Aldehydes and ketones are two important classes of organic compounds which are widely used in the synthesis of a variety of other organic compounds. Due to the polarity of the carbonyl group, they undergo nucleophilic addition reactions. Broadly speaking, these addition reactions are of two types- one in which simply the addition of the nucleophilic reagent occurs across \(>C=O\) bond and the other in which addition is followed by the elimination of a molecule of water. A number of ammonia derivatives such as hydroxylamine, hydrazine, phenylhydrazine, 2,4-dinitrophenylhydrazine, and semicarbazide belong to the second category of reactions. These derivatives are used for the identification and characterization of aldehydes/ketones. Both types of nucleophilic addition reactions are influenced by steric and electronic factors. In general, aliphatic aldehydes are more aromatic aldehydes. The aliphatic aldehydes reduce both Fehling's solution and Tollen's reagent but aromatic aldehydes reduce only Tollen's reagent. Like aldehydes, aromatic ketones are less reactive than aliphatic ketones towards nucleophilic addition reactions.

Phenols and carboxylic acids are acidic in nature. Both dissolve in \(NaOH\) solution and turn blue litmus red. However carboxylic acids are much stronger acids than phenols and decompose \(NaHCO_3\) with the evolution of \(CO_2\) but phenols do not. Both electron-donating and electron-withdrawing substituents influence the acid strength of aliphatic as well as aromatic acids. Unlike all other aliphatic acids, formic acid has reducing properties and also does not show reactions of the alkyl group.

Carboxylic acids do not give the characteristic reaction of the carbonyl group because of:

Resonance

The correct answer is (2) Resonance.

The correct explanation for why carboxylic acids do not exhibit the characteristic reactions of the carbonyl group to the same extent as other carbonyl compounds resonance.
1. Polar Nature: The polar nature of carboxylic acids is not the primary reason why they do not show characteristic reactions of the carbonyl group. In fact, the presence of the carbonyl group itself makes them polar.

2. Resonance:  The correct explanation is resonance. In carboxylic acids, the carbonyl carbon is part of a carbonyl group (\(>C=O\)), and it is also bonded to an oxygen atom of the hydroxyl group (\(-OH\)). The resonance stabilization involves the delocalization of the electrons between the oxygen of the carbonyl group and the oxygen of the hydroxyl group. This resonance stabilization makes the carbonyl carbon less electrophilic and less prone to nucleophilic attack compared to simple carbonyl compounds.

3. Symmetrical Structure: The symmetrical structure of carboxylic acids is not the primary reason for their behavior. The key factor is the resonance stabilization discussed above.

4. Attached Alkyl Group: While the presence of alkyl groups can affect reactivity, the primary reason in this context is the resonance stabilization involving the hydroxyl group.

Therefore, option 2, resonance, is the most accurate explanation for why carboxylic acids do not exhibit the characteristic reactions of the carbonyl group to the same extent as other carbonyl compounds.