Carboxylic acids are characterized by the presence of carboxyl group. The ¾COOH group which itself is made up of a carbonyl group (
Carboxylic acids may be aliphatic or aromatic
Comparison of resonating structures of carboxylic group and carbonyl group
Carbonyl group has two resonance structures (I and II)
However, for a carboxyl group, three resonance structures (A, B and C) can be written.
In both structures (A) and (C), the C – atom and the two O – atoms have eight electrons in their respective valence shells while in structure (B), C – atom has only six electrons. Therefore, structure (B) is less stable than structure (C), in other words the two important resonance structures of carboxyl group are structures (A) and (C). In both these structures, carboxyl carbon is electrically neutral. However in case of aldehydes and ketones, only one structure i.e. I is electrically neutral. As a result, the carboxyl carbon of the resonance hybrid is less positive and hence less electrophilic than the carbonyl carbon of aldehydes and ketones. However, it may be noted that like carbonyl group, carboxyl group is also polar due to resonance structures (B) and (C).
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