Key Stage 5 Optical activity
Your task
Pupil worksheet
1. Work in a pair. One person builds the molecule butan-2-ol and the
other, its mirror image.
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 superimposable.
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.
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.
http://www.oxfordsparks.net/animations/give-us-hand
5. Construct a rule which you can use to work out of a molecule is chiral
without having to build a model.
Optical isomers
Using a polarimeter
Enantiomers are also known as optical isomers. This is because they rotate
plane polarised light in different directions.
A polarimeter is an instrument which measures the degree of optical
rotation.
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.
Water (an achiral molecule) is placed inside. You look down polarising filter
B and rotate it until it is at its darkest.
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.
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.
A solution of the other enantiomer rotates
the plane of polarisation in an anticlockwise direction. This enantiomer is
known as the (-) form.
Questions
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)?
http://www.oxfordsparks.net/animations/give-us-hand
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?