6.1 Acidic character

  Alcohols, Phenols and Ethers

Acidic character: Alcohols are very weak acids $\left( {{K_a} = {{10}^{ - 16}} - {{10}^{ - 18}}} \right)$ even feeble than water $\left( {{K_a} = {{10}^{ - 14}}} \right)$ and they do not turn blue litmus red. Alcohols behave as an acid because of the presence of polar $O—H$ group.

Due to the greater electronegativity of an oxygen atom, the shared pair between `O’ and H is drawn towards the oxygen atom helping in the release of the $H^+$ ion. However, the release of $H^+$ ion in alcohol does not take place with ease and hence the alcohols act as very weak acids.

This can be explained on the basis of electron releasing nature of alkyl group present in alcohol. This electron releasing tendency of alkyl group increases electron density around the oxygen atom due to which polarity of $O—H$ bond decreases.

In addition, the alkoxide ion formed is also destabilized due to the concentration of negative charge on oxygen atom by electron releasing inductive effect of the alkyl group. Thus the release of $H^+$ ion is difficult. This explains the weak acidic characters of alcohols.

The order can be explained in terms of electron releasing inductive effect of alkyl groups. Greater is the number of alkyl group attached to carbon carrying $—OH$ group lesser shall be the polarity of $—OH$ group and hence lesser will be an acidic character.

In tertiary alcohols, the carbon atom carrying $–OH$ group is attached to three alkyl groups. Due to which electron density on oxygen is minimum in their case and hence the loss of ${H^ + }$ is most difficult. Thus tertiary alcohols are least acidic; whereas primary alcohols are most acidic

Alcohol’s reaction with metal

Note: Alcohols do not react with NaOH (due to the low acidic character)