In organic chemistry, with the racemic term - or racemic mixture - is meant a 1: 1 mixture (hence equimolar) of two enantiomers. Enantiomers are defined as mirror images of the same molecule that do not overlap with each other.
Organic molecules that cannot be superimposed on their mirror image are called chiral molecules; exactly as our left hand cannot be superimposed on our right hand (from the Greek cheir "hand", from which the chiral term was born).
Example of the two enantiomers of a raceme
An organic molecule is chiral if - within its structure - it has a tetrahedral atom (usually a carbon atom but can also be a different atom) linked to four different atoms or groups. A carbon atom that links four atoms or groups that are different from each other and lack symmetry elements is called the chiral center or chirality center .
To better understand the concept of chirality, the example of 2-butanol enantiomers is given below:
As you can see, the two molecules are a mirror image of the other. They possess the same atoms - bound in the same way - but oriented differently in space and this makes them not overlapping.
The enantiomers are distinguished from each other based on the absolute configuration of the chiral center. The system still used to assign the absolute configuration to chiral centers is called the Cahn-Ingold-Prelog convention or R, S convention, named after the scientists who designed it in the late 1950s.
Furthermore, enantiomers can be distinguished on the basis of their optical rotation power. In fact, the molecules having chiral centers in their structure have the ability to rotate the plane of polarized light; they are said to be optically active compounds. However, there are also chiral molecules that are unable to rotate polarized light.
If a molecule rotates the plane of light polarized clockwise - from left to right - it is called dextrorotatory or destrorotatory . If, on the other hand, the molecule rotates the light counterclockwise - then from right to left - it is called levorotatory or levorotatory .
A right-handed molecule is usually indicated by putting the sign " + " or the letter " d " before its name, while a levogyrate molecule is indicated by putting the sign " - " or the letter " l " before its name.
For each pair of enantiomers, one is dextrorotatory and the other is levorotatory, but the absolute value of the rotatory optical power is the same. Therefore, a raceme - in which there is an equimolar mixture of enantiomers and contains, therefore, the same number of destrogire and levogire molecules - does not possess any rotatory optical power, and is defined as optically inactive .
