Molecules, which have the same molecular formula, and the same structural formula, and same shape, can still exist in two different forms if they are asymmetric. The simplest form of molecular asymmetry is when four different groups are attached to a carbon atom as shown below. Molecules like this have mirror images which are not identical, and they are described as chiral molecules .

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But nature is chiral at the molecular level in cells However we twist or turn these forms, we cannot get them to overlap each other. The two forms are called enantiomers , and exhibit a form of isomerism known as optical isomerism . When alanine is produced in a laboratory under normal conditions a mixture is obtained, half of which is (S)-alanine and the other (R)-alanine. The synthesis is symmetrical in the sense that it produces equal amounts of both enantiomers. This 50:50 mixture is referred to as a racemic mixture. Asymmetric synthesis, on the other hand, aims to produce a significant excess of one form. Why is this so important? Nature is Chiral



Where chiral compounds occur in living things only one of the two forms is usually present.Examples of such compounds are amino acids, carbohydrates and nucleic acids. Enzymes, nature's catalysts, must also be chiral. They are extremely selective and produce or bind to only one of the enantiomers which fits the active site on an enzyme like a key fits a lock. So the two enantiomers often have totally different effects on cells and it is important to be able to produce pure enantiomers, and not just racemic mixtures. The two molecules can be difficult to distinguish at first glance. But our noses are more sensitive. One form smells of lemons and the other of oranges.

The images below show other examples of asymmetry leading to chirality. The relationship between the two forms is the same as between our two hands, as shown for the amino acids like alanine. Question Can you identify the chiral centre in the thalidomide molecule? Answer and the two enantiomers are In certain cases one form may even be harmful. This was the case with the drug thalidomide, which ws sold in the 1960's to pregnant women. (R) thalidomide helped against nausea, while (S) thalidomide could cause foetal damage. At the time the drug was only available as a racemic mixture.