Organic Chemistry

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Organic Chemistry - Ch 11 559 Daley & DaleyCHOCHOHOHHOHHOHHOHCH 2 OHCH 2 OHErythroseThreoseUse the term erythro when the similar groups are adjacent to eachother on the molecule. When these groups are identical, the moleculeis meso. Then name the molecule with the meso name. Use the termthreo when the similar groups are on opposite sides of the molecule.When these groups are identical, they are (±) or dl molecules. Thenuse (±), or dl. For example, the terms meso and dl are appropriate forsymmetric molecules such as 2,3-dichlorobutane or tartaric acid.COOHCOOHCOOHHOHHOHHOHHOHHOHHOHCOOHCOOHCOOHmeso-Tartaric acid(dl)-Tartaric acidFor molecules such as 2-bromo-3-chlorobutane or 3-chloro-2-butanol,erythro and threo are more appropriate. While the threo, erythro, meso,and dl terms are widely used, none of these terms are a part of IUPACnomenclature rules.CH 3CH 3HClClCH 3HHClClCH 3HHOHHOHHOHHOHCH 3CH 3Erythro 3-chloro-2-butanolCH 3CH 3Threo 3-chloro-2-butanolSolved Exercise 11.3There are two stereocenters in 1,3-dimethylcyclohexane and thus, accordingto the 2 n rule, there are four stereoisomers. Two of these stereoisomers havethe methyl groups cis and in the other two the methyl groups are trans. Whatis the stereochemical relationship between the two chair conformations of cis-1,3-dimethylcyclohexane? What about trans-1,3-dimethylcyclohexane?www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 11 560 Daley & DaleySolutionThe two chair forms of cis-1,3-dimethylcyclohexane are a pair ofnonsuperimposible nonmirror images. Thus, they are diastereomers.The two chair forms of trans-1,3-dimethylcyclohexane are a pair ofnonsuperimposible mirror images. Thus, the two are enantiomers.Exercise 11.8Draw the threo and erythro isomers of 2-bromo-3-chlorobutane.11.7 Resolution of EnantiomersResolving enantiomersis a method ofseparating a pair ofenantiomers.A resolving agent is achiral compound orapparatus used as anaid in separatingenantiomers.Because enantiomers have identical physical and chemicalproperties in a symmetric environment, separating, or resolving,them using normal separation methods such as distillation,chromatography, or crystallization do not work. Resolution ofenantiomers requires the use of a resolving agent. The firstresolution of a pair of enantiomers occurred in 1848, and the resolvingagents were a microscope and a pair of tweezers. After crystallizingsalts of (±) tartaric acid, Louis Pasteur observed through hismicroscope that there were two forms of crystals.COOHCOOHHOHHOHHOHHOHCOOHCOOHwww.ochem4free.com 5 July 2005
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Organic Chemistry

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