Optical Isomerism

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.