Problem 5 - A Complete Resource Book in Chemistry for JEE Main

Question

Cyanohydrin of which of the following will yield lactic acid? (a) \(\mathrm{HCHO}\) (b) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\) (d) \(\mathrm{CH}_{3} \mathrm{CHO}\)

Step-by-Step Solution

Answer

Lactic acid is formed from the cyanohydrin of acetaldehyde ( ce{CH3CHO} ).

Step 1: Understanding Cyanohydrin Formation

Cyanohydrins are formed when cyanide ions ( ce{CN^-} ) react with carbonyl groups in aldehydes or ketones. The typical reaction takes place at the carbonyl carbon, resulting in the addition of a hydroxyl group and a cyano group ( ce{CN} ). The general formula for a cyanohydrin is ce{R2C(OH)CN} , where ce{R} can be hydrogen or alkyl groups.

Step 2: Recognizing Lactic Acid Structure

Lactic acid has the chemical structure ce{CH3CH(OH)COOH} . It features an ce{OH} group on the second carbon of the ce{C3} chain. To produce lactic acid from a cyanohydrin, the cyanohydrin must contain both an ce{OH} and an ce{OH} group on the adjacent carbon atoms.

Step 3: Identifying the Correct Cyanohydrin Precursor

Among the given options, option (d) ce{CH3CHO} (acetaldehyde) forms the cyanohydrin ce{CH3C(OH)(CN)} . When this cyanohydrin undergoes hydrolysis, the cyano group is converted to a carboxylic acid group, yielding lactic acid ce{CH3CH(OH)COOH} .

Step 4: Conclusion

Only acetaldehyde ce{CH3CHO} forms the necessary cyanohydrin that converts to lactic acid upon hydrolysis. Other options do not form the appropriate structure to convert into lactic acid.

Key Concepts

Cyanohydrins

Cyanohydrins are special chemical compounds formed when cyanide ions interact with carbonyl groups in aldehydes and ketones. This process occurs through a nucleophilic addition reaction. The cyanide ion, ce{CN^-}, attacks the carbon atom of the carbonyl group, ce{C=O}, leading to the formation of a cyanohydrin.

These compounds are characterized by a hydroxyl group ( ce{OH}) and a cyano group ( ce{CN}) attached to the same carbon atom. The general structural formula for a cyanohydrin is ce{R2C(OH)CN}, where ce{R} represents either hydrogen or an alkyl group.

Key points to remember about cyanohydrins:

• They result from reactions between carbonyl compounds and cyanide ions.
• The reaction involves the addition of two distinct functional groups to the same carbon.
• This process can lead to further chemical transformations, such as hydrolysis.

Lactic Acid Formation

Lactic acid is a vital organic compound, renowned for its presence in sour dairy products and as a metabolic byproduct in muscles during intense exercise. It is structurally unique, featuring a hydroxyl group ( ce{OH}) and a carboxylic acid group ( ce{COOH}) on adjacent carbon atoms. This arrangement is essential for its biochemical role and interactions.

The molecular formula of lactic acid is ce{CH3CH(OH)COOH}. Its formation from cyanohydrins involves hydrolyzing the ce{CN} group into a ce{COOH} group. This transformation is crucial in organic synthesis, especially for creating biologically significant molecules.

To produce lactic acid:

• Start with a cyanohydrin precursor.
• Ensure the precursor has a hydroxyl group and a cyanide group on adjacent carbons.
• Apply hydrolysis to convert the cyanohydrin into lactic acid.

Acetaldehyde

Acetaldehyde ( ce{CH3CHO}) is a pivotal molecule in organic chemistry and the formation of cyanohydrins. With its simple structure, consisting of a carbonyl group ( ce{C=O}) linked to a methyl group ( ce{CH3}), acetaldehyde can efficiently form cyanohydrins.

When acetaldehyde reacts with cyanide ions ( ce{CN^-}), it forms a cyanohydrin with the formula ce{CH3C(OH)(CN)}. This compound is noteworthy because it can undergo further chemical changes, such as hydrolysis, to yield lactic acid.

Highlights about acetaldehyde:

• A common aldehyde formed during alcohol metabolism in the body.
• Plays a role in various industrial processes, including the production of acetic acid.
• Acts as a precursor in synthesizing complex organic compounds.

Aldehydes and Ketones

Aldehydes and ketones are two fundamental classes of organic compounds characterized by their carbonyl groups ( ce{C=O}). Aldehydes have the structure ce{RCHO}, where ce{R} can be a hydrogen atom or a hydrocarbon chain, while ketones have the formula ce{RCOR'}, with both ce{R} groups being hydrocarbon chains or rings.

These compounds are highly reactive due to the polar nature of the carbonyl group, making them ideal for various chemical reactions, including the formation of cyanohydrins.

Key characteristics:

• Aldehydes are typically more reactive than ketones due to less steric hindrance.
• Ketones often serve as excellent solvents in industrial applications.
• Both types of compounds undergo nucleophilic addition reactions easily.

Hydrolysis Reaction

Hydrolysis is a chemical reaction where water interacts with a compound, leading to a breakdown or transformation of the original structure. In the context of cyanohydrins, hydrolysis is pivotal for converting the ce{CN} group into a carboxylic acid ( ce{COOH}) group, thereby forming products like lactic acid.

This reaction occurs through a mechanism that involves the gradual addition of water molecules, breaking the cyano group bonds, and forming a more energetically stable acid formation. Hydrolysis is fundamental in many biochemical pathways and synthetic processes.

Notable aspects:

• Hydrolysis plays a crucial role in digestion, helping break down nutrients.
• It is essential for recycling biological molecules in cells.
• In synthetic chemistry, it's vital for transforming intermediate compounds into final products.