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Question
Phenols are compounds in which a hydroxyl group is bonded directly to an sp2 hybridized carbon atom of an aromatic ring. It is more acidic than an alcohol, but less acidic than a carboxylic acid. It is even less acidic than carbonic acid but more acidic than water. Thus, phenol is unable to protonate HCO−3 anion but easily protonates OH− anion. The OH− group activates the benzene ring so much that electrophilic substitution takes place readily.
The correct order of acidity of the following substituted phenols is:
Phenols are compounds in which a hydroxyl group is bonded directly to an sp2 hybridized carbon atom of an aromatic ring. It is more acidic than an alcohol, but less acidic than a carboxylic acid. It is even less acidic than carbonic acid but more acidic than water. Thus, phenol is unable to protonate HCO−3 anion but easily protonates OH− anion. The OH− group activates the benzene ring so much that electrophilic substitution takes place readily.
The correct order of acidity of the following substituted phenols is:
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Solution
The correct option is D I>III>II>IV
The acidity of phenol increases when electron withdrawing groups, such as −NO2 or chloro are present. This is due to inductive effect and mesomeric effect.
The acidity of phenol decreases such as electron releasing group, e.g., a methyl group, is present.
When nitro group is in the para position, the resulting phenol is most acidic.
When methyl group is in the para position, the phenol is least acidic.
Phenol with nitro group in meta position is more acidic than phenol with chloro group in para position.
The increasing order of pKa values is : 7.15(I)<8.39(III)<9.38(II)<10.2(IV).
The acid having a lower value of pKa is stronger acid.
Thus the correct order of acid strength is I>III>II>IV.
The acidity of phenol increases when electron withdrawing groups, such as −NO2 or chloro are present. This is due to inductive effect and mesomeric effect.
The acidity of phenol decreases such as electron releasing group, e.g., a methyl group, is present.
When nitro group is in the para position, the resulting phenol is most acidic.
When methyl group is in the para position, the phenol is least acidic.
Phenol with nitro group in meta position is more acidic than phenol with chloro group in para position.
The increasing order of pKa values is : 7.15(I)<8.39(III)<9.38(II)<10.2(IV).
The acid having a lower value of pKa is stronger acid.
Thus the correct order of acid strength is I>III>II>IV.
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