chirality center

Question

Assign R or S configurations to the chirality centers in ascorbic acid (vitamin C).

Answer 1

Hi Ivan, so first off chiral centers are where there are 4 unique groups. R/S is labeled based off how you rank the groups 1-4 where 1 is often the most EN or highest MW, and 4 is often a H or lowest priority group. A big rule is only invert your answer when 4th priority group is wedged. Another one is you mainly draw based off 1->2->3 (ex. if it's weird 1->2->4->3, just focus on 1,2,3)

Depending on how your professor gave you the model of ascorbic acid, realize that it can change because there is L(+) and D(-) ascorbic acid where the second OH is either wedged or dashed. For my explanation I'm going to be using L(+) so H is wedged and OH is wedged.

If you look at L-(+)- ascorbic acid, I will be starting from the lactone ring where the first chiral center we see is next to the ether bond: C-O, C-COH2, C-H (wedge), C-C-OH.

ranking priority:

1.C-O

2.C-COH2

3.C-COH

4.C-H (wedge, UNFAVORABLE)

*Note that you usually try to determine if you are going in clockwise direction or counterclockwise from the order. For this case, 1->2->4->3 making a counterclockwise (S), but since 4th priority is wedged it is now (R).

The following carbon (if you count it is 2 carbons away from OH and ester) has chiral center: C-O (wedge), C-H (dash), C-CH2OH, C-COH.

ranking priority:

1.C-OH

2.C-COH

3.C-CH2OH

4.C-H (dash, FAVORABLE)

This will be S.