Question

The absolute configuration of the following compound is:

(a)$2S,3R$
(b)$2S,3S$
(c)$2R,3S$
(d)$2R,3R$

Answer 1

An absolute configuration is the spatial arrangement of a chiral molecule and its stereochemical description that is R and S. R means Rectus and S means Sinister. These absolute configurations are mostly obtained by X-ray crystallography with some limitations also.

Complete answer:
If we rotate the curve in clockwise direction, the absolute configuration we get is R and if we rotate the curve in anti-clockwise direction, the absolute configuration we get is S. But some limitation is there. The lowest priority group should be present at the vertical position, otherwise the absolute configuration gets reversed. So from the above image, we see that H is the least priority group and $Cl$ is the highest priority group. So, we start from chlorine after that moves to the third carbon and then to the methyl group and finally to the hydrogen. We can see that this rotation is in clockwise direction but H is in horizontal position. So the absolute configuration for the $2$ carbon is $2S$. Similarly, if we look for third carbon, again the highest priority group is $Cl$. So we start from $Cl$ and move to the second carbon and then the methyl group. But H is present in the horizontal direction. So the absolute configuration is $3S$. So the absolute configuration of the compound is $2S,3S$.
Therefore, option B is the correct option.

Note:
There are also other configurations present in the stereochemistry which are D and L. D stands for dextro-rotatory and L is called levo-rotatory. Dextro-rotatory means the rotation is + means in clockwise direction and levo-rotatory means the direction is – means in anti-clockwise direction.