Question

In the presence of Grignard reagent (RMgX), which of the following is used to prepare a carboxylic acid?

• A. \(NH_{3}\)
• B. \(CO_{2}\) (Dry ice)
• C. \(H_{2}O\)
• D. \(O_{2}\)

Hint:

Important Grignard reactions: \[ \boxed{ \begin{aligned} RMgX+HCHO &\rightarrow 1^\circ \text{ alcohol}\\ RMgX+\text{Aldehyde} &\rightarrow 2^\circ \text{ alcohol}\\ RMgX+\text{Ketone} &\rightarrow 3^\circ \text{ alcohol}\\ RMgX+CO_2 &\rightarrow \text{Carboxylic acid} \end{aligned} } \] Always remember that dry ice (\(CO_2\)) is used to prepare carboxylic acids from Grignard reagents.

Answer 1

The Correct Option is B

Concept: Grignard reagents (\(RMgX\)) are highly reactive organometallic compounds. They behave as strong nucleophiles and react with carbon dioxide (dry ice) to produce magnesium carboxylates, which upon acid hydrolysis give carboxylic acids. The reaction is \[ RMgX+CO_2 \longrightarrow RCOOMgX \xrightarrow{H_3O^+} RCOOH. \] Thus, carbon dioxide is the reagent used for preparing carboxylic acids from Grignard reagents.

Step 1: Reaction of Grignard reagent with carbon dioxide.
Grignard reagent reacts readily with dry ice: \[ RMgX+CO_2 \rightarrow RCOOMgX. \] This forms a magnesium salt of the carboxylic acid.

Step 2: Hydrolysis of the intermediate.
The magnesium salt is hydrolysed using dilute acid. \[ RCOOMgX \xrightarrow{H_3O^+} RCOOH. \] Thus, the final product is \[ \boxed{\text{Carboxylic acid}.} \]

Step 3: Analyse the remaining options.

• \(NH_3\) does not produce carboxylic acids with Grignard reagents.

• \(H_2O\) simply decomposes the Grignard reagent.

• \(O_2\) forms different oxidation products.
Therefore, \[ \boxed{CO_2} \] is the correct reagent. Hence, \[ \boxed{\textbf{Option (B)}} \] is the correct answer.