Stereochem A - STEREOCHEMISTRY AN EXERCISE WITH MOLECULAR...

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1 STEREOCHEMISTRY AN EXERCISE WITH MOLECULAR MODELS Objective: To learn about the three-dimensional aspects of chemical structures and isomerism by using molecular models. Stereochemistry is a very important and reasonably subtle structural aspect of organic chemistry. Stereoisomers are isomers that have the same general connectivity but different configurations. Consider the two stereoisomeric forms of lactic acid shown here. We see that they have the same general connectivity; that is, there are three connected carbon atoms: one carbon is part of a carboxyl group; the next bears an H and an OH; and the third, three hydrogens. H C COOH CH 3 HO H C HOOC CH 3 OH (-)-Lactic acid (+)-Lactic acid mirror plane Careful scrutiny, however, shows that the structures of these two compounds are not the same. If (-)-Lactic acid were rotated 180 o , it would not be superimposable on (+)- Lactic acid, which would be the case if they were identical. This is a characteristic of a molecule that does NOT have a plane of symmetry . A plane of symmetry is an imaginary plane that divides an object or molecule into two equal halves that are mirror image reflections of each other. If an imaginary plane were placed so as to include H-C- COOH in (+)-Lactic acid, for example, then the CH 3 - on one side would not be the same as the -OH on the other. Lactic acid is said to be chiral since the two forms are related to each other like the left and right hands, Your left hand does not have a plane that divides it into two identical mirror image halves. Your left hand is therefore chiral. Because your left hand is chiral, it will not fit into a right-handed glove. An object or molecule that has a plane of symmetry is superimposable on its mirror image and is achiral . A ball, for example, has a plane of symmetry that cuts the ball in half: one side of the plane, the half-sphere is the exact mirror reflection of the half sphere on the other side of the plane. The ball is therefore achiral. The easiest way to detect a chiral molecule is to look for a plane of symmetry. If the molecule does not have a plane of symmetry, then it is chiral, and its mirror image will not be superimposable. Since the chiral molecule and its mirror image are not superimposable, they are not the same compound. The nonsuperimposable mirror images are known as enantiomers . A necessary condition for not having a plane of symmetry around a tetrahedral carbon is to have four different substituents (ligands). The carbon atom is a chiral carbon , a stereocenter , stereogenic center, or a chiral center (these words are used interchangeably). Molecules with ONE
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Stereochem A - STEREOCHEMISTRY AN EXERCISE WITH MOLECULAR...

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