Question

The configuration of the reactants and the product in the following reaction, respectively are:

A. R,R
B. R,S
C. S,R
D. S,S

Answer 1

The actual three dimensional arrangements of the groups in a molecule containing an asymmetric carbon is termed as absolute configuration and under this configuration, we state the R,S nomenclature. The groups are assigned the priorities on the basis of CIP sequence rule and then the direction of rotation is observed.

Complete step by step answer:
The rules for assigning the R,S nomenclature are stated as:
(i) Each group attached to the stereo centre is assigned a priority on the basis of CIP sequence rule.
(ii) After assigning the priorities, the least priority group is written at the position away from the observer.
(iii) Now, the order from top to second and then to third priority is observed. If the priority gives a clockwise direction then it is termed as R configuration and if it gives an anticlockwise direction then it is termed as S configuration.

Now, let us apply the above rules and find the configuration in the reactant side.
According to their atomic weight, Br is given the 1st priority, C being the 2nd and D being the third. As the 4th priority, i.e. H is in vertical position and the direction of rotation is anticlockwise, the configuration is ‘S’.

Similarly, let us apply these rules on the product side.
According to their atomic weight, both the carbons have the same priority. Whenever this type of case arises, we need to look to the neighboring atoms of the groups attached to the stereo centre. In this case, the carbon which has oxygen beside it will be given 1st priority and the one which has nitrogen beside it will be given 2nd priority. D will be given 3rd priority. Thus, as the direction of rotation is clockwise and the 4th priority is in horizontal position, the configuration will ‘S’.

Thus, the correct option is (D) S,S.

Note:
Here, in this question, one must have to learn the basic trick of:
VCR- Vertical (4th priority), Clockwise- R
VAS- Vertical (4th priority), Anticlockwise- S
HCS- Vertical (4th priority), Clockwise- S
HAR- Horizontal (4th priority), Anticlockwise- R
If someone were to learn this trick, the questions of R,S configuration would merely be a problem.