l10 - CH 203 O R G A N I C C H E M I S T R Y I...

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Stereoisomers © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I Stereoisomers Stereoisomers Consider the two structures shown below. H 3 C H H CH 3 H H H 3 C H H H CH 3 H A B Are these structures constitutional isomers of each other? No! They are con- stitutionally identical, that is, Structure A and B are made up atom-for-atom of the same number and kinds of bonds. Yet, Structures A and B are not ex- actly identical: they differ in the relative disposition of the methyl (CH 3 ) groups. The properties of the two compounds are not identical either: Struc- ture A is less stable by 2.5 kJ/mol due to steric strain. Thus, the two structures represent different compounds. If two structures have the same formula and are constitutionally identical, but differ in the arrangement in space of their constituent atoms, then those two structures are stereoisomers of each other. Problem Indicate whether each of the following pairs of structures represent (1) constitutional isomers, or (2) stereoisomers, or (3) the same structure merely drawn in two different ways, or (4) none of the above, that is, they are neither constitutional isomers nor stereoisomers nor identical. (a) H 3 C CH 3 CH 3 H 3 C H 3 C CH 3 (b) CH 3 (c) CH 3 (d) Cl Cl Cl Cl (e) (f)
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Stereoisomers © Bruno I. Rubio 2 (g) CH 3 H 3 C H 3 C H 3 C (h) CH 3 CH 3 (i) Cl CH 3 Cl CH 3 (j) H 3 C Br Br CH 3 H H H H 3 C CH 3 H Br Br (k) OH OH OH OH OH OH (l) O OH HO O OH HO Answer (a) constitutional isomers (b) constitutional isomers (c) constitu- tional isomers (d) same structure. If you flip one structure like a pancake, you get the second structure. (e) constitutional isomers (f) constitutional isomers (g) stereoisomers (h) same structure (i) stereoisomers (j) same structure (k) stereoisomers (l) stereoisomers Enantiomers and diastereomers Stereoisomers are divided into two categories: enantiomers and diastereomers. Two structures are enantiomers of each other if they are non-superimposable mirror images of each other. Two structures are diastereomers of each other if they are stereoisomers that are not enantiomers. Consider the two structures shown below: Cl Cl Cl Cl Cl Cl C D Both are 1,2,3-trichlorocyclohexane, but they don’t look exactly alike. How are they related? Are they (1) enantiomers, or (2) diastereomers, or (3) the same structure merely drawn in two different ways?
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Stereoisomers © Bruno I. Rubio 3 Imagine placing Structure C (called the object in the figure below) in front of a mirror. The chlorine atoms on dashes in Structure C point toward the mirror; in the mirror image of Structure C, these chlorine atoms will also point toward the mirror, but from our vantage point they appear to live on wedges. In like fashion, the chlorine atom that lives on a wedge in Struc- ture C points away from the mirror; in the mirror image of Structure C, this chlorine atom will also point away from the mirror, but from our vantage point it appears to live on a dash. It looks like Structure C and Structure D are mirror images of each other: Cl Cl Cl Cl Cl Cl C D the object the image a mirror
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This note was uploaded on 02/27/2012 for the course CH 203 taught by Professor Rubio during the Fall '07 term at BU.

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l10 - CH 203 O R G A N I C C H E M I S T R Y I...

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