Problem 205 - A Complete Resource Book in Chemistry for JEE Main

Question

p-cresol reacts with chloroform in alkaline medium to give a compound 'A' which on reacting with hydrogen cyanide gives, compound 'B'. The latter on acidic hydrolysis, gives chiral carboxylic acid. The structure of the carboxylic acid is [2005]

Step-by-Step Solution

Answer

The chiral carboxylic acid is 2-hydroxy-3-methylbenzoic acid.

Step 1: Identify the starting reactant

The starting compound is p-cresol, which is a phenol derivative with a methyl group at the para position. Its chemical structure can be represented as C7H8O or C6H4(OH)CH3.

Step 2: Apply Reimer-Tiemann Reaction

p-cresol is undergoing a Reimer-Tiemann reaction with chloroform (CHCl3) in an alkaline medium, which typically results in the formation of a formyl group. The product (Compound A) is 4-hydroxy-3-methylbenzaldehyde, also known as 4-formyl-m-cresol.

Step 3: React Compound A with Hydrogen Cyanide

4-Hydroxy-3-methylbenzaldehyde (Compound A) reacts with hydrogen cyanide (HCN) to form a cyanohydrin. This reaction adds a -CN group to the carbonyl carbon, resulting in the intermediate Compound B, which is 4-hydroxy-3-methyl-2-(hydroxymethyl)benzonitrile.

Step 4: Conduct Acidic Hydrolysis

The cyanohydrin (Compound B) undergoes acidic hydrolysis, converting the nitrile group (-CN) into a carboxylic acid group (-COOH). This process transforms Compound B into the corresponding chiral carboxylic acid, which is 4-hydroxy-3-methyl-2-(hydroxymethyl)benzoic acid.

Step 5: Identify the Structure

The final structure of the chiral carboxylic acid is 2-hydroxy-3-methylbenzoic acid, also known as o-hydroxy-p-toluic acid which is chiral due to the presence of a stereocenter at the benzylic position.

Key Concepts

Reimer-Tiemann Reaction

The Reimer-Tiemann reaction is a classic organic chemical reaction primarily used to introduce formyl groups into aromatic compounds such as phenols. It provides a straightforward method to convert phenols into aromatic aldehydes. The reaction involves the use of chloroform (CHCl₃) and a strong base, usually sodium hydroxide (NaOH).

• When p-cresol, a type of phenol derivative, is reacted with chloroform under these conditions, a new carbon-to-carbon bond is formed, attaching a formyl group to the aromatic ring.

This addition occurs at the ortho position relative to the hydroxyl group, resulting in the formation of 4-hydroxy-3-methylbenzaldehyde, known as 4-formyl-m-cresol. This transformation is essential in organic synthesis for preparing compounds that can undergo further modifications.

Cyanohydrin Formation

Cyanohydrin formation is a significant step in organic synthesis involving the addition of hydrogen cyanide (HCN) to aldehydes or ketones. This reaction results in the creation of a new cyanohydrin compound, which includes both a hydroxyl group (-OH) and a nitrile group (-CN) on the same carbon atom.

• Compound A, 4-hydroxy-3-methylbenzaldehyde, reacts with HCN, resulting in Compound B, a cyanohydrin, formally named 4-hydroxy-3-methyl-2-(hydroxymethyl)benzonitrile.

The process creates a new carbon-carbon bond, adding vital functional groups for further chemical transformations. Cyanohydrins are important intermediates because their cyanide and hydroxyl groups enable them to serve as building blocks for more complex molecules in processes like hydrolysis.

Acidic Hydrolysis

Acidic hydrolysis is a crucial conversion process in organic chemistry where a nitrile group is transformed into a carboxylic acid group. This reaction involves treating the nitrile compound with an acid and water to break down and replace the -CN group with a -COOH group, releasing ammonia.

• For the compound in our process, 4-hydroxy-3-methyl-2-(hydroxymethyl)benzonitrile (Compound B), acidic hydrolysis converts it to a chiral carboxylic acid.

The resulting product is 4-hydroxy-3-methyl-2-(hydroxymethyl)benzoic acid. The transformation is key for altering molecules into more reactive or biologically active forms, rendering them useful for further applications or studies.

p-Cresol Reactions

p-Cresol is a phenolic compound that serves as a starting material for various chemical transformations. In reactions, its phenolic group (the -OH) enables it to engage in electrophilic substitution reactions, facilitating the introduction of additional functional groups. In the context of this exercise, p-cresol first undergoes a Reimer-Tiemann reaction to introduce an aldehyde group.

• This transformation produces 4-hydroxy-3-methylbenzaldehyde, setting the stage for subsequent reactions like cyanohydrin formation and hydrolysis.
• Eventually, this pathway leads to the formation of a chiral carboxylic acid, demonstrating how p-cresol's reactivity can be exploited to create sophisticated organic structures with potential applications in pharmaceuticals and materials science.

p-Cresol's chemical nuance in reactivity showcases its importance in synthetic chemistry.