Chapter 51 Stereochemistry: Chapter 5 The Arrangement of Atoms in Space; The Stereochemistry of...
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Chapter 5 1 Stereochemistry: Stereochemistry: Chapter 5 Chapter 5 The Arrangement of Atoms in The Arrangement of Atoms in Space; The Stereochemistry of Space; The Stereochemistry of Addition Reactions Addition Reactions
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Transcript of Chapter 51 Stereochemistry: Chapter 5 The Arrangement of Atoms in Space; The Stereochemistry of...
Chapter 5 1
Stereochemistry:Stereochemistry: Chapter 5Chapter 5
The Arrangement of Atoms in Space; The Arrangement of Atoms in Space;
The Stereochemistry of Addition The Stereochemistry of Addition
ReactionsReactions
Chapter 5 2
Contents of Chapter 5 Conformational and Configurational Isomers Chirality Centers, Enantiomers Optical Rotation, Optical Purity Isomers with More Than One Chirality Center Separation of Enantiomers Reactions of Chiral Compounds Absolute Configurations Stereochemistry of Reactions
Chapter 5 3
Kinds of Isomers
Chapter 5 4
Conformational Isomers
Chapter 5 5
Configurational Isomers: Cis-Trans Diastereomers
Chapter 5 6
Isomers with One Chirality Center
A chirality center arises when four different substituents are bonded to a carbon
Only two isomers are possible, an R isomer and an S isomer.
Chapter 5 7
Chirality
R and S enantiomers are mirror images of each other, just as your right hand is the mirror image of your left hand
Chapter 5 8
Chirality
Any object that has a plane or point of symmetry is achiral (not chiral).
Chapter 5 9
Examples
chiral
CH3CH3H3C CH3
achiral
Chapter 5 10
Mirror Trick
Whenever two structures can be positioned around a symmetry plane if they aren’t identical they’re enantiomers.
Chapter 5 11
Drawing Enantiomers
Fischer Projections
N and S are down into plane, E and W are up out of plane of page or screen.
Chapter 5 12
Naming Enantiomers: The R,S System of Nomenclature
Rank groups by atomic number of the atom bonded to the chirality center. Use the same system that was used for the E and Z isomers of alkenes
Chapter 5 13
2. Orient molecule so that group (or atom) of lowest priority is directed into plane.
3. Draw a curve from group of highest priority through the group of second priority to group of third priority
Naming Enantiomers: The R,S System of Nomenclature
Chapter 5 14
Naming Enantiomers: The R,S System of Nomenclature R (Latin rectus) = right turn S (Latin sinister) = left turn
1
2 3
4
(S)-2-bromobutane
Chapter 5 15
If for any reason you ever wish a group were in a different position simply swap it with another group.
Swap simply reverses the chirality.
R for switched compound implies S for actual compound
Swap Trick
Chapter 5 16
If the low priority group in figure points up rather than down simply draw circular arrow and reverse chirality.
Low Priority Group Up Trick
R for reversed chirality implies S for actual chirality
Chapter 5 17
If low priority group on vertical line (into plane) draw normal rotating arrow.
If low priority group on horizontal line draw rotating arrow and reverse the R or S result.
Rule of thumb: If group 4 is Vertical, Very true. If group 4 is Horizontal, Horribly wrong.
(R)-2-bromobutane
Fischer Tricks
(S)-2-bromobutane
Chapter 5 18
90º rotation reverses all chiralities.
180º rotation maintains all chiralities.
Fischer Tricks
1
2 3
4 5
6 7
8 1
23
45
67
8
180°
everything winds up in opposite pos'n
Chapter 5 19
Can rotate 3 groups either CW or CCW without changing configuration
If 3 grps have same rel CW or CCW sequence C’s same
Multi-Carbon Fischer Trick
H
F
F
Cl
Cl
H
Cl
F
H
F
H
Cl
top carbons have same config
bottom carbons have opposite config's (swap H and Cl)
Chapter 5 20
For front C swap low priority group to back C If swap was necessary curved arrow now gives
reversed chirality; otherwise correct
Newman Projections
H
F
Cl H
F
Cl 1 2
3
4
swap 3 and 4
H
F
Cl H
F
Cl 1 2
3
4
Configuration is R
Chapter 5 21
For back C swap low priority group to front C If swap was necessary curved arrow now gives correct
chirality; otherwise reversed
Newman Projections
H
F
Cl H
F
Cl
1
2
34
swap 3 and 4
H
F
Cl H
F
Cl
1
2
34
Configuration is S
Chapter 5 22
Compounds with More Than One Chirality Center
Two Pairs of Similar Groups
Chapter 5 23
Compounds with More Than One Chirality Center
Three Pairs of Similar Groups
Chapter 5 24
Meso Compounds
Chapter 5 25
Drill on Meso CompoundsProblem 27
Does the following compound have a stereoisomer that is a meso compound?
Plane of symmetry
Meso form possible
Br Br
Chapter 5 26
Topology of Stereoisomers With Two Chiral Centers
Chapter 5 27
R,S System for Isomers with More Than One Chirality Center
Compound is (2S,3R)-3-bromo-2-butanol
H3C
CH3HO
H
H
Br
Chapter 5 28
R,S System for isomers with more than one Chirality Center
Compound is (2S,3R)-3-bromo-2-butanol
CH3
CH3
OHH
BrH
Chapter 5 29
Relative and Absolute Configurations (–) amphetamine is known to have the R- configuration Therefore the (+) form has S configuration
Chapter 5 30
Separation of Enantiomers Racemic lactic acid can be reacted with a
naturally occurring chiral base, such as morphine or strychnine
The product is a pair of diastereomers Properties of diastereomers are sufficiently
different from each other to allow separation After separation, each diastereomer can be
reacted with hydrochloric acid, yielding an optically pure R or S acid
Chapter 5 31
Enantiotopic, Diastereotopic, and Homotopic Hydrogens
Chapter 5 32
Enantiotopic, Diastereotopic, and Homotopic Hydrogens
Chapter 5 33
Regioselective, Stereoselective, and Stereospecific Reactions
A regioselective reaction is one in which multiple constitutional isomers possible, but more of some formed than others.
HBr
Br
Br
+
major product no measurable quantity formed
Chapter 5 34
Regioselective, Stereoselective, and Stereospecific Reactions
A stereoselective reaction can produce multiple stereoisomers theoretically, but more of some produced than others.
Br
Br
+
(2R)-2-bromo-1,1-dimethylcyclohexane+
(2S)-2-bromo-1,1-dimethylcyclohexane
base
(1Z)-3,3-dimethylcyclohexene
no E (trans) isomer is formed
Chapter 5 35
Regioselective, Stereoselective, and Stereospecific Reactions
A stereospecific reaction produces different stereoisomer products from different stereoisomer reactants.
Br2 (2R,3R)-2,3-dibromobutane+
(2S,3S)-2,3-dibromobutane
(no meso isomer formed)
Br2 (2R,3S)-2,3-dibromobutane
meso isomer(no R,R or S,S isomers formed)
Chapter 5 36
Syn Addition
When the two substituents add to the same side
Addition of H2 is a syn addition
Chapter 5 37
Anti Addition
When the two substituents add to opposite sides
Chapter 5 38
Stereochemistry of Addition
Unmodified only works for addition of X2
For addition of XY use to calculate X eclipses Y or X opposite to Y in Fischer projection.
CIS-SYN-ERYTHRO RULE
Chapter 5 39
Stereochemistry of AdditionCis-Syn-Erythro Example
Chapter 5 40
Stereochemistry of Addition
Pro-Fischer Analysis
Chapter 5 41
Stereochemistry of Addition
Pro-Fischer Analysis Example
