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 acid do not give the characteristic reaction of carbonyl group because:

carbonyl carbon of carboxyl group is less electrophilic 

The correct answer is option 4. carbonyl carbon of carboxyl group is less electrophilic  

Let us look into the explanation:

1. Carboxylic Acid Structure:

Carboxylic acid has the general structure \(RCOOH\), where \(R\) represents an organic group. The carbonyl group in a carboxylic acid is \(C=O\), and it is attached to a hydroxyl group (\(OH\)).

2. Influence of the Hydroxyl Group:

The presence of the hydroxyl group (\(OH\)) adjacent to the carbonyl carbon has a significant impact on the reactivity of the carbonyl group.
The oxygen atom in the hydroxyl group is more electronegative than carbon, and it can withdraw electron density from the carbonyl carbon through resonance and inductive effects.

3. Electrophilicity of Carbonyl Carbon:

The carbonyl carbon in a carboxylic acid is less electrophilic compared to the carbonyl carbon in simple carbonyl compounds (e.g., aldehydes or ketones) because of the electron-withdrawing influence of the adjacent oxygen atom.
The partial negative charge on the oxygen atom makes the carbonyl carbon less attractive to nucleophiles.

4. Characteristic Reactions of Carbonyl Compounds:

Characteristic reactions of the carbonyl group often involve nucleophilic attack by species with electron-rich centers.
In aldehydes and ketones, where the carbonyl carbon is more electrophilic, nucleophilic addition reactions are common.

5. Carboxylic Acids and Nucleophilic Attack:

Due to the less electrophilic nature of the carbonyl carbon in carboxylic acids, they are less prone to nucleophilic attack compared to other carbonyl compounds.
Instead of nucleophilic addition, carboxylic acids often undergo reactions involving the nucleophilic attack on the acidic proton of the hydroxyl group.

Conclusion:

The key point is that the presence of the hydroxyl group in carboxylic acids makes the carbonyl carbon less electrophilic. This, in turn, affects the characteristic reactions of the carbonyl group in carboxylic acids, making them less responsive to nucleophilic addition reactions compared to simple carbonyl compounds like aldehydes and ketones. Option 4, stating that the carbonyl carbon of the carboxyl group is less electrophilic, is the correct explanation.