exam queftion exchange Stereochemistry of Cyclic Hydrocarbons Robert R. perkins' University of British Cdumbia Vancouver, BC Canada V6T 1Y6 T h e topic of stereochemistry always poses great difficulties for many students in introductory organic chemistry. problems can he used as a tutorial question The after having introduced the various terms associated with stereochemistry. T h e question requires skills a t the applications level in the Bloom taxonomy. There are seven compounds of formula CsHn t h a t contain a cyclohutane ring. Draw the structures of the seven compounds and use them t o illustrate the following terms: (a) struutural isomers (b) positional isomen tc) itrrwisomere (dl geumetric isomrrs re) manticmers rfr diasterromrrs (g) chiral compound (h) achiral compound (i) meso compound
(i) racemic mixture
Acceptable Solution T h e structures of t h e seven compounds are shown below.
edited . hv: ,JOHN J. ALEXANDER University 01 Cincinnati Cincinnati. Ohio 45221
Positional lsomers Compounds that fall into this category must differ only in the position of a group on the main chain of carbon atoms. Compounds Band C, or C and D, or E and F, ete., will be positional isomers. Stereoisomers If two have the same name at this stage, then they are stereoisomers.Compounds, C, F, and G will be stereo~~~~~~~d~D and E will also be stereoisomers. Geometric Isomers These are stereoisomers that differ in the geometry of groups around a double bond or aring. We use the prefixes cis and trans (or E and Z ) to distinguish them. Compounds C (cis) and F (trans) are geometric isomers,as are D and E ior C and GI, Enantiomers These are stereoisomers that are "on-superimposable mirror images of each other. Compounds F and G represent a pair of enantiomers. Diastereomers These are rtereoisomer~rhar are nor relarrd by mirror image. Compounds C and F are dinsteremnerr.asare Dand F.(or C and G). Geometric isomers are examples of diastereomers Chiral Compound A chiral compound is one that will rotate the plane-polarized light. Structure F is a chiral compound, as is G. Both compounds contain chiral carbon atoms (four different groups attached). Achiral Compound An achiral compound is one that will not rotate the plane of planepolarized light. All carbon atoms in the molecule will he achiral. Compounds A, B, D, and E are all achiral compounds.
E
G
F
Structural lsomers I tell my students to name the compounds without specifyingany stereochemistry, those compounds with different names will be structural isomers. A = ethvlcvclobutane
