Chirality 3+4

Question 1

what is the equation to work out the number of possible stereoisomers ?

Answer 1

• 2n

Question 2

Why do molecules with two identical chiral centers have only 3 stereoisomers?

Answer 2

• Meso form (achiral) = (S,R and R,S ) are symmetrical = same subs per chiral centre
• (R,R) configuration
• (S,S) configuration

Question 3

what is a meso compound ?

Answer 3

• has a plane of symetry
• achiral
• same subs per chiral centre
• optically inactive

Question 4

what is meso-DMSA used for ?

Answer 4

• medically to chelate toxic metals in cases of metal poisoning

Question 5

do diastereiomers have the same chemical + physical properties ?

Answer 5

• mostly same chemical properties (same functional groups)
• different physical properties (eg. solubility)

Question 6

what is resolution ?

Answer 6

• the separation of enantiomers from either racemic
  mixture or enantiomerically enriched mixture

Question 7

how is chiral chromatography used to seperate enantiomers ?

Answer 7

• normally HPLC or GC where racemic solution is passed over a **chiral stationary phase **
• giving rapid and reversible diastereotopic interaction

Question 8

How can diastereoisomers be separated using their different physical properties?

Answer 8

• diastereoisomers have different solubilities in solvents
  1. use a solvent where one diastereoisomer is very soluble + the other is less soluble
  2. both diastereoisomers dissolve
  3. the less soluble one crystallises out and is filtered
  4. evaporate the solvent to get the other pure
  diastereoisomer

Question 9

How can the difference in physical properties of diastereoisomers be used to separate enantiomers?

Answer 9

• Enantiomers can be separated by converting them into diastereoisomers
• Racemic acid reacted with one enantiomer of a pure base, forming diastereomeric salts.
• one diastereoisomer crystallises + can be filtered off
• leaving other to be evaporated
• recover enantiomer by acidifying salt (with HCl) + breaking it
• can do same with racemic base and one enatiomer of pure acid

Question 10

resolution by esterification ?

Answer 10

• react racemic of acid (R and S) with S-alcohol
• makes (R,S)-ester and (S,S)-ester
• diaseteroisomers seperated by crystillisation
• recover enantiomers by ester hydrolysis using HCl

Question 11

pharmacological effects of chirality ?

Answer 11

• allows for appropriate recognition by targets
• strong binding to targets
• if only one enatiomer active : inactive form can have no adverse effects or be toxic

Question 12

what is a stereoselective reaction ?

Answer 12

• preferential formation of one stereoisomer over another

Question 13

what is a stereospecific reaction ?

Answer 13

• each stereoisomeric reactant produces a
  different stereoisomeric product or a different set of products

Question 14

what is the relationship between stereoselctive + stereospecific reactions ?

Answer 14

• All stereospecific reactions are stereoselective
• Not all stereoselective reactions are stereospecific

Question 15

what is a non-concerted reaction ?

Answer 15

• SN1
• bond making + breaking in different steps
• intermediate formed
• nucleophile can attack from above or below the plane
• results in formation of racemix

Question 16

what is a concerted reaction ?

Answer 16

• SN2
• walden inversion = inverts stereochemistry due to backside attack

Question 17

which reaction forms a racemate ?

Answer 17

• SN1 / non-concerted