Isomerism in Alcohols

Alcohols are a key functional group is synthesis (creating other organic compounds). We already met them in our Spectroscopy learning, with a main focus on classification (primary, secondary and tertiary).

FUNCTIONAL GROUP: -OH

GENERAL FORMULA: C_nH_2n+1OH (non-cyclic)

Nomenclature (Naming)

Alcohols have the suffix "-anol". For example, methanol has one carbon atom and an -OH group.

Once we get to three carbons, we need to identify where the -OH group is.

Note that we number the side chains after we number the carbons based upon where the -OH group is (put the -OH) group on the lowest possible number within the longest carbon chain.

Classification

The isomers above can be classified as primary, secondary or tertiary. This becomes very important when we look at the reactions and properties of alcohols.

To decide its classification, we look at the carbon the -OH group is bound to. How many other carbon atoms is it bound to?

 

1 = primary

2 = secondary

3= tertiary

Stereo-Isomerism (Optical Isomers)

Butan-2-ol is a good example of a chemical that exhibits optical isomerism. There are two possible structures (with identical structural formulae!!) that are different. How is that possible?

It is easiest to see this in 3D...

SOURCE: http://research.cm.utexas.edu/nbauld/fischer.htm

UNDERSTANDING STEREO-ISOMERISM

Stereo-isomers have a chiral carbon. This is a carbon atom bound to four different groups. If you recognise a molecule as having a chiral carbon, you need to be able to draw a 3D diagram of that molecule, with the chiral carbon at the centre of the diagram. The diagrams of 2-butanol (above) are good examples.