Consider the following SN2 reaction of optically pure
1-chloro-3-ethylcyclopentane (X).H3CH2C–(cyclopentane ring)–Cl + OH− → YX
Q.23 The absolute configuration of
1-chloro-3-ethylcyclopentane (X) shown above is
(A) (1S,3R)
(B) (1S,3S)
(C) (1R,3R)
(D) (1R,3S)
Absolute Configuration of 1-Chloro-3-Ethylcyclopentane
Determining the absolute configuration of chiral molecules
is an essential part of stereochemistry. In this question, we analyze
optically pure 1-chloro-3-ethylcyclopentane using
CIP priority rules to identify its correct
(R/S) configuration.
Step 1: Identification of Chiral Centers
A carbon atom is chiral when it is attached to four different groups.
In 1-chloro-3-ethylcyclopentane, chiral centers are present at:
- Carbon-1 (attached to Cl)
- Carbon-3 (attached to ethyl group)
Step 2: Configuration at Carbon-1
Priority order at C-1 (according to CIP rules):
- Cl
- Ring path toward C-2
- Ring path toward C-5
- H
The hydrogen atom is oriented away from the observer.
The sequence 1 → 2 → 3 is clockwise.
Therefore, configuration at C-1 is R.
Step 3: Configuration at Carbon-3
Priority order at C-3:
- Ethyl group
- Ring path toward C-2
- Ring path toward C-4
- H
Here, the hydrogen atom is pointing toward the observer.
The sequence 1 → 2 → 3 is clockwise.
Since the lowest priority group is toward the observer,
the configuration is reversed.
Therefore, configuration at C-3 is S.
Final Absolute Configuration
The absolute configuration of the compound is:
(1R,3S)
Final Answer
Correct Option: (D) (1R,3S)
Conclusion
Accurate application of CIP rules and careful observation of
molecular orientation allows correct determination of absolute
configuration. In cyclic systems, different ring paths must be
compared carefully to avoid mistakes.
