IDO 2 Stereochemistry

Question 1

enantiomers

Answer 1

non-superimposable stereoisomers (mirror images)
- 1 chiral centers and no alkenes: MUST be
- 2+ chiral centers or chiral center + alkene: CAN be

*some exceptions

Question 2

chiral molecules

Answer 2

contain at least one chiral center and lack a plane of symmetry–nonsuperimposable on mirror image!
- chirality center: sp3 atom bonded to 4 different groups

Question 3

types of stereoisomer for molecules with multiple chiral centers

Answer 3

• enantiomer: all chiral centers have 2 groups flipped
• diasteromers: not all chiral centers have 2 groups flipped

Question 4

requirements for diastereomers

Answer 4

they must have either:
- 2+ chiral centers
- chiral center + cis/trans alkene
- just cis/trans alkene

Question 5

switching one pair vs both pairs of groups attached to a chiral center

Answer 5

• switch one pair: enantiomer (or diastereomer if other chiral centers)
• switch both pairs: same molecule, just rotated

Question 6

rules for configuration of chiral centers

Answer 6

1. prioritize groups according to largest atomic number (if same atom, whichever has a bond to a larger atom)
2. standard perspective (lowest priority facing away)
3. trace circuit 1 to 3
   R = clockwise
   S = counterclockwise

Question 7

shortcuts for determining chiral center configuration

Answer 7

• if lowest priority atom is facing towards, count normally and then it’s the reverse
• use Fischer projection

Question 8

general rules for Fischer projection

Answer 8

• useful when lowest priority isn’t pointing away
• imagine a viewpoint going between wedge and dash
• wedge and dash are placed horizontally (pointing forward)
• bonds in plane are placed vertically (pointing away)
• after determining direction of arrow, remember that if lowest substituent is

Question 9

physical properties of chiral molecules

Answer 9

• chirality only affects physical properties in chiral environments
• e.g. certain solvents will have different interactions with the 2 forms of an enantiomer

Question 10

physical properties of enantiomers vs diastereomers

Answer 10

• enantiomers have identical physical properties (in nonchiral environments)
• diastereomers do not have identical physical properties

Question 11

racemic mixture

Answer 11

• 50/50 mixture of entantiomers
• each enantiomer rotates light a different way (cancels out)

Question 12

optical purity calculation for enantiomers

Answer 12

• divide specific rotation of sample by specific rotation of a pure enantiomer
• this gives the enantiomerical excess (depending on the sign of the sample)
• then divide the remaining fraction by 2 to determine proportions

Question 13

relative vs absolute configuration

Answer 13

• optical activity data gives you the relative configuration of enantiomers, but not 3D structure
• absolute configuration would require x-ray crystal structure

Question 14

molecules WITH chiral centers that are NOT chiral molecules

Answer 14

• mesocompounds: multiple chiral centers + plane of symmetry

Question 15

molecules WITHOUT chiral centers that ARE chiral molecules

Answer 15

• atropisomers: hindered rotation around a single bond
• allenes: 2 sets of double bonds (pi bonds) on C are 90 degrees from eachother
• helicenes: cyclohexane rings in big molecules make a twisted structure

Question 16

can mesocompounds rotate light?

Answer 16

no, they are not chiral!