Optical isomerism 1.

Central chirality – Axial chirality

Carbon atoms attached to four different atoms (or groups of atoms, ligands, substituents) possess central chirality and are named asymmetric or chiral carbons. These molecules have non-superimposable mirror images.

Try to rotate the two molecules (by pressing and dragging the left mouse button or by touching the image) until the two images look like mirror images. Next, try to align the molecules until they look exactly the same. Can you do it as well?


Answer

Using a real molecule (3-chloropentane) determine whether the two molecules are identical and/or mirror images:


Answer

Scrutinize now the 2-chloropentane molecule. This molecule does not have a plane of symmetry. Are the molecules the same or not?


Answer

A pair of non-superimposable mirror image mole­cules are called enantiomers. The 1:1 mixture of the two enantiomers is known as a racemic mixture or racemate.
The isomers of chiral compounds differ in their reactivity toward the isomers of another chiral compound, in the same way as a right and left foot toward a right shoe.

Excersizes:

How many asymmetric centres are present in the following structures?

 1
 2
 3

 1
 2
 3

 2
 3
 4


Estrone


 3
 4
 5

Paclitaxel

  9
 10
 11

Obviously, all tetravalent elements with tetrahedral geometry behave similarly and could be chiral. In case of trigonal bipyramidal geometry chirality is theoretically possible, for example amines (the lone electron pair takes the role of the "fourth valence"). These compounds usually does not show optical activity because of the quick interconversion. The sulfoxides are already more stable and their isomers can often be separated, though: