Enantiomer

In chemistry two stereochemical isomers are said to be enantiomers if they can be superimposed on the mirror image of the other. Stated differently: an enantiomer is a non-superimposable mirror image of itself. 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 as taken from http://www.chemistry.mcmaster.ca/berti/3f03/special_stereochemistry.pdf: 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;

That footnote also carries indepth information on the proper prefixes for naming an optically active isomer. Enantiomers will have the opposite prefixes of each other. D-... becomes L-..., dextro becomes levo, (+)... becomes (-)..., R-... becomes S-....This notation is used to denote molecules with chiral[?] centers. If the mirror image can be based around a carbon-carbon double bond, with two different ligands on each of the two carbon atoms making up the double bond then it is denoted as either a cis- or a trans- bond.

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. 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 spearmint and the other like caraway.

If you have ever read the phrase partially hydrogenated vegetable oil then you know of a health problem with enantiomers that comes from industry. Applying vegetable oil to toast is a little too messy for most people; they would prefer to have a thicker spread that they can apply to their bread. When you take vegetable oil and add hydrogen in the presence of a lead (not used for other health reasons), platinum or paladium catalyst many of the carbon-carbon double bonds are broken and hydrogen attachs to the carbon atom at each side of the double. This tends to clutter up the molecule and makes it more difficult for the various chains in the molecule to move about freely. This additional clutter translates to viscosity at the macro scale. Although this reaction runs close to completion a small part of it runs backward and creates double bonds. Perhaps you remember the news reports a few years ago on the health problem associated with trans-fatty acids. This is because mother nature only makes fatty acids in which the carbon-carbon double bond is of the cis- variety. As a result our bodies have only figured out how to metabolize the cis- variety of the double bond. When industry makes margarin out of oil, I mean, partially hydrogenates the oil they use a catalytic reaction as opposed to the enzymatic reaction Mother Nature uses when making them. When a double bond is created in a catalytic process there is a 50-50 chance of either trans- or cis-. The ratio is actually biased to favor the trans- variety because it has a slightly lower energy state than the cis- variety due to the overall electrical charge not being concentrated on the top or bottom half of the double bond.

The above comes from my Organic Chemistry class notes (exceptionally well taught by Dr. Richard Jagger aka Jag) and as such are copyright by me (Tres) and now licensed under the GNU FDL. Additional information may be found at:

Stereochemistry http://www.chemistry.mcmaster.ca/berti/3f03/special_stereochemistry.pdf http://www.chem.umd.edu/courses/chem233mazz/chapternotes/chapter9notes.pdf