Download Properties of optically

Properties of optically active
compounds
Optical activity
Glucose is an optically
active compound.
On the straight-chain form
of glucose shown here, all
four of the carbons in the
middle of the chain are
chiral centres – they each
have four different groups
attached to them.
Four different groups can be arranged around a central
atom in two different ways. These optical isomers are
mirror images of each other and cannot be superimposed —
just as your left and right hands are mirror images and
cannot be superimposed.
The structure for glucose shows four chiral centres,
which means a total of 16 different forms are possible.
Glucose is just one of those forms.
Optical isomers
Molecules which have one or more chiral centres
rotate plane-polarised light.
Molecules which are mirror images of each other
rotate plane polarised light in opposite directions.
Light waves of unpolarised light vibrate in many planes.
Here we see the yellow wave vibrating horizontally, while
the red wave vibrates vertically.
The metal grill acts like a polarising filter. The red wave
is able to pass through the grill, but the yellow wave is
blocked.
If we add a second grill at right angles to the
first, it will block the red wave.
Two polarising filters placed at right angles will
block all light.
Two sheets of polarising film are placed on an overhead
projector stage. The arrows on the sheets show their
orientation.
On the left we see that when the two sheets are
orientated in the same direction light passes through
them, but when the sheets are at right angles (right) the
light is blocked.
Beakers of water and sucrose solution (which is cheaper
than glucose and also optically active) are placed on a
sheet of polarising film sitting on the overhead stage. A
second sheet of polarising film is on top of the beakers,
at right angles to the first.
Water Sucrose
Although the film above the water beaker is dark, light
shines through the film above the sucrose solution.
The sucrose has rotated the light waves sufficiently so
that they are able to pass through the second film.
Water Sucrose
When we rotate the top sheet, the film above the
sucrose solution is now dark, while light passes through
everywhere else.
Different wavelengths (colours) of light are rotated by
different amounts, so that as the polarising film is
rotated by different angles, we see these different
colours.
Remember:
• Optically-active solutions rotate plane-polarised light
• optical isomers rotate plane-polarised light by equal
angles in opposite directions.