Answer to Question #175173 in Organic Chemistry for Sam

Question

Explain structural isomerism and the different types that exist (i.e. chain, positional and functional group). Give at least two examples of each type, including structural formulae and photographs of 3D models to illustrate more clearly what structural isomerism is. Compare different structural isomers which have the same molecular formula, considering the difference in physical or chemical properties.

Answer 1

In structural isomerism, the atoms are arranged in a completely different order.

Chain isomerism. For example, there are two isomers of butane, C₄H₁₀. (n-butane and isobutane), pentane, C₅H₁₂, has three chain isomers: n-pentane, isopentane, neopentane.

Position isomerism. In position isomerism, the basic carbon skeleton remains unchanged, but important groups are moved around on that skeleton. For example, there are two structural isomers with the molecular formula C₃H₇Br. In one of them the bromine atom is on the end of the chain, whereas in the other it's attached in the middle. Another similar example occurs in alcohols such as C₄H₉OH.

Functional group isomerism In this variety of structural isomerism, the isomers contain different functional groups - that is, they belong to different families of compounds (different homologous series).

For example, a molecular formula C₃H₆O could be either propanal (an aldehyde) or propanone (a ketone).

Another common example is illustrated by the molecular formula C₃H₆O₂. Amongst the several structural isomers of this are propanoic acid (a carboxylic acid) and methyl ethanoate (an ester).