Problem 82 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify \(\mathrm{R}\) in the following reaction. \(\mathrm{PhCOOH} \frac{\mathrm{LiAlH}_{4}}{\mathrm{PBr}_{3}} \mathrm{P} \stackrel{\mathrm{KCN}}{\longrightarrow} \mathrm{Q}\) \(\frac{\text { (i) partical hydrolysis }}{\text { (ii) } \mathrm{NaOBr}}=\mathrm{R}\)

Step-by-Step Solution

Answer

The compound R is benzylamine (PhCH2NH2).

Step 1: Understand the Reaction Sequence

We begin with benzoic acid (PhCOOH) and subject it to a series of reactions. The first step involves reducing the acid using lithium aluminum hydride (LiAlH4) and then converting it into another compound using PBr3.

Step 2: Reduction by LiAlH4

The benzene carboxylic acid (PhCOOH) is reduced by LiAlH4 to benzyl alcohol (PhCH2OH). This step involves the conversion of the carboxylic acid group to a primary alcohol group.

Step 3: Conversion with PBr3

Next, the alcohol (PhCH2OH) is reacted with phosphorus tribromide (PBr3) to convert the hydroxyl group (-OH) into a bromine group (-Br), forming benzyl bromide (PhCH2Br).

Step 4: Reaction with KCN

The benzyl bromide (PhCH2Br), obtained from the previous step, undergoes a nucleophilic substitution reaction with potassium cyanide (KCN). This results in the formation of phenylacetonitrile (PhCH2CN). We identify this as compound Q.

Step 5: Partial Hydrolysis of Q

Phenylacetonitrile (PhCH2CN) is subjected to partial hydrolysis, which converts the nitrile group (-CN) into an amide group (-CONH2) forming benzylamide (PhCH2CONH2).

Step 6: Hoffmann Bromamide Reaction

The amide (PhCH2CONH2) undergoes a Hoffmann bromamide rearrangement when treated with sodium hypobromite (NaOBr). This reaction removes the carbonyl group and results in primary amine formation. The final product is benzylamine (PhCH2NH2). This compound is R.

Key Concepts

Reduction Reaction

In organic chemistry, reduction reactions play a vital role in transforming compounds by changing their oxidation state. These reactions typically involve the gain of electrons or hydrogen atoms. A classic example is the reduction of carboxylic acids to alcohols.
In the original exercise provided, lithium aluminum hydride (LiAlH₄) is used. It is a powerful reducing agent often employed to convert carboxylic acids into primary alcohols.

• Reduction Step: The transformation involves the loss of oxygen atoms or the gain of hydrogen atoms to the carbon atom bonded to oxygen in the carboxylic acid.
• Example: Benzoic acid (PhCOOH) is reduced to benzyl alcohol (PhCH₂OH).

Reduction reactions are central to constructing more complex molecules in organic synthesis. They allow chemists to alter functional groups, thereby changing the chemical behavior of compounds.

Nucleophilic Substitution

Nucleophilic substitution is a fundamental reaction type in organic chemistry. It involves the replacement of one atom or group in a molecule with another nucleophile.
A nucleophile is a species that donates an electron pair to form a chemical bond. Potassium cyanide ( KCN) acts as the nucleophile in the exercise reaction sequence.

• Mechanism: The nucleophile attacks an electrophilic carbon, leading to the displacement of a leaving group.
• Example: Benzyl bromide (PhCH₂Br) undergoes nucleophilic substitution with KCN to form phenylacetonitrile (PhCH₂CN).
• Outcome: The original bromine atom is replaced by the cyanide group (-CN).

This reaction type is widely used in organic synthesis due to its ability to introduce a wide range of functional groups into a molecule.

Hoffmann Bromamide Rearrangement

This rearrangement is a notable reaction utilized to synthesize primary amines from amides. The Hoffmann bromamide rearrangement involves treating an amide with sodium hypobromite ( NaOBr), which leads to the loss of one carbon atom from the carbon chain.
The reaction proceeds via the formation of an isocyanate intermediate, followed by hydrolysis:

• Step 1: Formation of an isocyanate occurs when the amide is treated with NaOBr.
• Step 2: The isocyanate undergoes hydrolysis, resulting in the formation of a primary amine.
• Example: Benzylamide (PhCH₂CONH₂) is converted into benzylamine (PhCH₂NH₂).

This method is an efficient way to tailor organic molecules, offering a straightforward route to primary amines, which are valuable in pharmaceutical and agrochemical industries.