6.2.2 (c,d) Chirality
Syllabus
(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.
What does this mean?
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.
The exception to this is Glycine.
But this is only because the R group in Glycine is also a Hydrogen atom
Not that it is perfectly possible to have multiple chiral centres in a molecule and that chiral centres are generally indicated with a *.
With two chiral centres there will be 4 optical isomers.
We don't have to name optical isomers using any of the many notations:R/S, D/L, +/- etc
You're often asked to spot chiral centres, rarely more than 4 or 5.
So, you need to be able to spot what does and doesn't count as chiral, even from a skeletal formula.
It may help to draw in the H atoms!
Anything with more than 1 H atom can't be chiral.
In this molecule, all the C atoms marked (a) only show two bonds, meaning that there are two H atoms bonded.
They can't be chiral.
And all the C atoms marked (b) contain a C=C or C=O double bond.
They can't be chiral because the C atom can't be bonded to 4 different groups/atoms.
Every other C atom is chiral because they are bonded to 4 different groups/atoms.
No one will ever ask you how many optical isomers a molecule with 6 chiral centres has.
(it's 2 to the power 6 = 64)
And don't assume a big molecule must have lots of chiral centres.
This molecule has only one!