Problem 153 - A Complete Resource Book in Chemistry for JEE Main

Question

An alcohol \(\mathrm{C}_{5} \mathrm{H}_{11} \mathrm{OH}\), on dehydration, gives an alkene, which on oxidation yields a mixture of a ketone and an acid. The alcohol is (a) CCCC(C)O (b) CC(C)C(C)O (c) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{OH}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

The correct alcohol is (c) (CH3)3CCH2OH.

Step 1: Understand Dehydration Reaction

In the dehydration process, the alcohol removes a water molecule, producing an alkene. Consider that the resulting alkene can be evaluated to understand the possible structures of alcohols.

Step 2: Analyze Oxidation Reaction

The alkene formed from dehydration undergoes oxidation to yield a ketone and an acid. These products give insight into which groups were adjacent in the original alkene, helping to backtrack to the original alcohol structure.

Step 3: Test Each Option

Let's analyze each option: (a) CCCC(C)O gives alkenes with oxidation leading only to one product type (not ketone plus acid). (b) CC(C)C(C)O results in alkenes providing only one type of product upon oxidation. (c) (CH3)3CCH2OH dehydration yields (CH3)2C=CH2, forming (CH3)2C=O and HCOOH upon oxidation; thus, it matches the products needed. (d) CH3CH2CH(OH)CH2CH3 dehydration results in an alkene, which does not account for both ketone and acid on oxidation.

Step 4: Verify Best Answer

Evaluate the option (c) (CH3)3CCH2OH more closely. Dehydration forms an alkene where oxidation produces the correct two products: a ketone and an acid, confirming this option as correct.

Key Concepts

Alcohol Dehydration

Alcohol dehydration is a fundamental chemical reaction in organic chemistry, where an alcohol molecule loses a water molecule to form an alkene. This process involves the removal of the hydroxy group (-OH) and a hydrogen atom from adjacent carbon atoms in the alcohol structure. The result of this elimination reaction is a double bond formation, characteristic of alkenes.

• This reaction is typically catalyzed by acids such as sulfuric acid or phosphoric acid.
• The reaction conditions, like temperature and the presence of a catalyst, are crucial to ensure the dehydration proceeds smoothly.
• Often, multiple alkene products are possible depending on the structure of the initial alcohol. The more substituted alkene is generally favored due to greater stability, following Zaitsev's rule.

Understanding alcohol dehydration is important for predicting the structure of the resulting alkene, which can then undergo further reactions, such as oxidation.

Alkene Formation

The formation of alkenes from alcohols via dehydration is a key step in organic synthesis. Alkenes are hydrocarbons containing one or more double bonds, which confer increased reactivity compared to their saturated counterparts. This reactivity is due to

• the presence of the electron-rich double bond, making them excellent nucleophiles.
• They can participate in reactions like addition, oxidation, and polymerization.

Alkene formation from an alcohol starts with the protonation of the alcohol's hydroxyl group by an acid, making it a better leaving group. This step is followed by the elimination of a water molecule, leaving behind a carbocation. The stability of this carbocation is crucial as it determines the favored product. A hydrogen atom from a neighboring carbon atom is then removed, forming the double bond.
The nature of the resulting alkenes, including their position and structure, determines how they will react in subsequent transformations. In the context of this reaction, once formed, the alkene can be easily transformed through an oxidation process.

Oxidation Reactions

Oxidation reactions in organic chemistry involve the increase in the oxidation state of a molecule, typically by the addition of oxygen or the removal of hydrogen. In the context of alkene oxidation, the process can yield various functional groups, such as ketones and acids.
When an alkene is subjected to oxidation:

• The double bond is cleaved, and oxygen is introduced into the structure.
• This reaction can produce different products depending on the oxidizing agent used.
• Common oxidizing agents include potassium permanganate ( KMnO₄) and ozone ( O₃), with each leading to distinct products.

In our specific exercise, the oxidation of the alkene that resulted from the alcohol dehydration leads to a ketone and an acid. This outcome provides valuable clues about the original arrangement of carbon atoms in the starting alcohol. Recognizing the products of oxidation helps understand not just the transformation itself, but the potential retrosynthetic pathways to elucidate the starting material.