chirality

Question 1

what does chiral mean?

Answer 1

molecule that can’t be superimposed with its mirror image
two forms of the same molecule with same bonding pattern but different arrangement of atoms in space
contain at least one asymmetrical sp3 C atom -> 4 different groups attached to the C

Question 2

how are enantiomeric pairs calculated?

Answer 2

• N chiral centres represented by 2N stereoisomers
• 2N-1 represents enantiomeric pairs
• pairs not related by symmetry are diasteromers

Question 3

describe the configuration of chiral centers in D/L systems

Answer 3

• D/L system-> comparison of chiral centre with configuration of known enantiomer (D or L-glyceraldehyde), used for biological molecules
  
  • D-glucose
  • L-amino acids

Question 4

describe the steps to find the D/L configuration of chiral centres based on Fisher projections

Answer 4

1. C atom is placed at the centre
2. vertical lines represent bonds inside plane, horizontal bonds outside plane
3. most oxidised substituent placed at the top
4. longest C chain placed vertically
5. relative position of the groups attached to the central C is compared to D/L glyceraldehyde to assign chirality

Question 5

describe the configuration of chiral centers in R/S systems

Answer 5

based on Cahn-ingold-Prelog rules, used for generic organic molecules

Question 6

describe the steps to find the configuration R/S of chiral centres based on Cahn-ingold-Prelog rules

Answer 6

• assign priority nnumber to substituents of chiral centre (based on atomic number, double bond counts as 2 atoms directly attached so higher priority)
• orient molecule so lower priority if further from viewer
• if 1,2,3 order is clockwise -> R(rectus)
• if 1,2,3 order is anticlockwise -> S(sinister)
• Fisher model can be used with lowest priority at the bottom

Question 7

what are prochiral carbon atoms?

Answer 7

• prochiral carbon atoms: those thatcanbe converted from achiral to chiral in a single step
  
  • tetrahedral carbon can be converted to a chiral center by changing only one of the attached groups
  • H is substituted with another group (D)-> makes molecule chiral

Question 8

examples of chirality in biology

Answer 8

• sugars contain chiral centres
• proteins are composed of 20 amino acids, 19 have at least 1 chiral centre
  
  • L-amino acids used for synthesis of protein
• nucleic acids have phosphate-(deoxy)ribose backbone which has 3/4 chiral centres (1 combination is used)

Question 9

describe the case of talidomide

Answer 9

• used in the 50/60 as ipnotic to fight nausea and pregnant women
  
  • sold as racemic mixture of 2 enantiomers which caused teratogenic activity
  • R-talitomide is teratogenic (inhibitor of angiogenesis and synthesis of cytokinesis, stopped development of foetus or causes tumour), S talitomide wasn’t

Question 10

what are enantiomers?

Answer 10

compounds which rotate the polarised plane in different rotations

• can be dextrorotatory (d) or levorotatory (l)
• 50% mixture of enantiomers doesn’t rotate the plane