Introduction, classification, nomenclature and isomerism of alcohols

Introduction of alcohols

The hydroxy derivatives of aliphatic hydrocarbons (compounds having their carbon atoms in chains and not in the form of rings) are called alcohols. When one, two or more hydrogen atoms of a hydrocarbon are replaced by a corresponding number of hydroxyl groups (-OH), alcohols can be obtained.

Classification of alcohols

They can be classified as:

• Alcohols with one hydroxyl group - Monohydric alcohol

• Alcohols with two hydroxyl groups - Dihydric alcohol

• Alcohols with three hydroxyl groups - Trihydric alcohols

• Alcohols with four or more hydroxyl groups - Polyhydric alcohols

The saturated monohydric alcohols have the general formula C_nH_2n+1OH or ROH. Their functional group is -OH. They can be further classified into:

Primary Alcohol

Here the carbon atom bearing the hydroxyl group is attached to just one other carbon atom.

Secondary Alcohol

Here the carbon atom bearing the hydroxyl group is attached to two other carbon atoms.

Tertiary Alcohol

Here the carbon atom bearing the hydroxyl group is attached to three other carbon atoms.

Nomenclature of alcohols

In the common system, alcohols are named as alkyl alcohols. The word alcohol is added after the name of the alkyl group to which the hydroxyl group is attached. For e.g., CH₃OH is methyl alcohol.

In the IUPAC system, the names of saturated alcohols are derived from corresponding alkenes by replacing 'e' of alkenes by 'ol'

Some examples are shown below.

The numbering is done such that the carbon atom attached to the

-OH group gets the lowest number.

For naming polyhydric alcohols, the name of the alkane is retained and the ending -e is not dropped. Thus dihydric alcohols are named as alkane diols and trihydric alcohols are named as alkene triols.

The position of carbon atoms carrying -OH groups are indicated by locants written after the name of alkene. The number of hydroxyl groups is indicated by adding the multiplicative prefix di, tri, tetra etc., before the suffix-ol.

Isomerism of alcohols

Alcohols exhibit following types of isomerism:

1. Chain isomerism

Alcohols with four or more carbon atoms exhibit this type of isomerism in which the carbon skeleton is different.

2. Position isomerism

Alcohols with three or more carbon atoms can exhibit position isomerism. In this type of isomerism the position of the functional group i.e., the -OH group varies. In other words the carbon atoms to which the -OH group is attached is different.

3. Functional isomerism

Alcohols with two or more carbon atoms can exhibit functional isomerism with ethers. Thus ethers and alcohols have the same molecular formula but have different functional groups, hence they are called functional isomers.

4. Optical isomerism

Alcohols containing chiral centrescen exhibit enantiomerismor optical isomerism. The optical isomers can rotate the plane of plane polarized angles in different directions.

* represents an asymmetric carbon atom.