Problem 112 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which of the following reaction is possible? (i) \(\mathrm{CH}_{3} \mathrm{COCl}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{CH}_{3} \mathrm{COOH}+\mathrm{HCl}\) (ii) \(\mathrm{CH}_{3} \mathrm{COOCH}_{3}+\mathrm{HBr} \longrightarrow \mathrm{CH}_{3} \mathrm{COBr}+\mathrm{CH}_{3} \mathrm{OH}\) (iii) \(\mathrm{CH}_{3} \mathrm{CONH}_{2}+\mathrm{HBr} \longrightarrow \mathrm{CH}_{3} \mathrm{COBr}+\mathrm{NH}_{3}\) (iv) \(\mathrm{CH}_{3} \mathrm{COOCOCH}_{3}+\mathrm{H}_{2} \mathrm{O} \longrightarrow 2 \mathrm{CH}_{3} \mathrm{COOH}\) (a) \(\mathrm{i}\) and \(\mathrm{iv}\) (b) \(\mathrm{i}\), iii and iv (c) \(\mathrm{i}\), ii and iv (d) All the four

Step-by-Step Solution

Answer

(a) i and iv

Step 1: Reaction Type Analysis

Identify the types of reactions given. - Reaction (i) is the hydrolysis of an acyl chloride, which is known to produce a carboxylic acid and HCl. - Reaction (ii) suggests the conversion of an ester to an acyl bromide, which is not possible under normal conditions as it requires harsher conditions. - Reaction (iii) proposes the conversion of an amide to an acyl bromide in presence of HBr, which is generally not feasible without the presence of thionyl chloride or similar reagents. - Reaction (iv) is the hydrolysis of an acid anhydride, resulting in two carboxylic acid molecules.

Step 2: Verify Reaction Mechanism

- In reaction (i), the hydrolysis of \ \( \mathrm{CH}_{3} \mathrm{COCl} \ \) with \ \( \mathrm{H}_{2} \mathrm{O} \ \) is possible and commonly occurs, forming \ \( \mathrm{CH}_{3} \mathrm{COOH} \ \) and \ \( \mathrm{HCl} \ \).- For reaction (ii), converting an ester to acyl bromide by direct reaction with HBr is not a favorable or typical reaction pathway.- In reaction (iii), converting an amide to acyl bromide usually requires reagents like \ \( \mathrm{PBr}_{3} \ \), thus the direct reaction with \ \( \mathrm{HBr} \ \) is unlikely.- Reaction (iv) involves the hydrolysis of an anhydride which is typically easy and results in two carboxylic acids.

Step 3: Conclusion

Based on the analysis and standard reaction conditions: - Reaction (i) and reaction (iv) are feasible. - Reaction (ii) and reaction (iii) are not typically feasible under normal conditions without additional reagents or altered conditions.

Key Concepts

Acyl Chloride Hydrolysis

Acyl chloride hydrolysis is a fundamental organic chemistry reaction where an acyl chloride reacts with water to form a carboxylic acid and hydrochloric acid. This reaction is straightforward and often used in laboratories due to the high reactivity of acyl chlorides. When an acyl chloride, like acetyl chloride (\( \mathrm{CH}_3\mathrm{COCl} \)), encounters water, it undergoes nucleophilic acyl substitution. Here, the oxygen in water, acting as a nucleophile, attacks the electrophilic carbon of the acyl chloride. This results in the formation of the carboxylic acid (\( \mathrm{CH}_3\mathrm{COOH} \)) and hydrochloric acid (HCl).

• This reaction is fast and does not require any catalyst.
• It proceeds at room temperature.
• Highly useful in industrial applications for carboxylic acid preparation.

Ester Conversion

The conversion of esters into other functional groups, such as acyl bromides, is a reaction that might seem straightforward but usually demands specific conditions. Direct conversion of an ester, such as methyl acetate (\(\mathrm{CH}_3\mathrm{COOCH}_3\)), into an acyl bromide using hydrogen bromide (HBr) is not feasible. Esters are less reactive compared to acyl chlorides, requiring harsher conditions for such transformations. Typically, to convert an ester into an acyl halide, reagents such as thionyl chloride (\( \mathrm{SOCl}_2 \)) or phosphorus tribromide (\( \mathrm{PBr}_3 \)) must be employed together.

• Direct reaction with acids like \( \mathrm{HBr} \) is not enough.
• Requires external reagents and often heat.
• Such conversions are less common and more challenging.

Amide Conversion

Amides are generally less reactive toward hydrolysis or conversion to acyl halides without specific catalysts. Converting an amide like acetamide (\( \mathrm{CH}_3\mathrm{CONH}_2 \)) to an acyl bromide using just hydrogen bromide (HBr) is unusual and requires additional reagents or conditions. Typically, phosphorus tribromide (\( \mathrm{PBr}_3 \)) or thionyl chloride is used for such conversions, providing the chlorinating or brominating environment necessary.

• Amides resist hydrolysis due to resonance stabilization.
• Reactions require stronger, often Lewis acid catalysts.
• High temperatures or basic environments can assist hydrolysis.

Amide conversion to acyl bromides without these catalysts is not typically practiced in standard organic chemistry due to feasibility issues.

Acid Anhydride Hydrolysis

Acid anhydride hydrolysis is a reaction that involves splitting an acid anhydride into two carboxylic acids. This reaction occurs when the anhydride, such as acetic anhydride (\( \mathrm{CH}_3\mathrm{COOCOCH}_3 \)), interacts with water. In this process, each anhydride molecule cleaves to form two acetic acid (\( \mathrm{CH}_3\mathrm{COOH} \)) molecules.

• Hydrolysis is typically easy and occurs readily at room temperature.
• Acid anhydrides are more reactive than esters but less reactive than acyl chlorides.
• No catalyst is needed due to the inherent reactivity of acid anhydrides.

This reaction is a classic method for preparing carboxylic acids in both academic and industrial settings.