optical isomerism Flashcards
optical isomer (or enantiomer)
optically active isomers which are mirror images of each other that are non superimposable and contain chiral carbon centers
two subsets of stereoisomerism
geometric isomerism
optical isomerism
Chiral carbon
any carbon with 4 DIFFERENT groups attached to it.
What is a chiral molecule and what do you usually get asked about?
A molecule that contains a chiral carbon - usually asked to locate the chiral centre of the molecule so look for the chiral carbon.
How many optical isomers can you get
only ever two because they have to be mirror images of each other
What does it mean if a compound is optically active
compound that can rotate plane polarised light
What does a polarising filter do?
Plane polarises light so that it vibrates in the same plane
Talk about what actually be happenin once the light has been plane polarised
Pass the plane polarised light through the optically active isomer
the molecules interact with the light
rotating plane polarised light in a particular direction
Think of the little picture diagrams I drew and explain them
1) plane polarised light passing through a mixture of one type of optically active enantiomer, molecules interact with the light and the plane of polarised light is rotated CLOCKWISE
2) plane polarised light passing through a mixture of ANOTHER type of optically active enantiomer, molecules interact with the light and the plane of polarised light is rotated ANTICLOCKWISE
plane polarised light
- monochromatic light
- light that is travelling in a particular direction
single wavelength that is one frequency - all waves vibrating in the same plane
normal light
- white light
- unpolarised light so waves travel in all directions
- can only interact with an enantiomer if it is plane polarised light so first it must be passed through a polarising filter
what can optical isomers only act on
plane polarised light
because their molecules can only react with a particular plane
racemic mixture (racemate)
contains equal quantities of each enantiomer of an optically active compound
50/50 mix of each optically active ennatiomer
Cancelling out plane polarised light
one particular enantiomer will rotate a plane of plane polarised light in one direction (eg. clockwise) and the OTHER enantiomer will rotate plane polarised light in the opposite direction (eg. anticlockwise). - rotates light 180 degrees
racemic mixture - no optical activity - both enantiomers cancel out each others light rotating effects.
one pure enantiomer output rotation
output light will have rotated 180 degrees
Enantiomer uses
cancer drug cis platin
racemic mixtures used to treat patients but often given a double dose to get 50+ 50 = 100 of the useful enantiomer
difficult and expensive to separate racemic mixtures
thalidomide - 1950s - birth defects in pregnant women’s children after they were given it to help with morning sickness
see example image of optical isomer of 1-aminoethanol on paper
X
Chiral atom
- atoms with 4 different groups attached to it
- asymmetric
- results in formation of two enantiomers
enantiomers
two versions of the optical isomer
racemic mixture
- equal number of each enantiomer
- does not affect plane polarised light
polarimeter
uses plane polarised light to detect racemic mixtures
nomenclature for chiral compounds rotating the plane of polarisation
R-clockwise (+) ; S-anticlockwise (-)
?? ASK ABOUT
What happens with polarised light in a racemic mixture?
no rotation of polarised light
Sn1
- tertiary (and secondary) halogenoalkane hydrolysis
- nucleophile attacks trigonal planar carbocation intermediate from either side with equal probability
- generates racemic mixture; optically inactive
Sn2
- primary (and secondary) halogenoalkane hydrolysis
- this reaction inverts stereochemistry (what doe sthis mean)
- if reactant is optically active, so will the products be (same asymmetric arrangements)
What does optical activity do
- affects plane polarised light
- provides evidence for reaction mechanisms (Would be Sn2 if product optically active)
Why can there be a chiral centre on a ring?
Because although the carbon is attached to the same ring twice, the order of the atoms in the ring is different depending on which way round you go so they count as two different groups.
If a molecule has more than one chiral centre, what will be significant about its optical isomers?
It will have more than two optical isomers.
Are all naturally occuring amino acids D or L enantiomers?
L-amino acids.
Except glycine which isn’t chiral.
If a molecule CAN be superimposed on its mirror image what is it?
Achiral - and it doesn’t have a chiral carbon.
