R/S Configuration, Fischer Projections, Stereocenters

Question 1

What are the steps to determine the configuration of a stereocenter?

Answer 1

1. Identify a stereocenter
2. Assign group priority
3. “turn” molecule so bond with the lowest priority is going away (ON THE DASH)
   4.Draw an arrow going from 1-2-3 on the priority groups
4. Label either R-config or S-config

Question 2

For configuration, what if the lowest priority is on the wedge?

Answer 2

Switch your configuration answer.

Question 3

What kind of config is clockwise?

Answer 3

R

Question 4

What kind of config is counterclockwise?

Answer 4

S

Question 5

Which way do you rotate groups?

Answer 5

dash -> wedge -> in plane

Question 6

What kind of configuration will enantiomers have?

Answer 6

Opposite configs
(R) (S)
(R,R) (S,S)
(R,S) (S,R)

Question 7

What kind of stereocenters do cis/trans molecules have?

Answer 7

Diastereomers

Question 8

Meso

Answer 8

Symmetry in molecule.
*Put in-plane molecules on same side to determine

Question 9

What is a fischer projection

Answer 9

Helps drawing molecules with multiple stereocenters. Done by flattening out a 3D structure.

Question 10

How to draw a fischer projection

Answer 10

1. Get in-plane molecules on same side.
2. Take the end molecules that are in plane, and make them the vertical (“backbone”)
3. For each stereocenter inbetween the in-plane molecules, draw a horizontal line
4. For each stereocenter, put the wedge molecule on the RIGHT and dash group on the LEFT

Question 11

Do chiral compounds have symmetry?

Answer 11

No.

Question 12

Do achiral compounds have symmetry?

Answer 12

Yes. They are meso.

Question 13

Do achiral or chiral compounds rotate plane with polarized light?

Answer 13

Chiral

Question 14

How do you separate an enantiomer?

Answer 14

1. Convert an enantiomer -> diastereomer
2. Separate diastereomers
3. Convert enantiomers back

Question 15

How do you convert an enantiomer into a diastereomer?

Answer 15

Perform acid base chemistry. If the enantiomers are acidic, add a chiral base. If basic, add a chiral acid.

Question 16

How do you convert a diastereomer back into an enantiomer?

Answer 16

Add the opposite acid/base that you used for extraction.
If you added a chiral base, add an acid.

Question 17

Why does adding a chiral acid/base work to convert an enantiomer to a diastereomer?

Answer 17

The chiral acid/base has it’s own configuration that stays the same even when added. This means that now the enantiomers’ have an additional stereocenter that stays the same - making it a diastereomer.

Question 18

When do you flip the R/S configuration on a fischer projection?

Answer 18

When the H is in the horizontal