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
will readily leave the alcohol.
, 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
, it gains a stable state.
So alcohol will lose
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
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
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
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.