Question

The R/S configuration of the following compound is

• A. R
• B. S
• C. No chiral centre

Answer 1

The correct option is B S

In the given projection formula, there is only one stereogenic centre.
It is the carbon attached to the $-COOH$ group! Apart from the carboxylic acid group, there is an $-N{H}_2$ group, a $-H$ and a $CSH(Me{)}_2$. Since there are four different groups on the carbon, it is a chiral centre.

The carbon attached to two methyl groups is clearly not chiral.
Let us convert the projection into a wedge-dash format and take it from there:

As we know, hydrogen has the least atomic number. So, 'H' takes number '4' and in the wedge-dash notation shown above, hydrogen is below the plane.
It is clear that amine group takes number '1'.
The carbon attached to the sulphur (SCC) takes number '2'. It has a higher precedence than the carboxylic acid carbon (OOO) due to the presence of sulfur atom.
The only number left is '3' and it is given to the carbon in carboxylic acid group.
Now we can see that, the numbers are making an arc in the anti-clockwise direction. Hence, given compound is in S configuration.