6.2.2 (c,d) Chirality

(c) Optical Isomerism (an example of stereoisomerism, in terms of non-superimposable mirror images about a chiral centre)

(see also 4.1.3 c–d)

{Learners should be able to draw 3-D diagrams to illustrate stereoisomerism.}

(d) Identification of chiral centres in a molecule of any organic compound.

Optical isomers are those isomers which rotate plane polarised light in opposite directions - though exactly why or how is not relevent to us.

The simplest optical isomers have a central Carbon atom tetrahedrally bonded to 4 different groups - a Chiral carbon atom.

The easiest way to draw the second optical isomer (or enantiomer) is to imagine a mirror image.

And, in fact, if an examiner asks for a definition of optical isomerism your answer should include the phrase:

"Non-superimpossible mirror-image".

In most chemical reactions, both enantiomers would be made in equal amounts, making a 50:50 mixture which would not affect light at all since the opposite rotations would cancel out.

This is known as a racemic mixture.

Almost all alpha amino acids have a chiral centre since Carbon 2 is bonded to an H atom and an R group as well as an amine and an acid group.