Ch05Su08 - Stereochemistry This is study of the 3...

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Stereochemistry This is study of the 3 dimensional arrangement in space of molecules. In organic chemistry, subtle differences in spatial arrangements can give rise to prominent effects. E.g. the isomers of butenoic acid: The cis isomer (maleic acid) is toxic, whereas the trans isomer (fumaric acid) is an essential metabolite for plants and animals. Chirality Chirality means “handedness”. Every object has a mirror image, but if a molecule’s mirror image is different from the molecule, it is said to be a chiral molecule. Chiral objects include: hands, feet, gloves, screws, cork screws Achiral objects have mirror images that are identical to the object. Chirality in Organic Molecules If a mirror image of a molecule can be placed on top of the original, and the 3 dimensional arrangement of every atom is the same, then the two molecules are superimposable , and the molecule is achiral . If a molecule has a non superimposable mirror image, it is chiral . 619d512ab328c97f6a84dcab791477d523239340.doc Page1 CO 2 H H H HO 2 C CO 2 H H HO 2 C H trans isomer cis isomer
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Consider stereoisomers of 1,2-dichlorocyclopentane: The cis isomer has a superimposable mirror image – achiral. The trans isomer is non-superimposable on its mirror image – chiral. Such pairs of non superimposable mirror images are called enantiomers . Every chiral compound has enantiomers. Chiral compounds do not have enantiomers. Chiral Carbon Atoms Rings are not essential for chirality, consider 2-bromobutane: 619d512ab328c97f6a84dcab791477d523239340.doc Page2 H Cl H Cl H Cl H Cl cis isomer is achiral Cl H H Cl H Cl Cl H trans isomer is chiral CH 3 CH 2 CH 3 Br H CH 3 H 3 CH 2 C Br H
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2-Bromobutane cannot be superimposed on its mirror image – it is chiral, and it exists in two enantiomeric forms. Any carbon bound to 4 different groups will be a chiral center. Such a carbon is called a chiral carbon atom (asymmetric carbon atom or stereocenter), and is usually denoted with an asterix (*). The absolute law is whether or not a molecule’s mirror image is identical to the original, but a few quick rules are: 1) if the molecule has no chiral carbon, it is usually achiral 2) if the molecule has just one chiral carbon, it is usually chiral 3) if it has 2 or more chiral carbons, it may or may not be chiral. (There are exceptions…) Mirror Planes of Symmetry If a molecule possesses an internal mirror plane of symmetry ( σ ), then it cannot be chiral. (Note that the absence of a mirror plane does not imply chirality). 619d512ab328c97f6a84dcab791477d523239340.doc Page3 A D C B A D C B H Cl H Cl σ
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(R) and (S) Assignments To distinguish between the two enantiomers, one is called the (S) enantiomer, and the other the (R) enantiomer. The (R) and (S) assignments are designated via the Cahn-Ingold-Prelog Convention. Each chiral center is designated either (R) or (S).
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