Problem 23 - A Complete Resource Book in Chemistry for JEE Main

Question

Ketones \(\left(\mathrm{R}_{1} \mathrm{COR}_{2}\right) ; \mathrm{R}_{1}=\mathrm{R}_{2}=\) alkyl group, can be obtained in one step by (a) hydrolysis of esters (b) oxidation of primary alcohols (c) oxidation of secondary alcohols (c) reaction of acid halides and alcohlos

Step-by-Step Solution

Answer

Oxidation of secondary alcohols (option c) forms ketones.

Step 1: Identifying Reaction for Ketone Formation

To form ketones, we need to consider the possible reactions that result in a ketone structure (R1COR2). A common method for synthesis of ketones is oxidation of secondary alcohols, where the alcohol group is converted to a carbonyl group, resulting in a ketone.

Step 2: Analysis of Given Options

We need to evaluate each option: (a) Hydrolysis of esters generally gives carboxylic acids and alcohols, not ketones. (b) Oxidizing primary alcohols typically produces aldehydes and then further to carboxylic acids, not ketones. (c) Oxidizing secondary alcohols converts the hydroxyl group into a carbonyl group, resulting in ketones. (d) Reaction of acid halides and alcohols might produce esters or other compounds, not specifically ketones.

Step 3: Determining the Correct Option

Given the typical reactions and expected outcomes, oxidation of secondary alcohols is the only reaction that directly results in the formation of ketones.

Key Concepts

Oxidation of Secondary Alcohols

Turning a secondary alcohol into a ketone is a very common chemical process. Typically, secondary alcohols are oxidized using an oxidizing agent such as potassium dichromate ( K_2Cr_2O_7 ) or pyridinium chlorochromate (PCC). During oxidation, the hydroxyl group, which is the (-OH) group, is changed into a carbonyl group (=O) . This conversion results in the formation of a ketone, distinguished by its RCOR structure. It's important to note that during this reaction, no further oxidation occurs past the ketone, unlike primary alcohols, which can further oxidize to carboxylic acids.

Hydrolysis of Esters

When esters undergo hydrolysis, they break down into carboxylic acids and alcohols. Hydrolysis is essentially a reaction with water (H_2O) , and it can happen either in the presence of an acid (acidic hydrolysis) or a base (basic hydrolysis or saponification).
In acidic conditions, the ester is protonated, making it easier to break the ester bond, resulting in a carboxylic acid and an alcohol. In basic conditions, the ester reacts with a hydroxide ion, resulting in a carboxylate ion and alcohol. The main takeaway is that esters do not directly produce ketones through hydrolysis; that is why this reaction pathway is not suitable for ketone formation.

Primary vs Secondary Alcohols

Alcohols are organic compounds with one or more hydroxyl groups (-OH) attached to a saturated carbon atom. The classification of an alcohol as primary or secondary depends on the carbon that the hydroxyl group is attached to.

• **Primary alcohols**: The hydroxyl group is connected to a carbon atom that is only bonded to one other carbon atom. They typically oxidize to form aldehydes, and further oxidation can lead to carboxylic acids.
• **Secondary alcohols**: The hydroxyl group is bonded to a carbon atom that is attached to two other carbon atoms. Upon oxidation, these form ketones and cannot be oxidized further without breaking the carbon framework.

Understanding these differences is crucial for predicting the types of compounds produced through oxidation reactions.

Acid Halides and Alcohol Reactions

Acid halides, also known as acyl halides, possess a structure of (RCOX) where X is a halogen. When they react with alcohols, esterification takes place, usually forming an ester and releasing hydrogen halide as a byproduct. This reaction does not directly create ketones, as the ester formation predominates.
Furthermore, this reaction often requires a base to neutralize the byproducts and optimize the yield of esters. Although acid halides can participate in various reactions, their interaction with alcohols is not primarily aimed at forming ketones.