Question

What are B and C respectively in the given sequence of reactions?

• A. Cyclohexanol, Cyclohexylmethanol
• B. Cyclohexane, Cyclohexylmethanol
• C. Cyclohexane, Cyclohexylethane
• D. Ethoxycyclohexane, Cyclohexylmethanol

Hint:

Grignard reagents will always prioritize acting as a base over acting as a nucleophile if a proton donor (like water, alcohol, or acid) is present in the mixture.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
We are given a multi-step reaction starting with cyclohexyl bromide.
We need to determine the structures of the organic products B and C.

Step 2: Key Formula or Approach:
1. Alkyl halides react with Magnesium metal (Mg) in dry ether to form Grignard reagents (R-MgX).
2. Grignard reagents are extremely strong bases and nucleophiles. They react with active hydrogen sources (like alcohols) to undergo acid-base reactions, forming alkanes.
3. Grignard reagents react with formaldehyde (HCHO) to form a primary alcohol after acidic hydrolysis.

Step 3: Detailed Explanation:

• Step 1: Formation of A:
Cyclohexyl bromide reacts with Mg in dry ether to form cyclohexylmagnesium bromide (Grignard reagent A):
\[ \text{C}_6\text{H}_{11}\text{Br} + \text{Mg} \xrightarrow{\text{dry ether}} \text{C}_6\text{H}_{11}\text{MgBr} \quad (\text{A}) \]

• Step 2: Formation of B:
Grignard reagent A reacts with ethanol ($\text{CH}_3\text{CH}_2\text{OH}$).
Since ethanol has an acidic proton ($-\text{OH}$ group), it protonates the Grignard reagent:
\[ \text{C}_6\text{H}_{11}\text{MgBr} + \text{CH}_3\text{CH}_2\text{OH} \rightarrow \text{C}_6\text{H}_{12}\text{ (cyclohexane)} + \text{Mg(OCH}_2\text{CH}_3)\text{Br} \] Thus, compound B is cyclohexane.

• Step 3: Formation of C:
Grignard reagent A reacts with formaldehyde (HCHO):
The nucleophilic cyclohexyl group attacks the carbonyl carbon of formaldehyde:
\[ \text{C}_6\text{H}_{11}\text{MgBr} + \text{HCHO} \rightarrow \text{C}_6\text{H}_{11}-\text{CH}_2-\text{OMgBr} \] Subsequent acidic hydrolysis converts this adduct into a primary alcohol:
\[ \text{C}_6\text{H}_{11}-\text{CH}_2-\text{OMgBr} \xrightarrow{\text{H}_3\text{O}^+} \text{C}_6\text{H}_{11}-\text{CH}_2-\text{OH (cyclohexylmethanol)} \] Thus, compound C is cyclohexylmethanol.


Step 4: Final Answer:
The products B and C are cyclohexane and cyclohexylmethanol, respectively.