Are your hands identical? To all intents and purposes you would probably reply ‘yes’. But are they? Imagine you were describing your hand to an alien (!) – you would say there are four fingers, one thumb, 5 nails, etc and that you have two of these items. You would probably forget to say that they were mirror images – it’s obvious isn’t it, you see them everyday but you would probably overlook this fact. It would be feasible that the alien may imagine you therefore to have two left hands or two right hands?!
So what? Well Carbon, because of its four covalent bonds and tetrahedral arrangement, can form ‘left’ and ‘right’ handed molecules.
If the carbon atom is joined to four different substituents a left or right handed molecule can form (we call it a ‘D’ or ‘L’ optical isomer).
Does this matter? Well, many biological systems (enzymes in particular) will only work with one of the isomers. The enzyme may not work or it may cause unwanted biological process to occur (Thalidomide).
Drug manufacturing companies spend considerable amounts of time, effort and money ensuring that their processes only produce on of the desired optical isomer’s).