318M – Krische, Lecture 10: F - 09/19/08 • We began today’s lecture by reviewing isomerism. Thus far, we have encountered constitutional isomers (same molecular formula, different connectivity), conformational isomers (same molecular formula, same connectivity, different orientation in space, isomer interconversion achieved via rotation about single bonds), and cis-/trans-isomerism (same molecular formula, same connectivity, different orientation in space, isomer interconversion NOT achieved via rotation about single bonds). Cis-/trans-isomerism is part of a larger subset of isomers known as “stereoisomers.” Today, we discuss another type of stereoisomerism: enantiomers. • Any object that is not superimposable upon its mirror image is “chiral,” (e.g. your left and right hands are non-superimposable mirror images and, hence, are chiral). Enantiomers are a pair of non-superimposable mirror image isomers. An enantiomeric pair is composed of two chiral molecules. A 1:1 mixture of enantiomers is termed a “racemic” mixture. While molecular chirality can arise in
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