In chemistry two stereoisomers are said to be enantiomers if they can be superimposed on the mirror image of one other. Stated differently: an enantiomer is a non-superimposable mirror image of itself. A simple analogy would be that your left and right shoes are enantiomers of each other. Two molecules that are made up of the exact same atoms and having exactly the same neighbors and differ only in their spatial orientation are said to be stereoisomers. A test for enantiomers can be stated as: Do the above molecules possess mirror planes of symmetry? That is, is it possible to find a plane that cuts through the molecule such that the two halves are mirror images of each other? It has to bisect all of the chiral centers.
An enantiomer of an optically active isomer rotates plane polarized light in an equal but opposite direction of the original isomer. A solution of equal parts of an optically active isomer and its enantiomer is known as a racemic solution and has a net rotation of plane polarized light of zero. A more indepth explanation of this is in the footnotes for Optical isomerism;
Enantiomers will have the opposite prefixes of each other:
- D- becomes L-
- dextro becomes levo
- (+)- becomes (-)-
- R- becomes S-
Research is expanding into the field of chiral chemistry quite rapidly because, for the most part, only one enantiomer is active in a biological system. This is because most biological reactions are enzymatic and the enzymes can only attach to one of the enantiomers. (The left-shoe stretcher will only fit in the left shoe, not in the right shoe. Enzymes and their targets must fit together.) That is usually not a problem because mother nature only tends to make the one that you need. There are exceptions where both enantiomers are biologically active. One would be (+)-carvone & (-)-carvone; one smells like s/mint and the other like caraway.