Problem 74 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{CH}_{3} \mathrm{COCl}\) reacts with (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) (b) Salicylic acid (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\) (d) all of these

Step-by-Step Solution

Answer

Acetyl chloride reacts with all of these compounds.

Step 1: Analyze the Reactants

The compound \( \mathrm{CH}_{3} \mathrm{COCl} \) is an acyl chloride, specifically acetyl chloride. We need to determine if it will react with the compounds provided: \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \) (aniline), salicylic acid, and \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH} \) (phenol). Acyl chlorides react with nucleophiles to form acyl derivatives.

Step 2: Reaction with Aniline (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \))

Aniline is a primary amine, which is a good nucleophile. It can react with acetyl chloride to form an amide through the process of nucleophilic acyl substitution. This reaction proceeds readily as amines are strong nucleophiles.

Step 3: Reaction with Salicylic Acid

Salicylic acid contains both a carboxyl group and a phenolic hydroxyl group. Acyl chlorides typically react with the hydroxyl group to form an ester. Therefore, acetyl chloride can react with salicylic acid to form an ester (aspirin), with the phenolic hydroxyl group participating in the reaction.

Step 4: Reaction with Phenol (\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH} \))

Phenol can also react with acetyl chloride because the hydroxyl group on phenol can be acetylated to produce an ester. While less reactive than amines, phenols do react with acyl chlorides under the right conditions.

Step 5: Conclusion

Acetyl chloride \( \mathrm{CH}_{3} \mathrm{COCl} \) can react with all the given compounds ---- aniline, salicylic acid, and phenol. All these reactions involve the formation of acyl derivatives such as amides and esters.

Key Concepts

Nucleophilic Acyl Substitution

Nucleophilic acyl substitution is an important concept when discussing reactions with acetyl chloride. This type of reaction involves a nucleophile, a molecule or ion with a lone pair of electrons, attacking the electrophilic carbon atom in the acyl chloride. Here's how it works:
The carbon in acetyl chloride (\(\mathrm{CH}_{3}\mathrm{COCl}\)) is part of a functional group called a carbonyl group, represented by a carbon atom double-bonded to an oxygen atom. This carbon is electrophilic, meaning it is positively polarized and attracts nucleophiles.

• When a nucleophile like an amine or alcohol approaches, it donates its pair of electrons to the electrophilic carbon, forming a new bond.
• At the same time, the bond between the carbon and chlorine in the acyl group breaks, resulting in the release of a chloride ion.
• This process forms a tetrahedral intermediate, which quickly rearranges to produce the final acyl substitution product, either an amide or an ester.

The beauty of nucleophilic acyl substitution lies in its versatility and its ability to form various organic derivatives with different functional groups.

Ester Formation

When acetyl chloride reacts with compounds containing hydroxyl groups, the typical outcome is ester formation. Let's break it down:
An ester is a compound derived from the reaction of an acyl chloride with an alcohol or phenol. In our example:

• Acetyl chloride reacts with phenol (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{OH}\)) or the hydroxyl group in salicylic acid to produce an ester bond.
• The mechanism begins with the nucleophilic attack by the alcohol's oxygen on the acyl chloride's electrophilic carbon.
• This leads to the loss of the chloride ion and the establishment of an ester linkage.

Ester formation is notably important in industry and biology. Aspirin, for instance, is an ester formed from salicylic acid and acetyl chloride. This simple reaction illustrates how changing a functional group can vastly alter the properties and uses of a molecule.

Amide Synthesis

Amides are another class of acyl derivatives formed during reactions involving acetyl chloride. This process is known as amide synthesis, and it follows a series of precise steps:
Amides result when acyl chlorides react with amines, such as aniline (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{2}\)):

• The nucleophilic nitrogen of the amine attacks the carbon in the acyl chloride, establishing a new carbon-nitrogen bond while displacing a chloride ion.
• The resulting product is an amide, characterized by a carbonyl group bonded to a nitrogen.
• This reaction is quite favorable with primary amines because they are strong nucleophiles.

Amide synthesis is central to creating proteins and other biological molecules, as peptide bonds are essentially amide linkages. Through understanding this reaction, chemists can design and synthesize a variety of important compounds, illustrating the fundamental role of acyl chemistry.