Q.23 The Fisher projection that represents (2R,3S)-2,3-dihydroxybutanoic acid is
Given Compound
Molecular structure of 2,3-dihydroxybutanoic acid:
CH3–CHOH–CHOH–COOH
The molecule contains two chiral centers at carbon 2 and carbon 3.
Important Rules for Fischer Projection
- Vertical bonds point away from the observer.
- Horizontal bonds point towards the observer.
- If the lowest priority group (H) is on a horizontal bond, the configuration is inverted.
- Numbering of carbon starts from the COOH group.
Step 1: Configuration at C-2
Priority order at C-2:
- –OH
- –COOH
- –CH(OH)CH3
- –H
To obtain R-configuration (after applying inversion rule),
the –OH group must be on the right side.
Step 2: Configuration at C-3
Priority order at C-3:
- –OH
- –CH(OH)COOH
- –CH3
- –H
To obtain S-configuration (after inversion),
the –OH group must be on the left side.
Required Fischer Projection Pattern
| Chiral Carbon | Position of –OH |
|---|---|
| C-3 | Left |
| C-2 | Right |
This arrangement gives opposite configurations at the two chiral centers.
Final Answer
The correct Fischer projection of (2R,3S)-2,3-dihydroxybutanoic acid is Option (D).
Conclusion
By applying CIP priority rules and Fischer projection conventions,
the correct configuration is obtained when the hydroxyl groups are
placed on opposite sides. This results in the (2R,3S) configuration,
which corresponds to option (D).
