Q44E - Organic Chemistry (Mcmurry)

Question

Assign R or S configurations to each chirality center in the following molecules:

Step-by-Step Solution

Answer

Configuration of given molecule at C₁ and C₆ is S-configuration. 

Configuration of given molecule at C₂ and C₆ is S-configuration. 

Configuration of given molecule at C₂ is R-configuration and C₅ is S-configuration. 

Step 1: R and S- configuration.

Follow the direction of the top 3 priorities from highest to lowest priority.

A counterclockwise direction is an S (sinister) configuration and a clockwise direction is an R (rectus) configuration.

Sinister in latin is left and rectus in latin is right. 

 

Step 2: Sequence Rules for Specifying Configuration

• Rule 1

Number all the four atoms directly attached to the chirality center according to their atomic number. The atom with the highest atomic

number has the highest ranking (first), and the atom with the lowest

atomic number has the lowest ranking (fourth). 

.

• Rule 2

If decision can’t be reached by ranking the first atoms in the

Substituent then look at the second, third, or fourth atoms away from the

chirality center until able to find the first difference.

 

• Rule 3

If two atoms are multiple-bonded then it is counted as equivalent to the same number of single-bonded atoms.

 

 

• After ranking the four groups attached to a chiral carbon, we orient the lowest prority group to rear side (away from observer) and then describe the stereochemical configuration around the carbon by using three remaining substituents, which now appear to radiate toward us. 

 

• If a curved arrow drawn from the highest to second-highest to third-highest ranked substituent and it is clockwise, then  chirality center has the R configuration and if the arrow counterclockwise, then S configuration.

 

 

Step 3: Structure based on forth priority group location:

As it is mentioned earlier that the forth priority group should be on rear side so to do this following concepts can be used:

• If it is already on rear side: In this case molecule will remain as it is.
• If it is radiating outward or is in plane of paper: In these cases there will be double interchange. Change the position of forth priority group with rear group and also interchange position of remaining two groups.

Step 4: Orientation of given molecule after forth group on rear side for C 1 :

Based on priority rules and molecular weight the priority of group is as follows:

• H is forth priority group
• Side of ring with Cl farther is third priority group
• Side of ring with Cl closer is second priority group
• OH is first priority group

As here forth priority group is pointing outside the plane of paper so, double interchange will occur. H will change position with OH group present on rear side and other two will also exchange position. 

Step 5: Configuration of given molecule at C 1 :

As here on drawing curved arrow from the highest to second-highest to third-highest ranked substituent it is counter-clockwise in direction so chirality center has the S configuration.

Step 6: Orientation of given molecule after forth group on rear side for C 6 :

Based on priority rules and molecular weight the priority of group is as follows:

• H is forth priority group
• Side of ring with OH farther is third priority group
• Side of ring with OH closer is second priority group
• Cl is first priority group

As here forth priority group is in plane of paper so, change will occur. 

Step 7: Configuration of given molecule at C 6 :

As here on drawing curved arrow from the highest to second-highest to third-highest ranked substituent it is counter-clockwise in direction so chirality center has the S configuration.

 

Step 8: Orientation of given molecule after forth group on rear side for C 2 :

Based on priority rules and molecular weight the priority of group is as follows:

• H is forth priority group
• Side of ring with methyl farther is second priority group
• Side of ring with methyl closer is first priority group
• Ethyl is third priority group

As here forth priority group is pointing outside the plane of paper so, double interchange will occur. H will change position with ethyl group present on rear side and other two will also exchange position. 

Step 9: Configuration of given molecule at C 2 :

As here on drawing curved arrow from the highest to second-highest to third-highest ranked substituent it is counter-clockwise in direction so chirality center has the S configuration.

Step 10: Orientation of given molecule after forth group on rear side for C 6 :

Based on priority rules and molecular weight the priority of group is as follows:

• H is forth priority group
• Side of ring with ethyl farther is second priority group
• Side of ring with ethyl closer is first priority group
• Methyl is third priority group

As here forth priority group is into plane of paper so, no change will occur. 

Step 11: Configuration of given molecule at C 6 :

As here on drawing curved arrow from the highest to second-highest to third-highest ranked substituent it is counter-clockwise in direction so chirality center has the S configuration.

 

Step 12: Orientation of given molecule after forth group on rear side for C 2 :

Based on priority rules and molecular weight the priority of group is as follows:

• Methyl is forth priority group
• Side of ring with methyl farther is third priority group
• Side of ring with methyl closer is second priority group
• Hydroxyl group is first priority group

As here forth priority group is into the plane of paper so, double interchange will not occur. 

Step 13: Configuration of given molecule at C 2 :

As here on drawing curved arrow from the highest to second-highest to third-highest ranked substituent it is clockwise in direction so chirality center has the R configuration.

Step 14: Orientation of given molecule after forth group on rear side for C 5 :

Based on priority rules and molecular weight the priority of group is as follows:

• Methyl is forth priority group
• Side of ring with methyl farther is third priority group
• Side of ring with methyl closer is second priority group
• Hydroxyl group is first priority group

As here forth priority group is into plane of paper so, no change will occur. 

Step 15: Configuration of given molecule at C 5 :

As here on drawing curved arrow from the highest to second-highest to third-highest ranked substituent it is counter-clockwise in direction so chirality center has the S configuration.