Question

Cyclohexanol undergoes a series of reactions as given. Identify compound (iv).
\[ C_6H_{11}OH + CrO_3 \rightarrow (i) + C_6H_5MgI \rightarrow (ii) + \text{dil. }HCl \rightarrow (iii) + \text{Conc. }H_3PO_4 \rightarrow (iv) \]

• A. A
• B. B
• C. C
• D. D

Hint:

Secondary alcohols oxidise to ketones, ketones react with Grignard reagents to form tertiary alcohols, and tertiary alcohols undergo dehydration to give substituted alkenes.

Answer 1

The Correct Option is A

Step 1: Oxidation of cyclohexanol.
Cyclohexanol is a secondary alcohol. On oxidation with \(CrO_3\), it forms cyclohexanone.
\[ C_6H_{11}OH \xrightarrow{CrO_3} C_6H_{10}O \]
So, compound (i) is cyclohexanone.

Step 2: Reaction with Grignard reagent.
Cyclohexanone reacts with phenyl magnesium iodide, \(C_6H_5MgI\).
The phenyl group attacks the carbonyl carbon of cyclohexanone.

Step 3: Formation of magnesium alkoxide.
After nucleophilic addition, a magnesium alkoxide intermediate is formed.
This is compound (ii).

Step 4: Hydrolysis with dilute HCl.
On treatment with dilute HCl, the magnesium alkoxide gives tertiary alcohol.
\[ \text{magnesium alkoxide} \xrightarrow{\text{dil. }HCl} 1\text{-phenylcyclohexanol} \]
So, compound (iii) is 1-phenylcyclohexanol.

Step 5: Dehydration using concentrated \(H_3PO_4\).
Concentrated \(H_3PO_4\) causes dehydration of alcohol.
Thus, 1-phenylcyclohexanol loses water to form an alkene.

Step 6: Formation of major alkene.
The alkene formed is 1-phenylcyclohexene, which is more stable due to conjugation with the phenyl group.
\[ 1\text{-phenylcyclohexanol} \xrightarrow{\text{conc. }H_3PO_4} 1\text{-phenylcyclohexene} \]

Step 7: Final conclusion.
Therefore, compound (iv) is 1-phenylcyclohexene.
Final Answer:
\[ \boxed{\text{1-Phenylcyclohexene}} \]