Q5-5-50P - Organic Chemistry (Mcmurry)

Question

Xylose is a common sugar found in many types of wood, including maple and cherry. Because it is much less prone to cause tooth decay than sucrose, xylose has been used in candy and chewing gum. Assign R or S configurations to the chirality centers in xylose.

Step-by-Step Solution

Answer

Configuration of given molecule at chiral center is:

Chiral center 1:R- configuration

Chiral center 2: S- configuration

Chiral center 3: R-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 or second method is if you don’t want to do interchange just find the orientation as it is and the result you get right its opposite configuration that is your answer.

 

Step 4: Number of chiral carbon.

As the condition for carbon to be chiral is presence of four different groups on carbon so here it can be seen that only one chiral center is present as shown in figure below.

Step 5: Orientation of given molecule after forth group on rear side for chiral center 1.

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

• Hydrogen is forth priority group
• Group toward second chiral center is third priority group
• Hydroxyl group is first priority group
• Aldehyde group is second priority group

As here forth priority group is outside plane of paper so, double interchange will occur. H will change position with hydroxyl group and other two group interchange positions too. 

Step 6: Configuration of given molecule at chiral center 1

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 7: Orientation of given molecule after forth group on rear side for chiral center 2.

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

• Hydrogen is forth priority group
• Group toward third chiral center is third priority group
• Hydroxyl group is first priority group
• Group toward first chiral center is second priority group

As here forth priority group is outside plane of paper so, double interchange will occur. H will change position with hydroxyl group and other two group interchange positions too. 

Step 8: Configuration of given molecule at chiral center 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 9: Orientation of given molecule after forth group on rear side for chiral center 3:

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

• Hydrogen is forth priority group
• Side toward chiral carbon 2 is second priority group
• Methanol is third priority group
• Hydroxyl group is first priority group

As here forth priority group is outside plane of paper so, double interchange will occur. H will change position with hydroxyl group and other two group interchange positions too

Step 10: Configuration of given molecule at chiral center 3:

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.