Problem 104 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

\(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \frac{\text { (i) } \mathrm{SOCl}_{2}}{\text { (ii) 3-methylaniline }}\) (iii) LiAlH \(_{4}\), then \(\mathrm{H}_{3} \mathrm{O}^{+}\)

Step-by-Step Solution

Answer

The final product after these reactions is N-(3-methylphenyl)ethylamine.

Step 1: Identify the First Reaction

The first step involves the reaction of acetic acid (\[\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\]) with thionyl chloride (\[\mathrm{SOCl}_{2}\]). This reaction is a common method for converting carboxylic acids to acyl chlorides. The byproducts are sulfur dioxide (\[\mathrm{SO}_{2}\]) and hydrogen chloride (\[\mathrm{HCl}\]). Therefore, acetic acid is converted to acetyl chloride (\[\mathrm{CH}_{3}\mathrm{COCl}\]).

Step 2: Perform the Second Reaction

The newly formed acetyl chloride (\[\mathrm{CH}_{3}\mathrm{COCl}\]) now reacts with 3-methylaniline. This step is an example of nucleophilic acyl substitution where the chlorine atom in acetyl chloride is replaced by the aniline group, forming 3-methylanilide, specifically N-(3-methylphenyl)acetamide.

Step 3: Execute the Reduction

The amide formed in the previous step is then reduced using lithium aluminium hydride (\[\mathrm{LiAlH}_{4}\]), followed by hydrolysis with \[\mathrm{H}_{3} \mathrm{O}^{+}\]. \[\mathrm{LiAlH}_{4}\] is a strong reducing agent that converts the amide into an amine, specifically reducing the carbonyl group to yield N-(3-methylphenyl)ethylamine.

Key Concepts

Carboxylic Acid to Acyl Chloride Conversion

In organic chemistry, converting a carboxylic acid to an acyl chloride is a fundamental reaction. This transformation is achieved using reagents like thionyl chloride (\(\text{SOCl}_2 \)).
This specific reaction is crucial for preparing acyl chlorides, which serve as highly reactive intermediates for further synthetic applications.
When acetic acid (\(\text{CH}_{3}\text{CO}_{2}\text{H} \)) reacts with thionyl chloride, the \(\text{OH} \) group of the acid is replaced by a \(\text{Cl} \) atom from \(\text{SOCl}_2 \), forming acetyl chloride (\(\text{CH}_{3}\text{COCl} \)).
This reaction also produces sulfur dioxide (\(\text{SO}_2 \)) and hydrogen chloride (\(\text{HCl} \)) as byproducts.

• Acyl chloride formation is integral in synthesizing esters and amides.
• It allows access to other functional groups through nucleophilic acyl substitution processes.

With this conversion, chemists create a reactive molecule that can undergo further reactions.

Nucleophilic Acyl Substitution

In the realm of organic chemistry, nucleophilic acyl substitution is a pivotal reaction mechanism. This process involves substituting a leaving group in an acyl compound with a nucleophile.
In this exercise, the acetyl chloride (\(\text{CH}_{3}\text{COCl} \)), formed from acetic acid, is subject to nucleophilic attack by 3-methylaniline.
Aniline acts as the nucleophile, attacking the electrophilic carbon in the carbonyl group of acetyl chloride.
This results in the displacement of the chloride ion and the formation of an amide linkage, yielding N-(3-methylphenyl)acetamide.

• This reaction showcases how acyl chlorides can be used to synthesize amides efficiently.
• Nucleophilic acyl substitution is integral in the formation of various types of amides, esters, and other derivatives.

Understanding this mechanism allows chemists to manipulate and design targeted organic molecules.

Amide Reduction using LiAlH4

Lithium aluminium hydride (\(\text{LiAlH}_4 \)) is a robust reducing agent used in organic chemistry to convert various functional groups. Specifically, it is widely employed to reduce amides to amines.
In the final step of this exercise, amide N-(3-methylphenyl)acetamide undergoes such reduction. \(\text{LiAlH}_4 \) targets the carbonyl group of the amide, converting it into an amine, N-(3-methylphenyl)ethylamine, by replacing the carbonyl oxygen with hydrogen atoms.
Following this, hydrolysis with \(\text{H}_3\text{O}^+ \) ensures the complete reduction process by eliminating any remaining metal complex intermediates, leaving the desired amine.

• The use of \(\text{LiAlH}_4 \) is essential in synthesizing primary amines from amides.
• It offers a pathway to convert carbonyl-containing compounds into alcohols and amines effectively.

This reaction step is crucial for expanding the structural diversity of amines in organic synthesis.