Question

(P) has molecular formula ' \(\mathrm{C}_{8} \mathrm{H}_{8} \mathrm{O}\) ' which forms red-orange ppt with (+ve) 2,4-DNP. (Q) on acidification \(\mathrm{f}_{0 \mathrm{rm}} \mathrm{C}_{7} \mathrm{H}_{6} \mathrm{O}_{2}\) Choose the correct option(s): (A) (R) is used as antiseptic (B) Red- Orange ppt. is 2,4 -dinitrophenyl hydrazone (C) (P) is acetophenone (D) (S) evolves \(\mathrm{CO}_{2}\) gas with \(\mathrm{NaHCO}_{3}\)

Short answer

The correct options are (B) and (C).

Step-by-step solution

Identify the Molecular Formula

We know that compound (P) has a molecular formula: \(\mathrm{C}_{8} \mathrm{H}_{8} \mathrm{O}\) and compound (Q) has a molecular formula: \(\mathrm{C}_{7} \mathrm{H}_{6} \mathrm{O}_{2}\).

Analyze Reactions and Properties

From the information given about compound (P), we know that it forms a red-orange precipitate with 2,4-DNP. Let's understand what this reaction implies. Compounds that form a red-orange precipitate with 2,4-dinitrophenylhydrazine (2,4-DNP) are carbonyl compounds (aldehydes and ketones).

Identify Compound P

The molecular formula of compound (P) is \(\mathrm{C}_{8} \mathrm{H}_{8} \mathrm{O}\). Since it gives a red-orange precipitate with 2,4-DNP, it must be a carbonyl compound. Considering the molecular formula and the carbonyl functional group, compound (P) could be acetophenone. So, option (C) is correct.

Evaluate 2,4-Dinitrophenyl Hydrazone Formation

Since we confirmed that compound (P) is acetophenone and gives a red-orange precipitate with 2,4-DNP, it can form 2,4-dinitrophenyl hydrazone. So, option (B) is correct.

Evaluate Other Options

Let's evaluate the other options: (A) There is no information given about compound (R) in the problem, so we cannot determine if it is used as an antiseptic. (D) The problem does not mention anything about compound (S), so we cannot determine if it evolves \(\mathrm{CO}_{2}\) gas with \(\mathrm{NaHCO}_{3}\). Therefore, options (A) and (D) are incorrect. So the correct options are (B) and (C).

Key concepts

Carbonyl Compounds

Carbonyl compounds are a group of organic compounds characterized by the presence of a carbonyl group, which is a carbon atom double-bonded to an oxygen atom (C=O). They are broadly classified into two main categories: aldehydes and ketones. Aldehydes feature the carbonyl group at the end of a carbon chain, whereas in ketones, the carbonyl group is located within the carbon chain.
It's interesting to note that the reactivity of carbonyl compounds is greatly influenced by the nature of the carbon adjacent to the carbonyl group. Due to the electrophilic nature of the carbon atom in the carbonyl group, these compounds frequently engage in nucleophilic addition reactions.

• Aldehydes: Found in compounds like formaldehyde and acetaldehyde, where the carbonyl group is bonded to at least one hydrogen atom.
• Ketones: Feature carbon chains on both sides of the carbonyl group, examples include compounds like acetone and acetophenone.

Understanding these basic classifications helps in predicting the types of chemical reactions these compounds may undergo, as seen in the identification of acetophenone with the use of specific tests.

2,4-Dinitrophenylhydrazine (2,4-DNP) Test

The 2,4-dinitrophenylhydrazine (2,4-DNP) test is an important qualitative analysis technique used in organic chemistry to detect the presence of carbonyl compounds, such as aldehydes and ketones. This test involves the reaction of a carbonyl compound with 2,4-DNP reagent, leading to the formation of a hydrazone derivative.
When a compound reacts with 2,4-DNP, a colorful precipitate forms, which is usually red, orange, or yellow. This result is a clear indicator of the presence of a carbonyl group within the compound. The color and formation of the precipitate depend on the particular carbonyl compound tested.

• A positive 2,4-DNP test confirms that the compound contains a carbonyl group, distinguishing aldehydes and ketones from other functional groups such as alcohols or carboxylic acids.

This reaction is crucial not just for identification, but also for further confirming the molecular character of the compound, as it was the case with acetophenone in the exercise. This method is simple, efficient, and a staple in the analysis and identification of carbonyl-containing compounds.

Acetophenone Identification

Acetophenone is a specific type of aromatic ketone with the molecular formula of \(\mathrm{C}_{8} \mathrm{H}_{8} \mathrm{O}\). It's a noteworthy compound within the study of carbonyl-containing substances due to its structure, which includes a phenyl group attached to a carbonyl group.
This compound is identified by its distinctive reaction with chemical reagents such as 2,4-DNP, which leads to the formation of a red-orange precipitate, indicating it is a carbonyl compound. Here's how you can identify acetophenone:

• Molecular Formula: By calculating or analyzing the molecular formula, you can deduce its potential structure as an aromatic ketone.
• 2,4-DNP Test: A positive test with a resultant red-orange precipitate provides evidence for the presence of the carbonyl group, confirming it as a likely candidate for acetophenone.

Such tests and observations enable chemists to identify acetophenone rapidly. In the context of the original exercise, the positive 2,4-DNP test played a crucial role in deducing that compound (P) is indeed acetophenone.