Drawing Stereo Chemical Structures
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Drawing Stereochemical Structures Newman Projections, Fischer Projections and Sawhorse Structures 1. How do we determine absolute configuration from a Newman projection? Consider 2-bromo-3-methylpentane, a molecule with two chiral centers. Et H CH 3 CH 3 Br H rotate projection slightly to give a sawhorse projection... CH 3 Br H Et H CH 3 flatten the sawhorse to give a 3-D structure ... H 3 C Br H Et H CH 3 ... add wedges and dashes (S,S) The IUPAC name for this isomer is (2S, 3S)-2-bromo-3-methylpentane. There are 3 more stereoisomers; what if you were also given this one? Et H CH 3 CH 3 H Br Front carbon is inverted from the 1st Newman structure, back carbon is the same; must be (R, S) This stereoisomer is (2R, 3S)-2-bromo-3-methylpentane. 2. How do we convert a 3-D structure into a Fischer projection? Using the (2S, 3S)-isomer from above, since we already know the stereochemical assignments, CH 3 Et Draw the pentane backbone vertically. CH 3 Br H Et Add the other 2 groups to C-2 C-2 is (R). Switch H and Br to give (S). CH 3 H Br Et Assign C-3 to give (S) CH 3 H B Et CH 3 H r Remember, with Fischer drawings, if H is Horizontal, the assignment is Horribly wrong. This is because horizontal bonds are understood to be coming out of the page in a Fischer drawing, so when we assign priorities to the 4 groups, the actual configuration is opposite since H (lowest priority) is not directed into the page. 3. How do we convert a Fischer drawing into a Newman projection? Using the (2S, 3S)-isomer once again: CH 3 H Br Et CH 3 H CH 3 H Br CH 3 H Et CH 3 H Br Et H CH 3 Tilt Fischer drawing to produce a sawhorse projection Remember, Fischer drawing is eclipsed. Rotate around C2-C3 bond to get staggered conformation Rotate sawhorse until C2 and C3 are eclipsed Et H CH 3 CH 3 Br H
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Transcript of Drawing Stereo Chemical Structures
Drawing Stereochemical Structures Newman Projections, Fischer Projections and Sawhorse Structures 1. How do we determine absolute configuration from a Newman projection?
Consider 2-bromo-3-methylpentane, a molecule with two chiral centers.
Et
H CH3
CH3
BrH
rotateprojectionslightly togive asawhorseprojection...
CH3
BrHEt
H CH3 flatten thesawhorse togive a 3-Dstructure ...
H3C
BrH Et
H CH3
... add wedgesand dashes
(S,S)
The IUPAC name for this isomer is (2S, 3S)-2-bromo-3-methylpentane.
There are 3 more stereoisomers; what if you were also given this one?
Et
H CH3
CH3
HBr
Front carbon is inverted from the 1st Newman structure, back carbon is the same; must be (R, S)
This stereoisomer is (2R, 3S)-2-bromo-3-methylpentane.
2. How do we convert a 3-D structure into a Fischer projection?
Using the (2S, 3S)-isomer from above, since we already know the stereochemical assignments,
CH3
Et
Draw the pentane backbone vertically.
CH3Br H
Et
Add the other 2 groups to C-2
C-2 is (R).Switch H and Br to give (S).
CH3H Br
Et
AssignC-3 togive (S)
CH3H B
EtCH3 H
r
Remember, with Fischer drawings, if H is Horizontal, the assignment is Horribly wrong. This is because horizontal bonds are understood to be coming out of the page in a Fischer drawing, so when we assign priorities to the 4 groups, the actual configuration is opposite since H (lowest priority) is not directed into the page. 3. How do we convert a Fischer drawing into a Newman projection?
Using the (2S, 3S)-isomer once again:
CH3H Br
EtCH3 H
CH3
H BrCH3 H
Et CH3
H Br Et
H CH3Tilt Fischer drawing to produce a sawhorse projection
Remember, Fischer drawing is eclipsed. Rotate around C2-C3 bond to get staggered conformation
Rotate sawhorse until C2 and C3 are eclipsed Et
H CH3
CH3
BrH
