1. 1 Stereoisomers Review: –Structural Isomers: Compounds that have the same molecular formula, but differ in the structural arrangement of atoms. Examples: 1-butene, 2-methyl-propene –Geometric Isomers: Compounds that have the same molecular formula, the same connectivity between atoms, but differ in the arrangement of groups attached to the double bond. Examples: cis-2-butene, trans-2-butene Stereoisomers: Isomers with the same molecular formula, same connectivity of atoms, but different arrangements of the atoms in 3-D space. –Geometric Isomers are a form of stereoisomer. –Enantiomers: Chiral compounds that are nonsuperimposable mirror images make up a pair of enantiomers. Enantiomers behave the same in almost every aspect of physical and chemical properties. The only physical property that differs for a pair on enantiomers is optical activity. The only difference in chemical properties is in the way they react with other chiral compounds. –Diastereomers: Stereoisomers that are not enantiomers. Diastereomers do not have the same chemical and physical properties. Chirality: Objects that have handedness are said to be chiral. They can not be superimposed on their mirror image. Objects that can be superimposed on their mirror image are said to be achiral (without chirality). If a molecule has an internal symmetry plane, it is achiral. A lack of an internal mirror plane indicates chirality. See the examples below, and determine which are chiral and which are achiral: 2-D:AEFOPQR 3-D:

2. 2 Optical Activity Light travels in waves at all angles. Light can be plane polarized so that it travels only in one plane rather than at all angles. The light will appear the same to you and I. A chiral compound and its nonsuperimposable mirror image make up a pair of enantiomers. Enantiomers are often referred to as optical isomers because they are optically active. An optically active molecule will rotate plane polarized light either to the left (levorotatory) or the right (dextrorotatory). Enantiomers rotate plane polarized light in the exact opposite direction. If a molecule has two or more chiral centers, life gets more complicated due to the presence of meso compounds and diastereomers. In an attempt to make life a little easier, we will always draw these molecules as Fischer Projections.

3. 3 The Significance of Enantiomers Enantiomers react differently with chiral compounds. Life is made up of chiral compounds. Amino acids used for biosynthesis in animals are always of the L conformation. We can obtain energy only from the D forms of carbohydrates. Enantiomers can be the difference between life and death. Enantiomers are important to: Pharmaceutical companies Agriculture Food additive industries

4. 4 Problems Are the following pairs (1-3) Identical, Enantiomers, Diastereomers, Unrelated, and/or Meso 1. 2. 3. 4. Circle all compounds above that will be optically active.