Optical Isomerism
OCR
Chirality
(c) optical isomerism
(an example of stereoisomerism, in terms of non- superimposable mirror images
about a chiral centre)
(d) identification of
chiral centres in a molecule of any organic compound.
EDEXCEL
Topic 17A: Chirality
1. know that optical isomerism
is a result of chirality in molecules with a single chiral centre
2. understand that optical
isomerism results from chiral centre(s) in a molecule with asymmetric carbon
atom(s) and that optical isomers are object and non- superimposable mirror
images
3. know that optical activity
is the ability of a single optical isomer to rotate the plane of polarisation
of plane-polarised monochromatic light in molecules containing a single chiral
centre
4. understand the nature of a
racemic mixture
5. be able to use data on
optical activity of reactants and products as evidence for SN1 and SN2 mechanisms
AQA
3.3.7 Optical isomerism
(A-level only)
Compounds that contain an
asymmetric carbon atom form stereoisomers that differ in their effect on plane
polarised light. This type of isomerism is called 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.
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.
A mixture of equal amounts of
enantiomers is called a racemic mixture (racemate).
Students could be asked
• to recognise the
presence of a chiral centre in a given structure in 2D or 3D forms. They could
also be asked to draw the 3D representation of chiral centres in various
species.
• draw the structural
formulas and displayed formulas of enantiomers
• understand how racemic
mixtures (racemates) are formed and why they are optically inactive.
Optical Isomerism
There is an
incredible confusion about optical isomerism in A level courses and students’
thinking at this level.
The confusion
arises because we so often start in the wrong place to try and understand
what’s going on here.
So let’s begin
where it is best to begin with the experimental behavior of molecules and
optical activity. Then move to the
structural explanation.
What is optical
isomerism?
Optical Isomerism is a form of
isomerism in certain molecules that enables an aqueous solution of an organic
compound such as an amino acid to rotate the plane of plane–polarised light in
an instrument called a polarimeter.
The rotation can happen in one of two different directions.
Here is a diagram of a polarimeter:
Above you can see a picture of a typical polarimeter and the 10cm sample tube used in the instrument.
As you can see above the light is first polarized so that there is only one plane
for the light’s electric vector.
This is plane polarized light.
This plane polarized light is then passed through the polarizing solution
under test.
The solution of say the amino acid or sucrose is of a given concentration
and length (10cm). As the light passes
through the solution, the molecules of the amino acid or sugar rotate the plane
of the electric vector a given number of degrees either to the right or to the
left.
This rotation can be measured as the light emerges from the solution and
enters the analyser. You can see this
illustrated in the two diagrams below.
If the substance in solution rotates the plane of plane polarised light it
is said to be optically active.
The question is now why is it
optically active?
What are the structural features of molecules that enable them to rotate the
plane of plane polarized light?
There are several features of molecular structures that give rise to
optical activity in molecules but only
one that comes up at A level.
The one feature of molecules that give rise to optical activity in these A
level courses is that the molecule contains at least one chiral centre.
But what is a
chiral centre?
Amino acids are a prime example of chiral molecules because they have four different functional groups attached
to a central carbon atom.
Here we see how that works in practice:
Each type of amino acid is called an enantiomer.
An aqueous solution of one will rotate the plane–polarized light to the
left and the other to the right if both solutions are of the same length and
concentration.
Another thing, these two isomers are non–superimposable
mirror images of each other. (As in
topology no number of rotations of objects will resolve mirror images of
objects).
How do we tell them apart?
Here is a fool proof method in this illustration.
But note that the molecular structure does not correspond to the rotation
direction. But the structure of the D–
form of the acid does correspond to a reading of the functional groups so
reading the D– form functional groups clockwise spells CO R N.
This is a convenient way to remember the structure of a D– amino acid. However, it does NOT mean that the acid will
rotate the plane of polarized light to the right in your polarimeter. As in the illustration above the D– alanine
is laevorotatory—it rotates the plane polarized light to the left!!
R and S are now used to designate these two structures.
Any molecule that contains a carbon atom which has four different
functional groups attached will be optically active. Such a carbon atom is called an asymmetric carbon atom and
designated with a star symbol.
If there is a mixture of equal moles
of D– molecules and L– molecules then such a mixture/solution is called a racemic mixture and does not rotate
plane polarized light.
Optical activity can be used to distinguish Sn1 from Sn2 mechanisms. The next two images illustrate this effect quite
well.
