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Featured Golden Rules of Chemistry: 3. There are two possible arrangements of four different groups around a tetrahedral atom. The two different arrangements are mirror images of each other, a property referred to as chirality and often compared to handedness. 

1. An enantiomeris any molecule that cannot be superimposed on its mirror image (it does not have a plane or center of symmetry).

3. A meso compound is a molecule with two or more chiral centers yet is not chiral because it contains a symmetry element, usually a plane of symmetry.

4. To decide whether a pair of stereoisomers are enantiomers, diastereomers or even meso, assign R or S to all chiral centers and compare these. R,R and S,S are enantiomers, R,S and S,R are enantiomers, and all other combinations (i.e. R,R and R,S) are diastereomers. Meso compounds are always R,S = S,R for symmetric molecules.

5. Enantiomers have identical properties measured in an achiral way, but diastereomers have different properties measured in any fashion.

6. Samples of enantiomers rotate the plane of plane polarized light to an equal extent, but in opposite directions. Rotation is denoted as either clockwise ("+") or counterclockwise ("-"), which replaced the older designations "D" and "L", respectively.

8. We are chiral because our proteins, carbohydrates, DNA etc. are chiral, so different enantiomers of the same constitutional isomer may taste/smell differently to us. Also, for most chiral drugs, only one enantiomer usually has the desired effect.

HOMEWORK:

Read: There is no reading assignment because of the exam. This text is part of the Longhorn Textbook access program.

There is no daily quiz or homework because of the exam.