Lecture 10 - Stereochemistry 3/4
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Dextrorotatory/ + / d
rotate plane polarised light clockwise
Laevorotatory / - / l
rotate plane polarised light anticlockwise
In chiral molecules, what do we call their centres?
Stereogenic.
In the old system, how do we assign D/L labels?
If the molecule matches D-glyceraldehyde, it is labelled D. If it looks different, it is L.
We can label enantiomers R and S. Which is the clockwise configuration and which is the anticlockwise?
R = clockwise S = anticlockwise
When labelled R and S, how do we assign priority?
according to atomic number.
When labelling R and S, if a decision on priority cannot be made with 1st atoms, then what do we do?
refer to the 2nd atoms - those bonded to 1st atoms
When labelling R and S, double bonds count as….
two sigma bonds to the same atom- same for triple bonds
When labelling R and S, lone pairs…
counts as zero
When labelling R and S, we always orientate 4….. . The lowest priority is always….
At the back 4, at the back
R and S configurations have nothing to do with
plane polarised light.
Ibuprofen is chiral. It is sold as a racemic mixture- only S is active. Is this structure the S enantiomer or the R?
s enantiomer, anticlockwise configuration.
If the lowest priority group is not facing the back/ not 4*, what do we do?
We reverse it- for example, if it looks anticlockwise, it should be clockwise and vice versa.
If a nucleophile attacks a prochiral from the back we call it…
Re face
If a nucleophile attacks a prochiral from the front
Si face
Re face= (think reverse)
anticlockwise
Si face=
clockwise
Draw this molecule if the nucleophile attacked it from the back. Is it Re or Si face
Draw this molecule if the nucleophile attached it from the front. Is it Re or Si face?
