Optical isomerism Flashcards
asymmetric molecule
Asymmetric = carbon atom is bonded to 4 different groups
An asymmetric carbon atom is chiral and gives rise to optical isomers (enantiomers), which exist as non super-imposable mirror images and differ in their effect on plane polarised light.
symmetric molecule
Symmetric = carbon atom bonded to less than 4 different groups or has 2 or more of the same group
optical isomerism
Optical isomerism is a form of stereoisomerism and occurs as a result of chirality in molecules, limited to molecules with a single chiral centre.
Optical isomers are two compounds which contain the same number and kinds of atoms, and bonds (i.e., the connectivity between atoms is the same), and different spatial arrangements of the atoms, but which have non-superimposable mirror images. Each non-superimposable mirror image structure is called an enantiomer.
racemate
A mixture of equal amounts of enantiomers is called a racemic mixture (racemate)
chiral molecules
-asymmetric carbon molecule
-can have a different chiral centre other than carbon but would not be known as a chiral carbon molecule
drawing optical isomers
1) draw mirror plane
2) draw central atom
3) draw on atom closest to mirror plane
4) repeat step for rest o atoms
5) check all bonds
enantiomers
-non-superimposible mirror images of eachother
drawing enantiomers
1) Draw the displayed formula
2) identify the chiral centre
3) Draw 3D depiction of the molecule
4) Draw the mirror image of the molecule
polarised light
Light is a wave and from fronton it would look like a straight line
Light waves arranged at the same angle = this is called plane polarised light
Shine plane polarised light through a filter
Unpolarised light through filter = forms plane polarised light
rotation of plane polarised light
Solutions of enantimoers rotate plane polarised light where one enantimer rotates light clockwise and the other anticlockwise by the same amount
Solutions of non-chiral molecules don’t rotate plane polarised light
Optically active solutions rotate plane polarised light
50:50 mix
50:50 mix of enantimors doesn’t rotate plane polarsed light = optically inactive. This mixture is called racemates
what do chiral centres lead to
2 possible mirror images that are mirror images of eachother
2 isomers can form/racemates form via
nucelophilic addition reaction
nucelophilles attack carbonyl group
chiral atom
A carbon atom that has four different groups attached is called a chiral(asymmetric) carbon atom
These four groups are arranged
tetrahedrally around the carbon.
non-superimposible
This causes two
different isomers that
are not superimposable
to be formed. They are
mirror images
Two compounds that are optical isomers of
each other are called enantiomers.
nucelophilic addition
Nucleophilic addition of HCN to
aldehydes and ketones (unsymmetrical)
when the trigonal planar carbonyl group
is approached equally from both sides by
the CN- attacking species: results in the
formation of a racemate (equal amounts
of both enantiomers)
electrophilic addition
A racemate can also be formed in the reaction of the
electrophilic addition of HBr to an unsymmetrical alkene
describe how you would distinguish between separate samples of the 2 entantimoers
-use plane polarised light
-would rotate in opposite directions
explain how a structural feature of this enzyme enables it to catalyse hydrolysis of isomer G but not isomer F
enzyme has an active site
enzyme has the correct stereochemistry for isomer G
explain why the reaction produces a racemic mixture
-planar carbonyl group
-attack each site with equal probability
explain the meaning of a racemic mixture
equal mixture of enantiomers
state the relationship between 2 chiral molecules with the same structural formula
non-superimposible mirror images of eachother
molecules with planar profiles such as double bonds
make racemic products
e.g propanal and acidified potassium cyanide
attack carbonyl group of unsymmetrical ketones and aldehydes
as C=O is planar the CN- ion can attack either from above or below forming 2 different enantiomers
racemic mixture why would it not rotate plane polarised light (3 marks)
-one enantiomer rotates the plane polarised light anticlockwise
-the other rotates it clockwise by the same amount
-they cancel eachother out so has no overall effect on the light
2 different monodentate ligands =
cis trans isomerism
