Problem 105 - A Complete Resource Book in Chemistry for JEE Main

Question

A substance \(\mathrm{C}_{4} \mathrm{H}_{10} \mathrm{O}\) yields on oxidation a compound, \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}\) which gives an oxime and a positive iodoform test. The original substance on treatment with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) gives \(\mathrm{C}_{4} \mathrm{H}_{\mathrm{g}}\). The structure of the compound is (a) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH}\) (b) \(\mathrm{CH}_{3} \mathrm{CHOHCH}_{2} \mathrm{CH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{O}-\mathrm{CH}_{2} \mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

The compound is \(\mathrm{CH}_{3} \mathrm{CHOHCH}_{2} \mathrm{CH}_{3}\) (option b).

Step 1: Understand the oxidation outcome

The compound \(\mathrm{C}_{4} \mathrm{H}_{10} \mathrm{O}\) when oxidized gives \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}\), which forms an oxime and a positive iodoform test. This indicates that it is likely a ketone (since ketones form oximes and some carry out the iodoform reaction). The presence of a methyl group next to the carbonyl is essential for a positive iodoform test.

Step 2: Identify the possible oxidation products

For the subtype ketone, the possible oxidation product from \(\mathrm{C}_{4} \mathrm{H}_{10} \mathrm{O}\) could be butanone. Because butanone (also known as methyl ethyl ketone) fits \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}\), meets the oxime formation requirement, and tests positive for iodoform due to the \(\mathrm{CH}_{3}-\mathrm{CO}-\) group.

Step 3: Match the reactions with options

The original compound forms \(\mathrm{C}_{4} \mathrm{H}_{8}\) when treated with \(\mathrm{conc.}\, \mathrm{H}_{2} \mathrm{SO}_{4}\). This reaction indicates a dehydration process, typically happening in alcohols. Dehydration leading to an alkene suggests the structure \((\mathrm{a})\, (\mathrm{CH}_{3})_{3} \mathrm{COH}\), as this could form isobutylene \((\mathrm{C}_{4} \mathrm{H}_{8})\) upon dehydration.

Step 4: Final verification

When considering alcohol that dehydrates to \(\mathrm{C}_{4} \mathrm{H}_{8}\) and can oxidize to butanone, \((\mathrm{a})\, (\mathrm{CH}_{3})_{3} \mathrm{COH}\) matches all criteria: isobutylene can be formed from it via dehydration, and its oxidation would yield acetone. However, the option \((\mathrm{b})\, \mathrm{CH}_{3} \mathrm{CHOHCH}_{2} \mathrm{CH}_{3}\) leads directly to butanone, satisfying both the oxime formation and iodoform tests.

Key Concepts

Alcohol Reactions

In organic chemistry, alcohol reactions are crucial for understanding a wide variety of chemical transformations. Alcohols are compounds characterized by a hydroxyl group (-OH) bonded to a carbon atom. These compounds can undergo several types of reactions depending on the structure of the alcohol and the reaction conditions.

Some of the most common reactions include:

• Dehydration, where alcohols lose water to form alkenes.
• Oxidation, where alcohols are converted into aldehydes, ketones, or carboxylic acids.
• Esterification, a reaction between an alcohol and an acid to form an ester.

These reactions make alcohols versatile intermediates in organic synthesis, playing a significant role in both industrial and laboratory settings.

Oxidation Reactions

In the realm of organic chemistry, oxidation reactions usually involve the increase in the oxidation state of a molecule by the loss of hydrogen or gain of oxygen.

For alcohols, oxidation often refers to converting alcohol into ketones or aldehydes. Primary alcohols are oxidized to form aldehydes, which can further oxidize to carboxylic acids. Secondary alcohols oxidize to form ketones. Tertiary alcohols generally resist oxidation due to lack of hydrogen atoms bonded to the carbon with the hydroxyl group.

The importance of knowing the oxidation state and the strength of the oxidizing agent lies in predicting the products formed and adjusting reaction conditions for desired outcomes.

Iodoform Test

The iodoform test is a classical organic reaction used to identify methyl ketones and secondary alcohols with a methyl group adjacent to the carbon bearing the hydroxy group. After reacting with iodine and a base, a yellow precipitate of iodoform ( CH3I) indicates a positive test.

This test is particularly useful because it helps identify compounds with specific structural features, pivotal in organic synthesis and analysis.

For instance, acetone, ethyl alcohol, and isopropyl alcohol give positive iodoform tests, which significantly aids in distinguishing these compounds from others that do not react similarly.

Dehydration Reaction

Dehydration reactions are processes by which water is removed from a molecule, typically resulting in the formation of a double bond and forming alkenes from alcohols.

This reaction usually involves heating the alcohol with a strong acid, like sulfuric acid or phosphoric acid, which acts as a catalyst.

• For example, the dehydration of tert-butanol (( CH3) 3COH) releases water and forms isobutylene ( C8H8)

This type of reaction is essential in industrial chemistry, where alkenes serve as starting materials for various essential compounds.

Ketones

Ketones are an important class of organic compounds characterized by the presence of a carbonyl group ( C=O) bonded to two carbon atoms. This structural feature makes them different from aldehydes, where at least one carbonyl is bonded to a hydrogen atom.

Common ketones include acetone and butanone, both valuable in commercial and industrial applications.

Ketones form through several reactions:

• Oxidation of secondary alcohols.
• Ozonolysis of alkenes.
• Friedel-Crafts acylation.

The versatility of ketones lies in their relative stability and ability to participate in further reactions, making them central figures in organic synthesis pathways.