Problem 155 - A Complete Resource Book in Chemistry for JEE Main

Question

Compound (X) liberates hydrogen when treated with sodium metal. On oxidation, it gives a compound (Y) with the same number of carbons. Compound (Y) gives orange coloured crystalline 2,4 -dinitrophenyl hydrazone derivative with 2, 4-dinitrophenyl hydrazine hydrochloride. Compound (Y) on treatment with aqueous \(\mathrm{NaOH}\) and iodine gives compound (Z)Compound (Z) on treatment with dilute acid yields benzoic acid. The structure of compound \((\mathrm{X})\) is (a) (b) CC(O)c1ccccc1 C=CC1=CC=CCC1O (c) C=CC1=CC=CC(O)C1 (d) OCCc1ccccc1

Step-by-Step Solution

Answer

Compound (X) is (d) ce{OCCc1ccccc1} .

Step 1: Identify the Functional Group of Compound X

Compound (X) liberates hydrogen when treated with sodium metal, which indicates that it is likely an alcohol ( ce{R-OH} ). Sodium metal reacts with alcohol to form hydrogen gas.

Step 2: Identify the Oxidation Product Compound Y

When an alcohol is oxidized and the compound formed has the same number of carbons, it suggests the formation of an aldehyde or a ketone. These are the common oxidation products of primary or secondary alcohols.

Step 3: Determine the Reagent's Reaction with Compound Y

Compound (Y) reacts with 2,4-dinitrophenylhydrazine to form an orange crystalline derivative, indicating that it is a carbonyl-containing compound such as an aldehyde or ketone.

Step 4: Identify Haloform Reaction Product Z

Compound (Y) reacts with aqueous NaOH and iodine to give compound (Z). This suggests a methyl ketone is involved, as this reaction is characteristic of the haloform reaction.

Step 5: Acid Hydrolysis of Compound Z

Compound (Z) on treatment with dilute acid yields benzoic acid. This implies that methyl ketone structure must be further oxidized to a carboxylic acid upon acid treatment, which aligns with structures containing benzoic ring systems.

Step 6: Determine Structure of Compound X

Given the consistent number of carbons and reaction outcomes leading to benzoic acid, the structure is identified as (d) ce{OCCc1ccccc1} . This is the only option fitting all the described chemical reactions.

Key Concepts

Alcohol Oxidation

Alcohols are organic compounds that contain one or more hydroxyl groups \( \text{(-OH)} \) attached to a carbon atom. A common reaction involving alcohols is the oxidation reaction. During oxidation, the \( \text{C-OH} \) group can be converted into a carbonyl group \( \text{(C=O)} \). The type of carbonyl compound formed—either an aldehyde or a ketone—depends on the classification of the alcohol: primary, secondary, or tertiary.

• Primary alcohols: These are oxidized to aldehydes. Further oxidation can yield carboxylic acids.
• Secondary alcohols: These are oxidized to ketones. They cannot be further oxidized to carboxylic acids without breaking carbon-carbon bonds.
• Tertiary alcohols: These don't typically undergo oxidation because they lack a hydrogen atom attached to the carbon bearing the \( \text{OH} \) group.

Oxidation of alcohols usually employs oxidizing agents like chromic acid, potassium dichromate, or even sulfuric acid and chromate, in a reaction termed as Jones oxidation. Knowing the product of alcohol oxidation is essential in organic synthesis and in deducing reaction pathways.

Carbonyl Compound Identification

Carbonyl compounds are characterized by the presence of a carbon-oxygen double bond \( (\text{C=O}) \). This functional group imparts distinctive chemical properties to aldehydes and ketones, the most common classes of carbonyl compounds.
Detecting and identifying carbonyl compounds often involve reactions that exploit the reactivity of the \( \text{C=O} \) group. One such reaction is the formation of 2,4-dinitrophenylhydrazones.
When a carbonyl compound reacts with 2,4-dinitrophenylhydrazine, it forms a yellow, orange, or red crystalline precipitate. This distinctive color change confirms the presence of an aldehyde or ketone.

• Aldehydes generally give a more intense color due to the absence of electron-donating substituents.
• Ketones give a lesser intense color due to the presence of electron-donating alkyl groups that stabilize the \( \text{C=O} \) group.

The 2,4-dinitrophenylhydrazone derivative not only confirms the presence of a carbonyl group but also aids in identifying the specific compound by allowing determination of its melting point, which is specific for different carbonyl compounds.

Haloform Reaction

The haloform reaction is a significant reaction in organic chemistry due to its specificity towards methyl ketones. It involves the reaction of a methyl ketone with halogens (like iodine) and a base (like sodium hydroxide) to form a haloform, such as \( \text{CHI}_3 \) (iodoform), and a carboxylate ion. This reaction can identify the presence of a methyl group directly adjacent to the carbonyl group.
The key steps in the haloform reaction include:

• The halogenation of the methyl group adjacent to the carbonyl group.
• The formation of a trihalo intermediate compound.
• The cleavage of the \( \text{C-C} \) bond to separate the haloform, depicted by the trihalo structure \( \text{CHX}_3 \), where \( \text{X} \) is a halogen atom, leaving behind a carboxylic acid or ester.

For example, the haloform reaction of a methyl ketone like acetophenone with iodine and sodium hydroxide first forms iodoacetophenone and finally gives a yellow precipitate of iodoform and benzoate ion. This reaction is widely used in the laboratory to confirm the presence of methyl ketones by observing the characteristic color and odor of iodoform.