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 \( \mathrm{X} \) is (b) \( \mathrm{CH}_3 \mathrm{CHO} \).

Step 1: Understand Cyanohydrin Formation

Cyanohydrin is formed when a ketone or aldehyde reacts with hydrogen cyanide, \( ext{HCN} \). The carbonyl group (\( \text{C=O} \)) in the ketone or aldehyde reacts with the \( \text{CN}^- \) ion, forming a cyano group and a hydroxyl group at that carbon.

Step 2: Identify Possible Cyanohydrins

Consider each option to see which could form a cyanohydrin that hydrolyses to lactic acid. Recall that lactic acid has the structure \( ext{CH}_3 ext{CH(OH)COOH} \). Compare this with the possible hydrolysis products of the cyanohydrins from the options given.

Step 3: Compare Options

If option (a), \(( ext{CH}_3)_2 ext{CO}\), were to form a cyanohydrin and then hydrolyze, it would not result in lactic acid as the structure does not allow for the creation of a \( ext{CH}_3 ext{CHOHCOOH} \) framework. For option (b), \( ext{CH}_3 ext{CHO} \), this would work as it can form a cyanohydrin that leads to lactic acid structure upon hydrolysis. \( ext{HCHO} \) and \( ext{C}_6 ext{H}_5 ext{CH}_2 ext{CHO} \) in options (c) and (d) also cannot lead to lactic acid structure upon hydrolysis.

Step 4: Conclude the Right Option

Considering reactions involving formation of cyanohydrins and hydrolysis, option (b) \( ext{CH}_3 ext{CHO} \) (acetaldehyde) is the only compound where the cyanohydrin hydrolyzes to lactic acid.

Key Concepts

Hydrolysis

Hydrolysis is a chemical reaction involving the breaking of bonds in molecules using water. This process is like cutting a rope by pouring water over its fibers until they separate. In the context of cyanohydrin formation, hydrolysis plays an essential role in transforming cyanohydrins into other molecules like lactic acid.
When cyanohydrins undergo hydrolysis, the cyano group (\( \text{CN} \)) in the molecule reacts with water. This reaction replaces the cyano group with a carboxyl group (\( \text{COOH} \)), turning the molecule into an alpha-hydroxy acid, like lactic acid, depending on the original compound used.

• The process is often acid-catalyzed, where an acid speeds up the reaction.
• This conversion helps change the cyanohydrin into more stable and functional molecules.

In simpler terms, think of hydrolysis here as a changeover from a temporary structure (cyanohydrin) into something more permanent and useful (like lactic acid). It showcases how water is a vital factor in transforming chemical structures in organic chemistry.

Lactic Acid

Lactic acid is an organic compound with the formula \( \text{CH}_3\text{CH(OH)COOH} \). It's notable for its appearance in sour milk products but has significant chemical implications too. Known as an alpha-hydroxy acid, lactic acid has both a hydroxyl group (\( \text{OH} \)) and a carboxyl group (\( \text{COOH} \)) attached to the same carbon atom.
In the cyanohydrin formation and hydrolysis context, lactic acid is created as a result of the processes. The structural rearrangement during hydrolysis makes this compound possible from aldehyde-derived cyanohydrins.

• Lactic acid is categorized as a chiral molecule, meaning it can exist in two mirror-image forms, known as enantiomers.
• These forms have different properties and biological functions, hence the importance of precise chemical control.

Understanding lactic acid in this context highlights how small changes in structure, like adding or transforming functional groups, lead to vastly different molecules with unique properties.

Carbonyl Group

The carbonyl group (\( \text{C=O} \)) is a key feature in organic chemistry. It consists of a carbon atom double-bonded to an oxygen atom and is found in many different kinds of compounds, including ketones and aldehydes. The carbonyl group's reactivity is due to the polarity of the carbon-oxygen bond, making it a prime center for reactions such as cyanohydrin formation.

• The presence of the carbonyl group facilitates the attack by nucleophiles, like the cyanide ion (\( \text{CN}^- \)).
• This group is essential for forming the initial cyanohydrin by connecting with the nitrogen atom from the cyanide.

In simpler terms, imagine the carbonyl group as the spotlight on a stage, drawing chemical reactions toward it. By understanding how it interacts within these reactions, you gain insight into the fascinating world of molecular transformations. The carbonyl group's behavior and attributes are fundamental to predicting reaction outcomes and designing new synthetic paths in chemistry.