Properties of optically active compounds

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. Optical activity

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.

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. Optical isomers

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.

When we rotate the top sheet, the film above the sucrose solution is now dark, while light passes through everywhere else. Water Sucrose

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.