Problem 107 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO} \underset{(\mathrm{HCl})}{\stackrel{\mathrm{NaCN}}{\longrightarrow} \mathrm{A}} \stackrel{\mathrm{H}_{3} \mathrm{O}}{\longrightarrow} \mathrm{B}\) in the above sequence of reactions \(\mathrm{A}\) and \(\mathrm{B}\) are (a) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH}) \mathrm{CN},\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCOOH}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH}) \mathrm{CN},\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH})_{2}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH}) \mathrm{CN},\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH}) \mathrm{COOH}\) (d) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH}) \mathrm{CN},\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{O}\)

Step-by-Step Solution

Answer

The answer is (c): A is \((\mathrm{CH}_3)_2\mathrm{C}(\mathrm{OH})\mathrm{CN}\) and B is \((\mathrm{CH}_3)_2\mathrm{C}(\mathrm{OH})\mathrm{COOH}\).

Step 1: Identify the Starting Reactant

The given compound is acetone, \((\mathrm{CH}_3)_2\mathrm{CO}\). Acetone is a simple ketone with a carbonyl group.

Step 2: Addition of Sodium Cyanide (NaCN) in the Presence of HCl

When acetone reacts with \(\mathrm{NaCN}\) in an acidic medium, such as \(\mathrm{HCl}\), a cyanohydrin is formed. In this reaction, the \(\mathrm{CN}^-\) ion attacks the carbon of the carbonyl group (\(C=O\)) in acetone to form \(\( (\mathrm{CH}_3)_2\mathrm{C}(\mathrm{OH})\mathrm{CN}\)\), named compound A.

Step 3: Hydrolysis of Compound A with Water and Acid (\(\mathrm{H}_3\mathrm{O}^+\))

The cyanohydrin, \((\mathrm{CH}_3)_2\mathrm{C}(\mathrm{OH})\mathrm{CN}\), undergoes hydrolysis in the presence of an acidic medium (\(\mathrm{H}_3\mathrm{O}^+\)). During this process, the cyano group (\(\mathrm{CN}\)) is converted into a carboxylic acid (\(\mathrm{COOH}\)), resulting in the formation of \((\mathrm{CH}_3)_2\mathrm{C}(\mathrm{OH})\mathrm{COOH}\), referred to as compound B.

Step 4: Match the Reaction Products to the Options

From the identified products, compound A is \((\mathrm{CH}_3)_2\mathrm{C}(\mathrm{OH})\mathrm{CN}\) and compound B is \((\mathrm{CH}_3)_2\mathrm{C}(\mathrm{OH})\mathrm{COOH}\). By comparing with the given options, option (c) matches the compounds A and B.

Key Concepts

Acetone Reactions

Acetone, with its formula \((\mathrm{CH}_3)_2\mathrm{CO}\), is a common organic solvent and a simple ketone. Its structure includes a carbonyl group, characterized by a carbon double-bonded to an oxygen atom \((C=O)\). This functional group makes acetone reactive in various chemical scenarios, often serving as an electrophile. When reacting, the carbon in the carbonyl group typically acts as the site's focal point for nucleophilic attack.
In the presence of a nucleophile, such as the cyanide ion \((\mathrm{CN}^-)\), acetone can undergo transformations that modify its structure. This particular reactivity of the carbonyl group in acetone forms the starting point for many important organic reactions, including the formation of cyanohydrins. Understanding acetone's behavior in reactions is crucial for predicting and planning chemical syntheses involving ketones.

Cyanohydrin Formation

Cyanohydrin formation is a key reaction in organic chemistry where an aldehyde or ketone reacts with hydrogen cyanide \((HCN)\) or, as in this case, sodium cyanide \((\mathrm{NaCN})\) in an acidic environment, to form a cyanohydrin. The resulting product has an additional cyanohydrin functional group. The reaction begins with the nucleophilic cyanide ion \((\mathrm{CN}^-)\) attacking the electrophilic carbon atom of the carbonyl group. This results in the formation of an intermediate that adds an \((\mathrm{OH})\) group, creating a more complex structure \((\mathrm{CH}_3)_2\mathrm{C}(\mathrm{OH})\mathrm{CN})\). This compound, known as a cyanohydrin, is marked by an alcohol \((\mathrm{OH})\) and a nitrile \((\mathrm{CN})\) group on the same carbon.
Cyanohydrins are valuable intermediates in synthetic chemistry applications, often serving as junction points in the production of various chemicals, including pharmaceuticals.

Hydrolysis of Nitriles

The hydrolysis of nitriles involves converting the nitrile \((\mathrm{CN})\) group into a carboxylic acid \((\mathrm{COOH})\). This process typically occurs under the influence of acid or base catalysts, with the acidic pathway being more common in organic synthesis. This transformation is significant in creating carboxylic acids from simpler precursors.In the provided sequence, the cyanohydrin \((\mathrm{CH}_3)_2\mathrm{C}(\mathrm{OH})\mathrm{CN})\) undergoes hydrolysis in the presence of hydrogen ions \((\mathrm{H}_3\mathrm{O}^+)\). The reaction proceeds through several protic steps, where the nitrile group first converts into an amide and then further hydrolyzes into the acid.
The final product from this reaction is \((\mathrm{CH}_3)_2\mathrm{C}(\mathrm{OH})\mathrm{COOH})\), which is a useful carboxylic acid-containing compound. Understanding the hydrolysis of nitriles is essential for chemists working on synthesizing carboxylic acids from nitrile-containing starting materials.