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An enantiomer is a molecule that is a non-superimposable mirror image of another molecule. Similar to a person’s right and left hand, the two molecules look alike but differ in orientation. Enantiomers are isomers, which means that they contain the same chemical formula but a different structure. Within that category there are also stereoisomers, which means that the bonds between the atoms in each molecule are the same but the three-dimensional arrangement of the atoms is different.
One of the features that defines an enantiomer is the fact that it can rotate plane-polarized light. When light is plane-polarized, its waves are oriented in one plane which then passes through a mixture of the enantiomeric compound. While one enantiomer will rotate the light waves that pass through in one direction, the other enantiomer will rotate it in the opposite direction by the exact same magnitude. This is due to the physical structure of the molecules and their specific orientation.
Enantiomers can be classified by either the direction of light rotation or the physical orientation of the atoms in each molecule. There is no ultimate relationship between the two naming systems, but they are used depending on the context of the classification. Compounds are said to be either d- or l- compounds, or (+) or (-) compounds, depending on the direction they rotate plane-polarized light. The letters correspond to the words dextrorotary and levorotary, which mean clockwise and counterclockwise, respectively.
The second method of classification for an enantiomer depends on the structural orientation of the atoms around the central atom, where the bonding difference occurs. This atom is frequently a carbon atom, which is known as the chiral carbon. When categorizing with this method, the molecule is orientated such that the smallest atom is placed away from the viewer, while the other three atoms are oriented in a triangle. The direction is determined starting with the highest weight atom and moving around the triangle in order of decreasing weight. Clockwise directions correspond to R- enantiomeric compounds, while counterclockwise directions refer to S- compounds.
Enantiomeric compounds will respond differently in some chemical reactions, especially when mixed with other enantiomers. Various enzymes and signaling molecules occur in one enantiomeric form, and the other enantiomer will not elicit the same effects. Enantiomers are sometimes formed together in what is known as a racemic mixture, in which there will be no rotation of plane-polarized light becaise each compound will cancel out the other.