chiralityg - Copyright Arizona State University...

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Stereochemistry 1 Copyright, Arizona State University Copyright, Arizona State University Chirality Molecular "Handedness" (more...) • Stereoisomers differ in orientation of atoms in space Example (seen already) Diastereomers C H H C Cl Cl C H Cl C Cl H + trans-diastereomer (no dipole) cis-diastereomer NON-Superimposable!! C H Cl C Cl H C H H C Cl Cl X X NON-Superimposable!! • DIASTEREOMERS have same molecular formula but different physical and chemical properties, they are different molecules. They are also NON-SUPERIMPOSABLE 1 Enantiomers • Important NEW type of stereoisomer is the ENANTIOMER • Enantiomers are "mirror image" stereoisomers • Enantiomers have the property of chirality • Chiral objects have a "handedness" W C X Y Z W C X Y Z mirror image not the same (non-superimposable) W C X Z Y not the same asymmetric (chiral) center (starred) * * * • The molecule on the left is the enantiomer of that on the right, they are enantiomers • Both isomers are chiral (they differ from their mirror image), in this case as a consequence of a "chiral center" • A chiral center is an atom with four different groups attached to it (starred above) • The presence of a chiral center (also known as an asymmetric center) is the most common reason for a molecule to be chiral Example : lactic acid (the chiral/asymmetric centers are starred) H C H 3 C OH CO 2 H H C CH 3 HO HO 2 C * * "R-lactic acid" "S-lactic acid" Example : 4-bromooctane CH 3 CH 2 CH 2 CH 2 –C–CH 2 CH 2 CH 3 H Br * small difference between these groups, still chiral! • switch any two substituents on a chiral center to get the enantiomer F C Br Cl H F C Br Cl H F C Br H Cl = switch enantiomers
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Stereochemistry 2 Copyright, Arizona State University Example : substituted cyclohexane Me Et Et Me * * * * Me Et Et Me * * * * enantiomers conformers conformers Et Me Et Me = = enantiomers 1.1 Specifying Configuration (Chirality); Cahn-Ingold-Prelog Notation • Specifies the "configuration" at each asymmetric or chiral/asymmetric center, equivalent to "cis-" and "trans-" • Same rules used to decide priority of the substituent groups as used iwhen determining Z and E in alkenes 1) Assign priorities to groups attached to chiral center according to atomic # 2) Compare 2nd, 3rd etc. atom from center as necessary, look for first point of difference 3). Multiple bonds "add up" 4) Determine direction of rotation 1 > 2 > 3 looking with #4 group pointing "away" Example H C H 3 C OH CO 2 H H C CH 3 HO HO 2 C * * 4 1 2 3 4 1 2 3 OH CO 2 H H 3 C look towards 4 1 2 3 HO HO 2 C CH 3 1 2 3 clockwise (R)-lactic acid anticlockwise (S)-lactic acid Example H C C C CH 3 C CH 3 H H H CH 3 H this group has priority 1 3 4 clockwise - (R) * this group has priority 2 H C C C CH 3 C CH 3 H H H CH 3 H * C, C, H attached C, C H attached H, H, and C attached (back to the previous carbon, C=C) H, H, H attached C C 2 1 • treat each double bond as an additional single bond to count the 3 atoms that are further connected to each carbon, so in this case the -CH=CH2 "wins" over the CH(CH3)2 v Visualize How to Determine the DIrection of Rotation With Your Hand
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