting at carbon 3 is the same molecule as pent-2-ene numbered from the other end, so 'pent-3-ene' is not a correct name. There are two. For a six-carbon chain the same method gives hex-1-ene, hex-2-ene and hex-3-ene.Do not confuse this with other kinds of structural isomer. Methylpropene, CH₂=C(CH₃)₂, is also C₄H₈, but it has a branched carbon skeleton, so it is a chain isomer of but-1-ene. Cyclobutane is a functional group isomer, since it has no C=C at all.