Problem 132 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{OH}\) can be converted into \(\mathrm{RCH}_{2} \mathrm{CH}_{2}\) \(\mathrm{COOH}\). The correct sequence of reagent is (a) \(\mathrm{PBr}_{3}, \mathrm{KCN}, \mathrm{H}^{+}\) (b) \(\mathrm{HCN}, \mathrm{PBr}_{3}, \mathrm{H}^{+}\) (c) \(\mathrm{KCN}, \mathrm{H}^{+}\) (d) \(\mathrm{PBr}_{3}, \mathrm{KCN}, \mathrm{H}_{2}\)

Step-by-Step Solution

Answer

The correct sequence is (a) \(\mathrm{PBr}_{3}, \mathrm{KCN}, \mathrm{H}^{+}\).

Step 1: Understand the Transformation

To convert \(\mathrm{RCH}_{2}\mathrm{CH}_{2}\mathrm{OH}\) into \(\mathrm{RCH}_{2}\mathrm{CH}_{2}\mathrm{COOH}\), we need to first convert the alcohol group (\(-\mathrm{OH}\)) into a halide group (typically bromide \(-\mathrm{Br}\)) using a reagent like \(\mathrm{PBr}_{3}\). Then, the halide can be converted into a nitrile \(-\mathrm{CN}\) through a reaction with \(\mathrm{KCN}\), and finally, the nitrile is hydrolyzed to carboxylic acid \(-\mathrm{COOH}\) using acidic conditions \(\mathrm{H}^{+}\).

Step 2: Determine Reagents for Each Step

- The first step requires \(\mathrm{PBr}_{3}\) to replace the \(-\mathrm{OH}\) with \(-\mathrm{Br}\).- The second step requires \(\mathrm{KCN}\) to convert \(-\mathrm{Br}\) into \(-\mathrm{CN}\).- The third step requires \(\mathrm{H}^{+}\) to hydrolyze the nitrile \(-\mathrm{CN}\) into the carboxylic acid \(-\mathrm{COOH}\).

Step 3: Find the Correct Option

Analyze the options: - Option (a) \(\mathrm{PBr}_{3}, \mathrm{KCN}, \mathrm{H}^{+}\) matches our sequence of required reagents: it converts \(-\mathrm{OH}\) to \(-\mathrm{Br}\), then \(-\mathrm{Br}\) to \(-\mathrm{CN}\), and finally \(-\mathrm{CN}\) to \(-\mathrm{COOH}\).- Option (b) and (c) have incorrect order or missing reagents.- Option (d) doesn't provide the necessary acid for hydrolysis. Thus, option (a) is correct.

Key Concepts

Organic Chemistry Transformations

In organic chemistry, transformations are fundamental processes that involve converting one functional group into another, enabling the creation of complex molecules from simpler ones. These transformations are crucial for synthesizing a wide variety of organic compounds. In our specific exercise, we are focused on transforming an alcohol into a carboxylic acid.

This process is not just a simple change. It requires a comprehensive understanding of the conversion steps involved. In our example, we start with an alcohol, which is then systematically transformed through a series of chemical reactions:

• The initial alcohol group is converted to a halide using reagents like phosphorus tribromide ( PBr_3 ).
• This halide is subsequently transformed into a nitrile group ( CN ) via a reaction with potassium cyanide ( KCN ).
• Finally, the nitrile group undergoes hydrolysis in acidic conditions to form the carboxylic acid ( COOH ).

Understanding these transformations gives us the ability to methodically alter chemical structures and is foundational for complex organic synthesis.

Conversion of Alcohols to Carboxylic Acids

The conversion of alcohols to carboxylic acids is a multifaceted process involving several distinct steps. Each step employs specific reagents and conditions to ensure the resulting compound is synthesized correctly and efficiently.

In our exercise, we transformed the alcohol group ( OH ) to a carboxylic acid ( COOH ) through the following sequence:

• The alcohol group is replaced with a halide using PBr_3 . This transformation is crucial as it prepares the molecule for subsequent reactions.
• Next, the formed alkyl halide reacts with the cyanide ion from KCN , resulting in the substitution of the halide with a nitrile group ( CN ).
• Finally, the nitrile is hydrolyzed under acidic conditions ( H^+ ) to convert it into a carboxylic acid. This usually involves the addition of water, breaking the nitrile bond to form the desired COOH group.

Each step requires careful reagent selection to ensure the transformation process goes smoothly without undesired side reactions.

Reaction Mechanisms

Reaction mechanisms are pathways by which chemical reactions proceed, detailing the step-by-step movement of electrons and changes in the molecular structure necessary for transforming reactants into products. Understanding these mechanisms is critical for predicting the outcomes of reactions and for troubleshooting synthesis problems.

In the transformation from alcohol to carboxylic acid, each reaction stage can be broken down into details of the mechanism:

• Initially, when PBr_3 is used, it converts the alcohol to an alkyl bromide via a substitution reaction, where the hydroxyl group ( OH ) is replaced by a bromide ( Br ).
• The next reaction involves a nucleophilic substitution where the cyanide ion ( CN^- ) attacks the carbon atom, replacing the bromide group, and forming a nitrile.
• Finally, the nitrile conversion to a carboxylic acid involves hydrolysis, a process where water adds across the nitrile bond facilitated by an acid ( H^+ ), breaking it to form the carboxylic acid group.

Each step requires precise conditions to ensure the proper movement of electrons and completion of the reaction as expected. Understanding these mechanisms deepens comprehension of chemical transformation and aids in addressing any challenges that may arise during such synthetic processes.