Problem 164 - A Complete Resource Book in Chemistry for JEE Main

Question

Compound 'A' (molecular formula \(\mathrm{C}_{3} \mathrm{H}_{\mathrm{s}} \mathrm{O}\) ) is treated with acidified potassium dichromate to form a product 'B' (molecular formula \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}\) ). 'B' forms a shining silver mirror on warming with ammonical silver nitrate. 'B' when treated with an aqueous solution of \(\mathrm{H}_{2} \mathrm{NCONHNH}_{2}, \mathrm{HCl}\) and sodium acetate gives a product ' \(C^{\prime}\). Identify the structure of ' \(\mathrm{C}^{\prime}\)

Step-by-Step Solution

Answer

The structure of 'C' is propane \( \mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_3 \).

Step 1: Analyze the Reaction with Potassium Dichromate

Compound 'A' with molecular formula \( \mathrm{C}_3 \mathrm{H}_8 \mathrm{O} \) is an alcohol because it undergoes oxidation. When treated with acidified potassium dichromate, it forms compound 'B' with the formula \( \mathrm{C}_3 \mathrm{H}_6 \mathrm{O} \). This indicates that the alcohol is oxidized to a carbonyl compound, either an aldehyde or a ketone.

Step 2: Determine Type of Carbonyl Compound

Compound 'B' forms a silver mirror with ammonical silver nitrate (Tollens' reagent). This means 'B' is an aldehyde, as Tollens' reagent is specific for aldehydes. Therefore, 'B' is an aldehyde with the formula \( \mathrm{C}_3 \mathrm{H}_6 \mathrm{O} \), likely propanal.

Step 3: Analyze Reaction with Hydrazine Derivative

When 'B' is treated with \( \mathrm{H}_2 \mathrm{NCONHNH}_2 \), \( \mathrm{HCl} \), and sodium acetate, it undergoes a Wolff-Kishner reduction. This reaction reduces the carbonyl group (aldehyde) to a hydrocarbon. Thus, the product 'C' is the corresponding alkane.

Step 4: Identify the Structure of Compound 'C'

Since 'B' is propanal \( \left(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CHO}\right)\), and it undergoes Wolff-Kishner reduction, the aldehyde carbonyl is reduced to a methylene group, resulting in propane \( \mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_3 \). Thus, 'C' is propane.

Key Concepts

Oxidation of Alcohols

Alcohols, which are organic compounds containing the hydroxyl (\(-OH\)) functional group, can undergo various transformations. One such important reaction is oxidation. Oxidation of alcohols refers to the process where an alcohol is converted into either an aldehyde, ketone, or carboxylic acid by removing hydrogen or adding oxygen. The outcome depends on the type of alcohol: primary, secondary, or tertiary.

• Primary Alcohols: When oxidized, primary alcohols typically yield aldehydes, which can further oxidize to carboxylic acids if oxidation continues.
• Secondary Alcohols: These usually oxidize to form ketones, which are generally resistant to further oxidation.
• Tertiary Alcohols: Due to the lack of a hydrogen atom on the carbon attached to the OH group, tertiary alcohols resist oxidation under mild conditions.

One common oxidizing agent used in laboratories is acidified potassium dichromate (\(K_2Cr_2O_7\)), which is effective in converting alcohols to carbonyl compounds such as aldehydes and ketones.

Tollens' Test

Tollens' test, a classic organic chemistry experiment, exploits the ability of aldehydes to be oxidized, distinguishing them from ketones. This test utilizes Tollens' reagent, which consists of an ammoniacal silver nitrate solution. When an aldehyde is present, the test results in the deposition of a silver mirror on the inside of the test vessel. Here's how it works:

• The aldehyde group (\(-CHO\)) is oxidized to a carboxylate ion (\(-COOH\)), while the silver ion is reduced to metallic silver, forming the mirror.
• Ketones do not produce a silver mirror since they do not easily oxidize under these conditions.

Thus, Tollens' test is a valuable tool for identifying aldehydes, as it clearly differentiates aldehydes from most ketones. This makes it an indispensable reaction in organic chemistry for verifying the identity of carbonyl compounds.

Wolff-Kishner Reduction

The Wolff-Kishner reduction is a widely used chemical reaction in organic chemistry for transforming carbonyl groups (such as those in aldehydes and ketones) into methylene groups, resulting in saturated hydrocarbons. This reduction is noteworthy for removing the oxygen atom from the carbonyl group, effectively replacing it with two hydrogen atoms.In this reaction:

• The carbonyl compound reacts with hydrazine (\(H_2N-NH_2\)), forming a hydrazone.
• Subsequent heating with a strong base, like sodium or potassium hydroxide, completes the reduction, replacing the carbonyl with a \(CH_2\) group.

The Wolff-Kishner reduction is particularly advantageous because it allows for complete reduction of a carbonyl compound without affecting other functional groups sensitive to metals or acids. This makes it a valuable method for synthesizing hydrocarbons from aldehydes or ketones.