Q.11 Among the following reactions, the one that produces achiral alcohol (after hydrolysis) is
Introduction
Grignard reagents are powerful nucleophiles that react with carbonyl compounds to form alcohols after hydrolysis. However, not all such reactions lead to chiral alcohols. In stereochemistry-based exam questions, identifying whether the resulting alcohol is chiral or achiral is crucial.
Let’s analyze the given question step by step.
The Question
Among the following reactions, the one that produces an achiral alcohol (after hydrolysis) is:
Each option shows a carbonyl compound reacting with ethyl magnesium bromide (CH₃CH₂MgBr) followed by hydrolysis.
Key Concept: When Is an Alcohol Achiral?
An alcohol is achiral if the carbon bearing the –OH group does not have four different substituents.
-
If all four substituents are different → chiral
-
If at least two substituents are identical → achiral
Option-wise Explanation
Option (A): Acetaldehyde (CH₃CHO) + CH₃CH₂MgBr
Reaction outcome:
Grignard addition to acetaldehyde forms 2-butanol after hydrolysis.
Structure of alcohol carbon:
-
–OH
-
CH₃
-
CH₂CH₃
-
H
All four substituents are different, so the carbon is a stereogenic center.
Conclusion:
❌ Chiral alcohol formed
Option (B): Acetophenone (CH₃COPh) + CH₃CH₂MgBr
Reaction outcome:
Gives a tertiary alcohol after Grignard addition.
Structure of alcohol carbon:
-
–OH
-
CH₃
-
CH₂CH₃
-
Phenyl (Ph)
Again, all four substituents are different.
Conclusion:
❌ Chiral alcohol formed
Option (C): Ethyl acetate (CH₃COOEt) + CH₃CH₂MgBr
Reaction outcome:
Esters react with two equivalents of Grignard reagent, forming a tertiary alcohol.
Structure of alcohol carbon:
-
–OH
-
CH₃
-
CH₂CH₃
-
CH₂CH₃
Here, two substituents are identical (ethyl groups).
Conclusion:
✅ Achiral alcohol formed
✔ This is the correct answer
Option (D): 2-Butanone (CH₃COCH₂CH₃) + CH₃CH₂MgBr
Reaction outcome:
Produces a tertiary alcohol.
Structure of alcohol carbon:
-
–OH
-
CH₃
-
CH₂CH₃
-
CH₂CH₃CH₃ (propyl)
All substituents are different.
Conclusion:
❌ Chiral alcohol formed
✅ Final Answer
Correct Answer: Option (C)
Because the tertiary alcohol formed contains two identical ethyl groups, the carbon bearing the –OH group is not chiral.
