Acidity of Alcohols and Phenols

Organic compounds containing OH group is termed as alcohol

In OH group, O is more electronegative than H. So it starts pulling on the shared electrons

A partial negative charge develops on O and partial positive charge develops on H

Now if a suitable base is introduced in alcohol, the $H^{\oplus }$ will readily leave the alcohol.

On losing $H^{\oplus }$, Alcohols gives alkoxide ion

Since alcohols lose hydrogen ion, they are said to show acidic character

However acidic strength of all alcohols is not same

The acidity of alcohols depend on the stability of alkoxide ion left behind

Having more stable alkoxide ion means that when alcohol loses $H^{\oplus }$, it gains a stable state.

So alcohol will lose $H^{\oplus }$ readily. Hence it will be more acidic

We can use this stability to compare strengths of alcohols

Let's first compare acidic strength of primary, secondary and tertiary alcohol

These will be the alkoxide ions when these alcohols lose $H^{\oplus }$

Alkyl is an electron releasing group. It will destabilise the negative charge on O

More number of alkyl groups will mean more destabilizing effect on the negative charge on O. So the stability of alkoxide ion will be $1^0>2^0>3^0$

This means strength of acidic order in these alcohols will be primary > secondary > tertiary.

Similarly we can compare strength of aromatic and aliphatic alcohols

On comparison, phenol is found to be more acidic than most aliphatic alcohols

This is because the phenoxide ion is highly stable

The phenoxide ion has 4 resonating structures. This makes it more stable than alkoxide ions

Revision

Acidity of alcohols depend on the stability of alkoxide/phenoxide and due the polar nature of OH bond.

More resonating structure means more stable alkoxide/phenoxide.

Phenoxide is more stable than alkoxide because phenoxide has more resonating structure than alkoxide.

The End