Question

The absolute configurations of the two centers in the following molecule are :

• A. $2\left(R\right),3\left(S\right)$
• B. $2\left(R\right),3\left(R\right)$
• C. $2\left(S\right),3\left(S\right)$
• D. $2\left(S\right),3\left(R\right)$

Answer 1

The correct option is A $2\left(R\right),3\left(S\right)$

$R$ configuration is when the decreasing priority order of groups are found in clockwise direction. If it is anticlockwise, it is $S$. It is applicable when least prior group(usually it is $H$ atom) is below the plane. But if the least prior group is above the plane(as shown below), we should check incresing order and if it is clockwise, it is $R$ otherwise $S$.

Hence, (a) is correct.