Q. 29 E - Organic Chemistry (Mcmurry)

Question

Assign R or S configuration to each chirality center in pseudoephedrine, an over-the-counter decongestant found in cold remedies (blue = N)

Step-by-Step Solution

Answer

C₁ has R- configuration

 C₂has S- 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 dashedbond:

▪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: 

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

(ii) between the remaining two groups.

▪Draw an arrow from 1 to 3 via 2.

Step 2: Convert the structure from ball and stick model into atomic form.

Red sphere representsoxygen,black spheres—carbon,and grey spheresrepresenthydrogen.

According to the representation of the atoms, draw the structure of the compound.

Step 3: Number of Chiral center present.

There are two chiral centre presents in the molecule C₁ and C_2.

Step 4: Assign the ranking.

According to the atomic numbers of the groups attached to the chiral center,assign the priority to the atoms attached.

Step 5: Assign the configuration.

 As fourth priority is in solid bond position, there is a need of double interchange as per the given rules. After double interchange the configuration of C₁, chiral center is R and C₂ chiral center is S.