organic chemistry for medicine and biology students chem 2311 chapter 5 stereochemistry by prof. dr....
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Organic chemistry for medicine and biology students
Chem 2311Chapter 5
Stereochemistry
By Prof. Dr.Adel M. Awadallah
Islamic University of Gaza
StereochemistryIsomers: different compounds having the same molecular formula
CH3CH2OH CH3OCH3
ClI
Br
H
ICl
Br
HCH3 CH3CH3CH3
CH3
Et
Cl
Cl HH
CH3
Et
Cl
ClH
H
ClH
Cl H
ClH
H Cl
Cl H
ClH
Cl Cl
HH
IsomersDifferent compounds that have the same molecular formula
Constitutional isomers Stereoisomers
Atoms are attached to one another in different ways
Atoms have the same constitution but different arrangement in space
Enantiomers Diasteriomers
Non superimposable mirror- image stereoisomers
Non mirror-image stereoisomers
Conformational Configurational
Interconvertable by rotation
Interconvertable by bond breaking
SR
cis- trans-
cis trans
• A chiral molecule is one that is optically active
• A racemic mixture (racemic modification) is one with equal ratios of enantiomers (50:50 mixture)
Configuration and the R-S Convention
Examples
BrI
Cl
H
BrCl
CH3
H
CH3Et
H
F
CH3
H
OH
O
CH3
CH2CH3
H
CH3CH2
HI
Cl
Br
BrH
CH3
Et
HEt
CH3
F
H
OH
CH3
O
CH3
CH3
CH2OH
CH2CH3CH2
R S R S S
S R R R S
Compounds with more than one stereogenic center; (Diasteriomers)
CH3
Et
H
Cl
Cl
H
CH3
Et
Cl
H
H
Cl
CH3
Et
H
H
Cl
Cl
CH3
Et
Cl
Cl
H
H
1
2
3
(2R,3R) (2S,3S) (2R,3S) (2S,3R)
1 2 3 4
1 and 2 are enantiomers, 3 and 4 are also enantiomers
1 and 3 , 1 and 4, 2 and 3, 2 and 4 are diastereomers
Diastereomers are non mirror image stereoisomers
If a molecule has n different stereogenic centers, it may exist in a maximum of 2n stereisomeric forms. There will be a maximum of 2n / 2pairs of enantiomers.in the above case 2 stereogenic centers, 2 pairs of enantiomers, 4 stereoisomers
CH3
CH3
H
Cl
Cl
H
CH3
CH3
Cl
H
H
Cl
CH3
CH3
H
H
Cl
Cl
CH3
CH3
Cl
Cl
H
H
1
2
3
(2R,3R) (2S,3S) (2R,3S) (2S,3R)
1 2 3 4
1 and 2 are enantiomers, 3 and 4 are the same compound (Meso structure)
1 and 3 , 2 and 3, are diastereomers
Diastereomers are non mirror image stereoisomers
A meso compound: is an achiral ( optically inactive) diastereomer of a compound with stereogenic centers. Its stereogenic centers have opposite configurations.One half of the molecule is the mirror image of the other half
1 pair of enantiomers and a total of 3 isomers
same compound
CH3Cl
Br
H
HCl
CH3
Br
CH3Cl
H
H
BrCH3
CH3H
Cl
Br
HCH3
CH3Cl
H
H
BrCH3
HCH3
Cl
H
BrCH3
CH3Cl
H
H
BrCH3
HCH3
Cl
Br
HCH3
ExampleWhat is the relation between the following compounds
R R
same compound
1
2
3 4
(2S,3S) (2R,3S)
Diastereomers
(2S,3S) (2R,3R)
Enantiomers
(2S,3S) (2S,3S)
Same compound
• Properties of enantiomers and diastereomers• A) Enantiomers• Have the same physical properties except for
direction of rotation of plane polarized light• Have the same chemical properties except for
their reaction with other chiral molecules• (This usually seen in their biological activity)• B) Diastereomers• Have different physical properties• Similar chemical properties
• Chemical reactions of stereoisomers• 1) Generation of a chiral center
CH3
CH2Ch3
HH
CH3
H CH2CH3
CH3
CH2Ch3
HCl
CH3
CH2Ch3
ClHCl2
CH3
Et
BrH
CH3
Et
HBrEt
CH3H
heat
.
planar radicalequal ratios
Cl2 2Cl.
Cl.
-HCl
HBr +
planar carbocation equal ratios
Br -
Reaction away from a chiral center
CH3
CH2Ch3
H COOH
EtOH / acid CH3
CH2Ch3
H COOEt
CH2
CH2Ch3
H Cl
OH
CH2Ch3
H Cl
OH
retention of configuration
retention of configuration
KMnO4
Generation of a second chiral center
CH3
ClH
CH3
CH3
Cl
H
H
Br
CH3
CH3
Cl
Br
H
H
HBr+
Diastereomeric products at different rates and in unequal amounts
Resolution of enantiomers
• Enantiomers + chiral molecules gives diasteriomers which can be sepearated, then we get red of the chiral molecules