chirality enantiomers: non superimposible mirror images Me Me
Br Br H Et H Et formalism: wedge (forward) “stereogenic center” dashed (back)
Not all mirror images are chiral. These are the same (not chiral) enantiomers: non superimposible mirror images Me Me Me Me Et Et H Br Br Et H Et H Et H Et
How to tell if molecules are chiral? Clue: many (but not all*) chiral carbons have 4 different things attached
chiral Me OH R1 Br Cl Et R3 R4 N R2
achiral Et R1 OH R3 R1 R2 NH2 enantiomers have same mp, bp, NMR, IR, etc
* exceptions to be discussed When plane-polarized light interacts with a chiral molecule, the plane of polarization of the light is rotated to the left or right. This effect is called optical rotation and the molecule is said to be optically active. Optical activity is measured using a polarimeter. This device contains a light source, a polarizer to produce the plane-polarized light, a sample cell, and an analyzer to determine the amount of rotation.
Cahn Ingold Prelog rules 4) if atoms are equal, go to first point of difference prioritize by: Higher priority: this carbon is connected to Br,C,H,H Br 1) Highest atomic # a c D 2) then by isotope atomic weight H b d this carbon 3) then by substituents, one atom at a time is connected to C,C,C,H this carbon is connected to C,C,H,H Br b CH b 3 c D c D CH H a H 3 d Higher priority: this carbon d a is connected to C,C,C,H
this carbon Cl is connected to C,C,H,H
b Cl c D H a Higher priority: this carbon d is connected to Cl, C, H, H 5) multiple bonds View the molecule with the lowest priority group in the back
c b ~ a H C C C a clockwise 3 H3C H a H R d H H c c b (Latin, rectus) b d
c b ~ opposite enantiomer: H3C H C C 3 C H a H d C C b d counterclockwise S c a c a (Latin, sinister-left) b O O O O c a ~ H C 3 O H3C O H b H O d H H
Chiral molecules are not necessarily optically active
c c CH3 CH3 CH3 b b Br H H a Br d C3H7 Bra
R same rules for alkenes = 0
higher priority higher priority higher priority
I (E) Br CH3 Br (Z)
Pile of Latvian Mittens (50% Left, 50% Right) higher priority
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See here: www.rigasummit.lv/en/?id=newsin&nid=115 What happens if an enantiomer interacts with another Racemates are not optically active element of chirality? L + R L + L
CH3
H Br CH3 CH3
Br H = 0 Br H CH3
H Br CH3 CH3 CH 3 H H Br Br Br H diastereomers CH3
H Br
What happens if an enantiomer interacts with another diastereomeric pairs up the nose element of chirality?
C3H7 Cl C3H7 Cl
C2H5 F C2H5 Br Br H3C H3C F R R R S
diastereomers O O have different mp, bp, NMR, IR, etc
R-carvone S-carvone spearment caraway diastereomers meso compounds
Meso compounds contain an internal mirror plane (or center of symmetry) which divides the two easy to see in ring compounds halves of the molecule which are mirror images of each other. NOT OPTICALLY ACTIVE! R R Br Br
I R S I
easy to see in acyclic compounds if we are consistent in our drawings
R R Br Br
I R S I Vitruvian man = meso man. He has symmetrically substituted elements of chirality, equal and opposite
meso compounds meso compounds
recognizing planes and centers of symmetry easy to see in ring compounds R S Me Me A A A B C C C C S Me Me R B B B A
easy to see in acyclic compounds if we are consistent in our drawings plane of symmetry center of symmetry R S Me A A A B Me C C C C Me Me B B B A Rs R R S R S
in general, look for R & S stereocenters in the same molecule attached to exactly the same thing CH3 Br (R) Cl H