Question

The compounds A and B are respectively:

• A.
• B.
• C.
• D.

Answer 1

The Correct Option is B

The reaction sequence starts with an alcohol reacting with \(SOCl_2\) (thionyl chloride) in the presence of pyridine. This is followed by a reaction with \(Mg\) in ether, and finally reaction with \(CO_2\) followed by acidification. Let's break down each step to identify compounds A and B.

Step 1: Reaction with \(SOCl_2\) and Pyridine 

The reaction of an alcohol with \(SOCl_2\) in the presence of pyridine is a classic method for converting an alcohol (\(R-OH\)) to an alkyl chloride (\(R-Cl\)) with inversion of stereochemistry at the carbon bearing the alcohol. Pyridine acts as a base to neutralize the \(HCl\) generated in the reaction. Thus, compound A is the alkyl chloride.

Step 2: Reaction with \(Mg\) in Ether

The reaction of an alkyl halide (in this case, the alkyl chloride formed in step 1) with magnesium (\(Mg\)) in dry ether forms a Grignard reagent (\(R-MgX\), where X is a halogen). The Grignard reagent is a strong nucleophile and a strong base. Therefore, compound B is a Grignard reagent.

Step 3: Reaction with \(CO_2\) followed by Acidification

The Grignard reagent (compound B) reacts with carbon dioxide (\(CO_2\)) in a carboxylation reaction. The Grignard reagent attacks the carbon of \(CO_2\), forming a carboxylate salt. Upon acidification (\(H_3O^+\)), the carboxylate salt is protonated to form a carboxylic acid. This step isn't needed to identify A and B, but helps to understand the full sequence.

Based on the starting alcohol structure and the reactions, we can determine A and B.

Compound A (alkyl chloride):

 

Compound B (Grignard reagent):

 

The compounds A and B are respectively:

Answer 2

The reaction sequence involves the formation of a Grignard reagent from MeBr and Mg in dry ether. The Grignard reagent reacts with CO₂ (from dry ice) to form a carboxylate, which upon protonation with H₃O⁺ gives a carboxylic acid.

The correct structures of A and B are shown in option (3).