Org. Chem I Handout #5 Copyright © Doug E. Frantz The University of Texas at San Antonio STEREOCHEMISTRY There are two major classes of isomers you should be intimately familiar with: 1) Constitutional isomers (or structural isomers) – compounds with the same molecular formula but differ in the way atoms are bonded to each other. • different IUPAC names • same or different functional groups • different physical properties • different chemical properties 2) Stereoisomers – compounds with the same molecular formula AND atom connectivity but differ in the 3D spatial arrangement of atoms. • differ only in the way the atoms are oriented in space • have identical IUPAC names (except for a prefix like cis or trans) • always have the same functional group(s) • differ in configuration (its particular three‐dimensional arrangement) EXAMPLES: CHIRALITY A molecule (or anything for that matter) that is not superimposable on its mirror image is CHIRAL. A molecule that is superimposable on its mirror image is said to be ACHIRAL. The most common and simple example is to look at your hands. Your hands are mirror images of each other. But, when you try to lay them on top of each other (palms up), they do not overlap (non‐superimposable). What about your whole body? Think about it and see if you can rationalize if your entire body is chiral or achiral. both have the same molecular formula and the IUPAC names except for the prefix (cis vs. trans). Thus, they are stereoisomers since the only thing that is different is the spatial arrangement of the ethyl groups. H 3 C CH 3 CH 3 CH 3 CH 3 CH 3 (1R,3S)-1,3-dimethylcyclohexane Chemical Formula: C 8 H 16 Molecular Weight: 112.21 (E)-3,4-dimethylhex-3-ene Chemical Formula: C 8 H 16 Molecular Weight: 112.21 both have the same molecular formula but........... very different IUPAC names. Thus, they are constitutional isomers. H 3 CH 2 C CH 2 CH 3 H 3 CH 2 C CH 2 CH 3 trans-1,2-diethylcyclobutane cis-1,2-diethylcyclobutane Chemical Formula: C 8 H 16 Molecular Weight: 112.21 Chemical Formula: C 8 H 16 Molecular Weight: 112.21