Lecture 3 Flashcards
manipulation of stereocentre for qsignment of configurationmanipulation of stereocentre for assignment of configuration
Manipulation of Fischer Projections
- If one group of a Fischer projection is held steady, the other three groups can be rotated clockwise or counterclockwise.
If you are trying to get the group of lowest priority to the vertical line don’t hold it in place. Make sure it can be rotated to one of the two vertical positions.
Note: This is the same principle as the umbrella manipulation on the 3D structure earlier.
Multiple Chiral Centres in a Molecule
If a compound has ‘n’ chiral centres, there are 2n possible stereoisomers
1 chiral centre - 2 stereoisomers i.e. R and S
2 chiral centres - 4 stereoisomers i.e. RR, SS, RS, SR
Fischer projections are most useful when a molecule has more than 2 chiral centres and these centres are adjacent to each other. Fischer projections are commonly used to illustrate carbohydrates.
Compounds with More than One Chiral Centre
2 chiral centres
Each chiral centre could have R or S configuration
4 possible stereoisomers
Compounds with More than One Chiral Centre
Compounds with More than One Chiral Centre
I and II are non-superimposable mirror images, i.e. enantiomers.
III and IV are non-superimposable mirror images, i.e. enantiomers
Compare I and III
List of diastereomer pairs: I and III; I and IV; II and III, II and IV
Diastereomers
Stereoisomers that are not mirror images of one another
They have the same configuration at one chiral centre
but different at the other
They have different physical and chemical properties
(i.e. melting and boiling points, solubility)
They have different effects on the plane of polarised light
The specific angles of rotation will be unequal
and may or may not differ in sign
Compounds with More than One Chiral Centre
Some molecules with chiral centres are achiral
i.e. they are not optically active
e.g. tartartic acid
Tartaric acid has three stereoisomers (rather than 4) because each of its
two asymmetric centres have the same set of four substituents
Tartaric Acid HOOCCH(OH)CH(OH)COOH
Note:
Lowest priority
group on
horizontal
Plane of symmetry
On either side of the plane drawn in the molecule above are identical groups. This is a plane of symmetry.
image
III and IV are superimposable mirror images and are identical (rotate III by 180).
III is achiral (optically inactive) since it has a plane of symmetry.
Tartaric Acid Summary
I and II are enantiomers: (non-superimposable mirror images)
I and III are diastereomers
II and III are diastereomers
III has two chiral centres but is achiral (optically inactive)
and is called a meso compound.
Note: the chiral centres in the meso-compound have opposite configurations.
2,3-Dichlorobutane: Another example to Try Yourself
Applications of Chirality
Molecules with different shapes fit into different receptors.
A receptor shaped like a right glove, for example, would interact only with a right-handed molecule.
The two enantiomers of limonene bind to different receptors in the olfactory organ and perceived as different smells by our brains.
Chirality in Action: Adrenaline
Thalidomide
Occupation: To induce sleep
and to prevent nausea during pregnancy.
Side effects: Teratogen
i.e. causes foetal abnormalities
Future roles: FDA approved for the treatment of leprosy.
Summary of Isomerism
Isomers: Compounds with same molecular formula but ….
Summary of Isomer Types
