Question

The absolute configuration of:

(1) (2R, 3R)
(2) (2R, 3S)
(3) (2S, 3R)
(4) (2S, R)

Answer 1

Hint:When we move in a clockwise direction, it is called R configuration, and when we move in an anti-clockwise direction, it is called an S configuration. The letter R is Rectus and the S is Sinister..

Complete step by step answer:
The Atomic number is given the priority for selecting the direction, and hence the rotation. After numbering the compound

Here, the 2nd position and the 3rd carbon atom of this compound have chiral centers, looking at the third position, the highest atomic number containing atom in the compound is the chlorine atom (17), followed by the carbon (8), methyl group (8) and hydrogen (1). It is an anti-clockwise rotation, according to the rule, but the 4th priority is on the horizontal line, hence it is R configuration. Looking at the 2nd position, the hydroxyl group is given the most priority, followed by chlorine, carboxylic, and hydrogen. Due to the 4th priority being on the horizontal it is S configuration. Hence, the configuration at the second chiral position is S and at the third chiral position is R.
Thus, the absolute configuration of: 2-hydroxy-3- chloro butanoic acid is option (C), (2S, 3R).

Note:
The absolute configuration of chiral compounds was obtained after 1951. If a compound does not possess a chiral center, it does not show absolute configuration. The compounds such as (+) isoserine, (+) glyceraldehyde show absolute configuration. Alternative techniques are optical rotatory dispersion, vibrational circular dichroism, ultraviolet-visible spectroscopy, use of chiral shift reagents in proton NMR, and Coulomb explosion imaging.