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 \(\mathrm{RCH}_{2} \mathrm{CH}_{2}\) 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

(a) \( \mathrm{PBr}_{3}, \mathrm{KCN}, \mathrm{H}^{+} \)

Step 1: Understand the Starting Material

The starting material is an alcohol with the structure \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{OH} \). We need to convert this compound to a carboxylic acid \( \mathrm{RCH}_{2}\mathrm{CH}_{2}\text{COOH} \).

Step 2: Identify the Target Transformation

The conversion required is from an alcohol group \( \mathrm{OH} \) to a carboxylic acid \( \mathrm{COOH} \). To achieve this, we need to understand that the alcohol must first be converted into a functional group that can easily be turned into a carboxylic acid, such as a nitrile.

Step 3: Conversion to Nitrile

To convert \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{OH} \) to a nitrile, one common procedure involves converting the alcohol to a bromide or iodide using \( \mathrm{PBr}_{3} \), then replacing the halide with a cyano group using \( \mathrm{KCN} \). The sequence is: \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{OH} \to \mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{Br} \to \mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{CN} \).

Step 4: Conversion of Nitrile to Carboxylic Acid

After forming the nitrile \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{CN} \) by using \( \mathrm{KCN} \), it can be hydrolyzed to a carboxylic acid using aqueous acid \( \mathrm{H^+} \). This process converts the \( \mathrm{CN} \) group into \( \mathrm{COOH} \).

Step 5: Evaluate the Correct Sequence of Reagents

We need to select a sequence where we can convert \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{OH} \) to \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{CN} \) first, and then use \( \mathrm{H}^+ \) to form \( \mathrm{R}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{COOH} \). The correct answer is (a) \( \mathrm{PBr}_{3}, \mathrm{KCN}, \mathrm{H}^{+} \).

Key Concepts

Alcohol to Carboxylic Acid Conversion

Transforming an alcohol into a carboxylic acid involves several chemical steps. Each step is carefully orchestrated by using specific reagents. In chemical terms, conversion from alcohol (OH) to carboxylic acid (COOH) means functionalizing the alcohol so it can be easily turned into a more reactive group like a nitrile.
This process usually starts by taking the alcohol and reacting it with reagents such as phosphorus tribromide (\(\mathrm{PBr}_{3}\)).
Phosphorus tribromide converts the alcohol into an alkyl bromide, which is a key intermediate compound that provides an excellent bridge to the carboxylic acid group.
Once the alcohol is transformed into an alkyl bromide, the path to the carboxylic acid becomes much more straightforward.

Nitrile Formation

The formation of nitriles is a crucial step in organic synthesis, especially when converting alcohols to carboxylic acids. This step typically follows the conversion of alcohols to alkyl halides.
To achieve this, the alkyl halide must be reacted with a cyanide ion, typically provided by potassium cyanide (\(\mathrm{KCN}\)). This process replaces the bromide ion (from the alkyl bromide made in the first step) with a cyano group (\(\mathrm{CN}\)).

• This conversion allows for extension of the carbon chain, crucial when synthesizing longer chain carboxylic acids.
• It forms a new molecule called a nitrile, which is a compound containing the cyano functional group.

The nitrile itself is an excellent precursor to many other functional groups in organic chemistry, including carboxylic acids.

Hydrolysis of Nitriles

Nitriles are versatile compounds that can be processed further to yield carboxylic acids through hydrolysis. This important reaction involves breaking the carbon-nitrogen triple bond of the nitrile in the presence of water and an acid like hydrochloric acid (H\(^+\)).
During hydrolysis, the cyano group (\(\mathrm{CN}\)) is converted into the carboxyl group (\(\mathrm{COOH}\)), completing the conversion of the alcohol into a carboxylic acid.

• The \(\mathrm{H}^{+}\) ions in the aqueous solution facilitate this transformation by adding a proton to the cyano group, eventually leading to the formation of a carboxyl group.
• Hydrolysis usually requires heat for the conversion process to be efficient, ensuring the complete transformation of nitriles.

This step is the final conversion stage, turning all initial components into the desired carboxylic acid.

Reagent Sequence in Organic Synthesis

Choosing the right sequence of reagents is key to successfully synthesizing complex compounds in organic chemistry. This sequence often requires strategic planning.

• For the conversion we've discussed, starting with phosphorus tribromide (\(\mathrm{PBr}_{3}\)) ensures transformation of alcohol to the more reactive alkyl bromide.
• Potassium cyanide (\(\mathrm{KCN}\)) then facilitates the formation of a nitrile, indispensable for chain extension.
• Finally, the application of a strong acid such as \(\mathrm{H}^{+}\) transforms the nitrile to the target carboxylic acid via hydrolysis.

By selecting the correct sequence of these reagents, we can ensure a smooth and efficient synthetic process, achieving the desired chemical transformation without unnecessary side reactions. This systematic process illustrates the logical flow from starting alcohol to final carboxylic acid, underlining the importance of reagent choice and order in organic chemistry reactions.