17.1.1 Chirality and enantiomers & 17.1.2 Optical activity

optical isomerism

non-superimposable mirror images about a chiral centre

chiral

an atom in a molecule that allows it to exist as non-superimposable forms. it can also refer to the molecule itself

enantiomer

if a molecule has a chiral centre, it can exist as optical isomers and each one is an enantiomer.

optical activity

substance show this if it rotates plane-polarised light

unpolarised light

transverse waves of light oscillating in all directions perpendicular to the direction travel of the wave.

plane-polarised light

monochromatic light that has oscillations in only one plane.

polarimetry

the use of a polarimeter to measure the amount of optical activity of a substance.

polarimeter

apparatus used to measure the angle of rotation caused by a substance.

polariser

material that converts unpolarised light to plane-polarised light.

analyser

a material that allows plane-polarised light to pass through it.

step 1 of polarisation

monochromatic light source passes through a polarising filter.

step 2 of polarisation

the now polarised light passes through a sample tube containing the substance in solution.

step 3 of polarisation

if the substance is optically active (because it contains an enantiomer) then the plane of polarisation will rotate.

step 5 of polarisation

the angle of rotation is measured and quoted as either + or - value.

step 4 of polarisation

the second polariser (analyser) is rotated to a position where the maximum light intensity can be seen.

properties of enantiomers

two enantiomers have identical physical properties, with one exception, they rotate the plane of polarisation of plane-polarised light by equal angles but in opposite directions.

racemic mixture

an equimolar mixture of two enantiomers that has no optical activity.