Transcript
Organic Chemistry 4th Edition
Paula Yurkanis Bruice
Irene LeeCase Western Reserve University
Cleveland, OH©2004, Prentice Hall
Chapter 5
Stereochemistry
The Arrangement of Atoms in Space;
The Stereochemistry of Addition Reactions
Isomers
Nonidentical compounds having the same molecular formula
Cis-Trans Isomers
Achiral compounds have superimposable mirror images
Chiral compounds have nonsuperimposable mirror images
Enantiomersnonsuperimposable mirror-image molecules
Drawing Enantiomers
Perspective formula
Fischer projection
A stereocenter (stereogenic center) is an atom at which the interchange of two groups produces a stereoisomer
Naming Enantiomers
Rank the groups (atoms) bonded to the chirality center
The R,S system of nomenclature
Orient the lowest priority (4) away from you
Clockwise = R configuration
Counterclockwise = S configuration
Naming from the Perspective Formula
1
23
4
1. Rank the groups bonded to the asymmetric carbon
2. If the group (or atom) with the lowest priority is bonded by hatched wedge,
3. If necessary, rotate the molecule so that the lowest priority group (or atom) is bonded by a hatched wedge
4. You can draw group 1 to group 2, passing group 4, but never 3
Naming from the Fischer Projection
1. Rank the groups (or atom) that are bonded to the asymmetric carbon and draw an arrow with the highest priority to the lowest priority
Cl
H
CH2CH2CH3CH3CH2 (R)-3-chlorohexane
2. If the lowest priority is on a horizontal bond, the naming is opposite to the direction of the arrow
CH3
CH2CH3
OHH (S)-2-butanol
A Fischer projection can only be rotated 180° in the planeof the paper to yield the same molecule
3. The arrow can go from group 1 to 2, passing group 4, but not group 3
CO2H
OH
CH3H
(S)-lactic acid
Chiral compounds are optically active; they rotate the plane of polarized light.
Clockwise (+) Counterclockwise (-)
Different from R,S configuration
Achiral compounds do not rotate the plane of polarized light. They are optically inactive.
A polarizer measures the degree of optical rotation of a compound The observed rotation ()
T is the temp in °C
is the wavelength
is the measured rotation in degrees
l is the path length in decimeters
c is the concentration in grams per mL
Each optically active compound has a characteristic specificrotation
A racemic mixture, which contains an equal amount of the two enantiomers, is optically inactive
optical purity = observed specific rotation
specific rotation of the pure enantiomer
enantiomeric excess = excess of a single enantiomer
entire mixture
Diastereomers are stereoisomers that are not enantiomers
Isomers with more than one chiral carbon: a maximum of2n stereoisomers can be obtained
CH3CHCHCH3
Cl OH
* *
Identification of Asymmetric Carbons in Cyclic Compounds
H
CH3
H
Br*
*these two groupsare different
H
CH3
H
Br
H
Br
CH3
H
H
CH3
H
Br
CH3
H
H
Br
cis-1-bromo-3-methylcyclohexane trans-1-bromo-3-methylcyclohexane
Meso CompoundsHave two or more asymmetric carbons and a plane of symmetry
They are achiral molecules
As long as any one conformer of a compound has a plane of symmetry, the compound will be achiral
plane ofsymmetry
plane ofsymmetry
The R,S nomenclature of isomers with more than one asymmetric carbon
Reactions of compounds that contain an asymmetric carbon.
CH2CH2CH2Cl
CH3CH2 CH3
H
CH2CH2CH2OH
CH3CH2 CH3
HOH-
No reaction at the asymmetric carbon; both the reagent and theproduct have the same relative configuration.
CH
CH3CH2CH2 CH3
H
CH2 CH2CH3
CH3CH2CH2 CH3
H
H2
Pd/C
If a reaction breaks a bond at the asymmetric carbon, you need toknow the reaction mechanism in order to predict the relative configuration of the product.
Resolution of a Racemic Mixture
(R)-acid (S)-acidenantiomers
(S)-base(R,S)-salt (S,S)-salt
diastereomers
(R,S)-salt (S,S)-salt
HCl HCl
(S)-baseH+
+(R)-acid
(S)-baseH+
+(S)-acid
Discrimination of Enantiomers by Biological Molecules
Terminologies Associated with Stereochemistry
pro-R-hydrogen
pro-S-hydrogen
Enantiotopic hydrogens have the same chemicalreactivity and cannot be distinguished by achiral agents,
but they are not chemically equivalent toward chiral reagents
Diastereotopic hydrogens do not have the same reactivity with achiral reagents
Atoms other than carbon can be asymmetric
N+CH3CH2CH2 CH2CH3
H
CH3
N+CH2CH2CH3CH3CH2
H
CH3
Br- Br-
O
POCH2CH3
H
O
PCH3CH2O OCH3
HCH3O
Amine Inversion
A regioselective reaction: preferential formation of oneconstitutional isomer
A stereoselective reaction: preferential formation of a stereoisomer
A stereospecific reaction: each stereoisomeric reactant produces a different stereoisomeric product or a different set of products
All stereospecific reactions are stereoselectiveNot all stereoselective reactions are stereospecific
Stereochemistry of Electrophilic Addition Reactions of Alkenes
What is the absolute configuration of the product?
Addition reactions that form one asymmetric carbon
CH3CH2C CH2 HBr CH3CH2CHCH2Br
2-methyl-1-butene
CH3
+peroxide
CH3
*
1-bromo-2-methylbutane
Addition reactions that form an additional asymmetric carbon
A carbocation reaction intermediate
Addition reactions that form two asymmetric carbons
Two substituents added to the same side of the double bond: synTwo substituents added to opposite sides of the double bond: anti
C C
CH3CH2 CH2CH3
CH3H3C
+ HCl CH3CH2CH
CH3
CCH2CH3
Cl
CH3
C C
CH3CH2
H3CCH2CH3
CH3
H
C C
C C
CH3CH2
H3CCl
CH2CH3
CH3
H
Cl
CH3CH2
CH2CH3
HCl
H3C
CH3 C C
CH3CH2
CH2CH3
HCH3
H3C
Cl
A radical reaction intermediate
Addition reactions that form two asymmetric carbons
Stereochemistry of Hydrogen Addition (Syn Addition)
Hydrogenation of Cyclic Alkenes
CH(CH3)2
H3C
+ H2Pt/C
H
CH3
H
CH(CH3)2
H3C
H
CH(CH3)2
H
Only cis isomers are obtained with alkenes containing fewer than eight ring atoms
Both cis and trans isomers are possible for rings containing eight or more ring atoms
Stereochemistry of Hydroboration–Oxidation
Addition reactions that form a bromonium ionintermediate (anti addition)
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