Key Stage 5 -Optical activity

Pupil worksheet

Chirality

Chiral comes from a Greek word

meaning 'handed'.

Your left and right hand are mirror images of each other and may look identical but they are not the same.

However you move them you cannot make both of your hands

look identical, they are not super-imposable.

A chiral object is one that is not super-imposable on its mirror image.

Conversely, an achiral object is one that is identical (super-imposable) to its mirror image.

Molecules can be chiral and will exist as two isomers known as enantiomers. Chirality is a form of stereoisomerism; the atoms in the enantiomers are joined up in the same order but they have a different spatial arrangement.

Your task

1. Work in a pair. One person builds the molecule butan-2-ol and the other, its mirror image.

2. Are the two models super-imposable?3. Is butan-2-ol chiral or achiral?4. Repeat steps 1-3 for 2-chloro-2-methylbutane.

5. Construct a rule which you can use to work out of a molecule is chiral without having to build a model.

http://www.oxfordsparks.net/animations/give-us-hand

Optical isomers

Enantiomers are also known as optical isomers. This is because they rotate plane polarised light in different directions.

Plane polarised light is light where the electromagnetic vibrations are in one plane only. It can be produced by passing light through polarising filters, like that found in polarising sunglasses.

Image: Kaidor, 老陳

A solution of one enantiomer rotates the plane of polarisation in a clockwise direction. This enantiomer is known as the (+) form.

A solution of the other enantiomer rotates the plane of polarisation in an anti-clockwise direction. This enantiomer is known as the (-) form.

Questions

1. What would you see if you held two polarising filters at right angles to each other? Why?

2. What would happen to the rotation of light if plane polarised light was passed through a 50/50 mixture of both enantiomers of a chiral molecule (called a racemic mixture or racemate)?

Using a polarimeter

A polarimeter is an instrument which measures the degree of optical rotation.

Water (an achiral molecule) is placed inside. You look down polarising filter B and rotate it until it is at its darkest.

Then, the water is replaced with the sample you wish to measure. The light rottes as it goes through the sample. Some light will now be seen through polarising filter B.

Questions

3. Describe how you would use the polarimeter to work out the direction of rotation.

4. Explain what you would add to the apparatus in order to measure the degree of rotation.

5. What variables will affect the degree of rotation of a sample? Why?

http://www.oxfordsparks.net/animations/give-us-hand