Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

The cyanohydrin of a compound \(\mathrm{X}\) on hydrolysis gives lactic acid; the \(\mathrm{X}\) is (a) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO}\) (b) \(\mathrm{CH}_{3} \mathrm{CHO}\) (c) \(\mathrm{HCHO}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CHO}\)

Step-by-Step Solution

Answer

The compound \(X\) is \(\text{CH}_3\text{CHO}\), option (b).

Step 1: Understand Cyanohydrin Formation

Cyanohydrins are formed when a carbonyl group (in an aldehyde or ketone) reacts with hydrogen cyanide (HCN). This reaction adds a cyanide group (\(-CN\)) and a hydroxyl group (\(-OH\)) across the carbon-oxygen double bond.

Step 2: Relationship to Lactic Acid

Lactic acid is an \(\alpha\)-hydroxy acid, which has both an alcohol (\(-OH\)) group and a carboxylic acid (\(-COOH\)) group on adjacent carbons. When a cyanohydrin undergoes hydrolysis, it converts the \(-CN\) group into a \(-COOH\) group, forming a carboxylic acid.

Step 3: Identify the Hydrolysis Product

The formation of a carboxylic acid with an adjacent hydroxyl group occurs when the starting carbonyl compound (\(X\)) forms a cyanohydrin, which upon hydrolysis gives lactic acid. This is characteristic of ethanal (\(\text{CH}_3\text{CHO}\)) as \(\text{CH}_3\text{CHO}\) can form this specific cyanohydrin and subsequent hydrolysis product.

Step 4: Analyze the Options

Each option represents a different carbonyl compound:(a) \(\left(\text{CH}_3\right)_2\text{CO}\) is acetone, a ketone that doesn't match the conditions for forming lactic acid via this mechanism.(b) \(\text{CH}_3\text{CHO}\) is ethanal, which can form lactic acid upon correct cyanohydrin formation and hydrolysis.(c) \(\text{HCHO}\) is formaldehyde, forming formic acid derivative upon cyanohydrin mechanism.(d) \(\text{C}_6\text{H}_5\text{CH}_2\text{CHO}\) is phenylacetaldehyde, producing phenylacetic acids sequence.

Step 5: Conclusion

Since ethanal (\(\text{CH}_3\text{CHO}\)) can form the cyanohydrin that hydrolyzes to give lactic acid, option (b) is correct.

Key Concepts

Cyanohydrin Formation

Cyanohydrins are fascinating chemical structures formed through a simple yet intriguing reaction process. These compounds emerge when a carbonyl group, which is present in aldehydes and ketones, reacts with hydrogen cyanide (HCN). This reaction is an addition reaction, where the

• cyanide group ([-CN])
• hydroxyl group ([-OH])

are added across the carbon-oxygen double bond.
This results in the formation of a new molecule called a cyanohydrin, which has both a [-CN] and a [-OH] group.

The reaction starts when the [-CN] group attacks the electrophilic carbon of the carbonyl group, opening up the double bond.
Subsequently, the oxygen carries a negative charge and takes up a proton from the reaction medium to form the hydroxyl group.

Understanding cyanohydrin formation is essential for studying how various compounds, such as lactic acid, can be synthesized through further chemical reactions.

Hydrolysis Reaction

Hydrolysis is a key chemical process involving the reaction of water with another compound, leading to the breakdown of bonds in the compound.
Cyanohydrins undergo hydrolysis effectively, turning the [-CN] group into a carboxylic acid group ([-COOH]).
This happens through a series of steps characterized by the addition of water, which splits the cyanohydrin and transforms it into an [\[\alpha\]-hydroxy acid\u00b].

In the case of cyanohydrins derived from aldehydes like ethanal, hydrolysis produces lactic acid, a notable [\[\alpha\]-hydroxy acid\u00b].
This conversion happens because water effectively breaks the cyano group and introduces an oxygen to form a hydroxyl and carboxyl group.

This mechanism is crucial in organic chemistry because it demonstrates how a seemingly small molecular modification can lead to entirely new compounds with significant biological and industrial relevance.

Lactic Acid Synthesis

The synthesis of lactic acid from cyanohydrins is a well-known chemical process involving two major steps: cyanohydrin formation and hydrolysis.
Lactic acid, identified as [\[\alpha\]-hydroxypropanoic acid\u00b], is an [\[\alpha\]-hydroxy acid\u00b] distinguished by having both an alcohol ([-OH]) and a carboxylic acid ([-COOH]) group on adjacent carbons.

When the cyanohydrin formed from ethanal undergoes hydrolysis, the [-CN] group is transformed to a [-COOH] group, resulting in lactic acid.
This method of synthesis is valuable due to its efficient conversion of simple aldehydes into more complex molecules.

Lactic acid is important in various sectors, including food, pharmaceuticals, and cosmetics.
Understanding its synthesis through such mechanisms helps chemists develop better methods for large-scale production, thereby making it more accessible for diverse applications.

• Effective transformation from simple compounds
• Applications in food and health industries
• Enhances the understanding of fundamental organic reactions