Problem 91 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

(i) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH} \quad \stackrel{\mathrm{PCl}_{5}}{\longrightarrow} C \stackrel{\mathrm{NH}_{3}}{\longrightarrow}\) $$ D \stackrel{\mathrm{P}_{2} \mathrm{O}_{5}}{\longrightarrow} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CN} \quad \stackrel{\mathrm{H}_{2}^{/ \mathrm{Ni}}}{\longrightarrow} E $$ Identify \(C, D\) and \(E\). [1991 - 2 Marks] (ii) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}=\mathrm{CH}-\mathrm{CHO} \stackrel{\mathrm{NaBH}_{4}}{\longrightarrow} F\) $$ \frac{\mathrm{HCl}}{\mathrm{ZnCl}_{2}} G \underset{\mathrm{H}^{+}}{\stackrel{\mathrm{KCN}}{\longrightarrow}} \mathrm{H} $$ Identify \(F, G\) and \(H\).

Step-by-Step Solution

Answer

(i) C: \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCl} \), D: \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CONH}_{2} \), E: \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2}\mathrm{NH}_{2} \). (ii) F: 2-buten-1-ol, G: 3-chlorobutene, H: 3-butenenitrile.

Step 1: Convert Benzoic Acid to Benzoyl Chloride

The first reaction in part (i) involves treating benzoic acid (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH} \)) with \( \mathrm{PCl}_{5} \). This converts the carboxylic acid group into an acyl chloride, specifically forming benzoyl chloride (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCl} \)). This is \( C \).

Step 2: Convert Benzoyl Chloride to Benzamide

Next, \( C \), benzoyl chloride, is treated with ammonia (\( \mathrm{NH}_{3} \)) to form benzamide \( (\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CONH}_{2}) \). This conversion involves the substitution of the chloride with an amide group, which is compound \( D \).

Step 3: Dehydrate Benzamide to Benzonitrile

Compound \( D \), benzamide, undergoes dehydration when treated with \( \mathrm{P}_{2} \mathrm{O}_{5} \), leading to the formation of benzonitrile (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CN} \)). This transformation involves the loss of water from the amide, forming compound \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CN} \).

Step 4: Reduce Benzonitrile to Benzylamine

The nitrile group in \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CN} \) is reduced using hydrogen in the presence of nickel (\( \mathrm{H}_{2}/\mathrm{Ni} \)), resulting in the formation of benzylamine (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2}\mathrm{NH}_{2} \)) which is compound \( E \).

Step 5: Reduce Aldehyde to Primary Alcohol

In part (ii), the compound \( \mathrm{H}_{3} \mathrm{C}-\mathrm{CH}=\mathrm{CH}-\mathrm{CHO} \) is reduced using sodium borohydride (\( \mathrm{NaBH}_{4} \)). This reduces the aldehyde group to a primary alcohol, forming 2-buten-1-ol (\( \mathrm{H}_{3} \mathrm{C}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\mathrm{OH} \)), known as \( F \).

Step 6: Convert Alcohol to Alkyl Chloride

Compound \( F \), 2-buten-1-ol, is treated with hydrogen chloride in the presence of zinc chloride (\( \mathrm{HCl}/\mathrm{ZnCl}_{2} \)) to form an alkyl chloride. This generates 3-chlorobutene (\( \mathrm{H}_{3} \mathrm{C}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\mathrm{Cl} \)), referred to as \( G \).

Step 7: Convert Alkyl Chloride to Nitrile

Finally, compound \( G \), 3-chlorobutene, is treated with potassium cyanide (\( \mathrm{KCN} \)) in the presence of a proton source. This substitution reaction replaces the chloride with a nitrile, forming 3-butenenitrile (\( \mathrm{H}_{3} \mathrm{C}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\mathrm{CN} \)), which is compound \( H \).

Key Concepts

Reaction Mechanism

A reaction mechanism is a step-by-step detailed description of the sequence of events in a chemical reaction. It describes how reactants are transformed into products, including the intermediate steps that may not be immediately visible. Understanding the reaction mechanism is crucial because it allows chemists to predict the products of a reaction and to understand how different reaction conditions affect the outcome.

• In the given exercise, the transformation of benzoic acid to benzylamine involves multiple steps, each with its own distinct mechanism.
• First, the carboxylic acid group in benzoic acid is converted to an acyl chloride using phosphorus pentachloride (\( \mathrm{PCl}_5 \)). This involves the substitution of the hydroxyl group with a chlorine atom, a common step in converting acids to acid chlorides.
• This is followed by the reaction of the acyl chloride with ammonia, wherein the chlorine atom is replaced by an amide group, forming benzamide.
• Benzamide is then dehydrated to form a nitrile group through the removal of water, which is facilitated by phosphorus pentoxide (\( \mathrm{P_2O_5} \)).

By breaking down each reaction step and understanding the transformations, we see a clear sequence of mechanisms that lead from a simple carboxylic acid to a complex amine.

Functional Group Transformation

Functional group transformations are fundamental in organic chemistry as they allow the conversion of one type of chemical group into another. This is a key concept because it enables the synthesis of a wide variety of compounds by altering specific functional groups in a molecule.

• In the problem, we see several functional group transformations across different stages of the reactions.
• The initial transformation from a carboxylic acid to an acyl chloride involves converting the \( -COOH \) group into a \( -COCl \) group.
• Then, the acyl chloride is converted into an amide (\( -CONH_2 \)) group when it reacts with ammonia. This is another classic transformation where the electronegative chlorine atom is replaced with an amide group.
• The change from an amide to a nitrile (\( -CN \)) is a dehydration reaction, demonstrating yet another transformation from a different functional group.
• In part (ii) of the exercise, the use of sodium borohydride illustrates the reduction of an aldehyde (\( -CHO \)) to an alcohol (\( -CH_2OH \)).

These transformations showcase the flexibility organic chemists have to design synthetic routes to desired molecules.

Reduction Reactions

Reduction reactions are a type of redox reaction where a molecule gains electrons or adds hydrogen atoms. In organic chemistry, reduction typically alters the functional groups, as seen in the provided exercise.

• Reduction of benzonitrile to benzylamine using hydrogen and nickel catalyst (\( \mathrm{H}_2/\mathrm{Ni} \)) is a classic example. Here, the nitrile group (\( -CN \)) is reduced to a primary amine (\( -CH_2NH_2 \)), effectively adding hydrogen across the multiple bond.
• Similarly, in part (ii) of the exercise, the use of sodium borohydride (\( \mathrm{NaBH}_4 \)) reduces the aldehyde group to an alcohol. This is a selective reduction, as \( \mathrm{NaBH}_4 \) primarily targets aldehydes and ketones, leaving other functional groups like alkenes intact.

These steps emphasize the importance of reduction in altering and converting functional groups to achieve the desired structural and functional properties of the final compound. Understanding which reducing agents to use and how they interact with different functional groups is key in synthetic organic chemistry.