Sequence of Reactions Analysis

The given reaction sequence involves a carboxylic acid undergoing two steps: first with SOCl₂ (thionyl chloride), then with aniline. Let's analyze step by step.

Step 1: Reaction with SOCl₂

The starting compound is a carboxylic acid: $C H_{3} - C H - C O O H$ (2-methylpropanoic acid). When carboxylic acids react with thionyl chloride (SOCl₂), they form acid chlorides. The reaction is:

$R - C O O H + S O C_{l} 2 \to R - C O C l + S O_{2} + H C l$

So, the product A is the acid chloride: $C H_{3} - C H - C O C l$ (2-methylpropanoyl chloride).

Step 2: Reaction with Aniline

Acid chlorides react with amines (like aniline, C₆H₅NH₂) to form amides. This is a nucleophilic acyl substitution. The reaction is:

$R - C O C l + C_{6} H_{5} N H_{2} \to R - C O N H C_{6} H_{5} + H C l$

So, product B is the amide: $C H_{3} - C H - C O N H C_{6} H_{5}$ (N-phenyl-2-methylpropanamide).

Final Answer

Comparing with the options, the correct structures for A and B are shown in the second option:

A: $C H_{3} - C H - C O C l$ and B: $C H_{3} - C H - C O N H C_{6} H_{5}$

Related Topics

Carboxylic Acid Derivatives
Nucleophilic Acyl Substitution
Reactivity of Acid Chlorides
Amide Formation

Important Formulae and Theory

Conversion of Carboxylic Acid to Acid Chloride:

$R - C O O H + S O C_{l} 2 \to R - C O C l + S O_{2} + H C l$

Reaction of Acid Chloride with Amine:

$R - C O C l + R_{'} N H_{2} \to R - C O N H R_{'} + H C l$

Theory: Acid chlorides are highly reactive due to the good leaving group ability of Cl^- and the strong polarization of the C=O bond. They readily undergo nucleophilic substitution with amines to form amides, which are stable compounds.