Q. 27-b - Organic Chemistry (Mcmurry)

Question

Assign R or S configurations to the chirality centers in the following molecules (blue = N)

Step-by-Step Solution

Answer

(b).  R configuration

Step 1:Rules for R/S nomenclature when molecule drawn as perspective formula are given below.

1. Determine the priority of the atoms or groups bonded to the chiral center, and label them as 1, 2 3, and 4.

2. If the lowest priority (4) group is bonded by a dashed bond, draw an arrow from 1 to 3 via 2.

• If the arrow points clockwise, then the centre has the R configuration.
• If the arrow points counter-clockwise, then the centre has the S configuration.

3. If the lowest priority (4) group is not bonded by a dashed bond, make double interchange: 

(i) between 4 and group bonded by hatched wedge,and

(ii) between the remaining two groups.

4. Draw an arrow from 1 to 3 via 2.

Step 2: Convert the given structure into atomic form

The red spheres represent oxygen, blue spheres represent nitrogen, black spheres—carbon, and grey spheres—hydrogen.

Step 3: Assign the ranking.

In this case assign the ranking according to the atomic numbers of the functional group attached to the chiral center.

Step 4: Assign the configuration

In this case the fourth priority is at dashed bond. So, there is no need of double interchange. Hence, the configuration of the chiral center is R.