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 compounds \( \mathrm{A} \) and \( \mathrm{B} \) are (c) \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH})\mathrm{CN} \) and \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH})\mathrm{COOH} \).

Step 1: Identify the Starting Material and Reagents

The starting material is acetone, or \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO} \), and it is reacted with NaCN in the presence of HCl. This is a nucleophilic addition reaction where the cyanide ion (\( \mathrm{CN}^{-} \)) acts as a nucleophile.

Step 2: Reaction of Acetone with NaCN and HCl

In this reaction, the cyanide ion adds to the carbonyl carbon of acetone, and a proton from \( \mathrm{HCl} \) adds to the oxygen. This forms a cyanohydrin compound, \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH})\mathrm{CN} \), which we label as compound \( \mathrm{A} \).

Step 3: Hydrolysis of Compound A

Compound \( \mathrm{A} \), \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH})\mathrm{CN} \), is then treated with \( \mathrm{H}_{3}\mathrm{O}^+ \), which is an acidic medium. Under these conditions, the cyano group (\( \mathrm{C}\equiv\mathrm{N} \)) undergoes hydrolysis to form a carboxylic acid group (\( \mathrm{COOH} \)). The resulting compound \( \mathrm{B} \) is \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH})\mathrm{COOH} \).

Step 4: Confirm the Products

Based on the reactions described, the product \( \mathrm{A} \) is \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH})\mathrm{CN} \), and \( \mathrm{B} \) is \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{OH})\mathrm{COOH} \). This matches option (c) in the given choices.

Key Concepts

Cyanohydrin Formation

Cyanohydrin formation is a key reaction under the broader category of nucleophilic addition reactions. Imagine you're taking a molecule with a carbonyl group, like acetone, and you want to attach something new. In our example, the cyanide ion \( \mathrm{CN}^{-} \) plays the role of a nucleophile. It seeks to donate a pair of electrons to stabilize its surrounding by forming a new bond.

Here's the interesting part: when the cyanide ion approaches the carbonyl group, it targets the carbon atom. Why? Because the carbon is slightly positive due to the electron-withdrawing nature of the oxygen atom attached to it. This electron-loving behavior facilitates the attachment of the cyanide, morphing the molecule into a new *cyanohydrin structure*.

• The carbonyl group of acetone (\( \left(\mathrm{CH}_3\right)_2\mathrm{CO} \)) is the target.
• In the presence of HCl, a hydrogen ion joins the oxygen.
• This results in the formation of \( \left(\mathrm{CH}_3\right)_2\mathrm{C}(\mathrm{OH})\mathrm{CN} \), our cyanohydrin.

Cyanohydrin compounds are noteworthy for their dual functionality: they have both alcohol and nitrile groups, giving them fascinating chemical properties and reactivity.

Carbonyl Compounds

Carbonyl compounds are an essential part of organic chemistry. These compounds contain a carbon atom double-bonded to an oxygen atom, forming a functional group known as a carbonyl group. This simple structure opens up a world of chemical possibilities due to the highly reactive nature of the carbonyl group.

A carbonyl compound, such as acetone, is known for its role in nucleophilic addition reactions. The presence of the carbonyl group makes the carbon atom electrophilic, meaning it is prone to attacks by nucleophiles. This nature is exploited in countless organic reactions, including the formation of cyanohydrins.

• Carbonyl groups possess a polar bond because oxygen is more electronegative than carbon.
• This polarity creates an electron-deficient carbon atom, a perfect site for nucleophilic attack.
• Reactions involving carbonyl compounds often result in the transformation of both the carbon-oxygen double bond and its surrounding structure.

Understanding carbonyl compounds and their reactivity is central to grasping complex organic mechanisms and transformations.

Organic Reaction Mechanisms

Organic reaction mechanisms offer a detailed look into how and why reactions occur the way they do. They explain the step-by-step transformations that take place in a reaction, featuring intermediates and transition states that elucidate the pathways from reactants to products.

Consider the sequence in our exercise: Acetone first reacts with NaCN and HCl to form a cyanohydrin. This is an example of a nucleophilic addition mechanism where the nucleophile, \( \mathrm{CN}^{-} \), adds to the carbonyl carbon. The reaction's progression illustrates the concept of electron flow and bond formation.

• Mechanisms illuminate each stage of a reaction, helping you predict the outcomes of similar reactions.
• They unveil transient species, intermediates that can't be isolated but are crucial in understanding transformation steps.
• They explain why certain conditions, like the presence of acid or base, are needed for particular transformations.

Unraveling an organic mechanism is like following a detailed map through a chemical landscape, ensuring you understand not just where you're going, but how you're getting there.