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CSEC Chemistry: Structural Isomerism

Previously, we discussed the naming of compounds based on their fully displayed structural formulae. We do this because there are examples of organic compounds with the same molecular formula but different structural formulae. These compounds are called isomers. The existence of these compounds is known as isomerism.

Isomers may belong to the same homologous series or different homologous series. Isomers within the same homologous series exhibit different physical properties but similar chemical properties. However, isomers belonging to different homologous series display different physical and chemical properties.


CSEC often asks questions regarding identifying or drawing the structural formulae of the isomers for a certain molecular formula. For example, if you were asked to draw the structural formula of all the alkanes containing six carbon atoms, you would start by identifying the molecular formula of the alkane with six carbon atoms. Since the general formula of alkanes is CnH2n+2, the molecular formula used here would be C6H14.

A good way to ensure that you include all possible isomers is to draw them starting with isomers with longer chains then working upwards to isomers with more branches.


The first isomer (with the longest main chain) would be the straight-chained alkane, hexane.


hexane

Then, to create branched chain compounds, move one methyl group (leaving the main chain with 5 carbon atoms, creating pentane) to the 2 position and the then the 3 position on the main chain to create 2 branched pentane compounds, 2-methylpentane and 3-methylpentane.

2-methylpentane 3-methylpentane

2 more compounds can be created by moving another methyl group from the pentane branched compound (leaving a branched butane compound) and appending that methyl group to either the 2 or 3 positions on the butane main chain, creating 2,2-dimethylbutane and 2,3-dimethylbutane.


2,2-dimethylbutane 2,3-dimethylbutane

Note: only 2,2 and 2,3-dimethylbutane are created here because you always number from the side that will result in the lowest numerical position of the alkyl groups. The same goes for the branched pentane compounds. Also note that an alkyl group in the '1' position will not create a new compound, as this will create an uninterrupted carbon chain when traced with a pencil:



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