Stereo Chemistry Introduction Ws10-11 d6k3c
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Chirality
Chiral objects don’t have a plane of symmetry.
Problem 2.2: chiral objects • screw, scissors • glove, shoe, jacket • your hand, foot, ear, nose, yourself
© Prentice Hall 2001
Objects with a plane of symmetry are achiral.
achiral (non chiral) objects nail, knife sock, pullover
Chapter 4
1
Introduction to Stereochemistry
Arrangement of Atoms in Space
© Prentice Hall 2001
Chapter 4
2
Contents
Stereoisomers: Conformation and Configuration
Chirality Centers and Enantiomers
Optical Rotation and Optical Purity
Separation of Racemates into Enantiomers
(lab
experiment)
© Prentice Hall 2001
Chapter 4
3
Kinds of Isomers
© Prentice Hall 2001
Chapter 4
4
Conformation Rotation of C-C Single Bonds
Different spatial arrangements of atoms that result from rotation about carbon-carbon single bonds are known as conformations.
Different conformations also are called conformational isomers or conformers:
© Prentice Hall 2001
Chapter 4
5
Conformation Rotation of C-C Single Bonds
A convenient way to describe conformation isomers is to look at the molecule along the axis of the bond of interest.
A Newman projection is a graphical representation of such a view:
© Prentice Hall 2001
Chapter 4
6
Conformations of Alkanes: Rotation of C-C Single Bonds
When ethane molecules rotate about the carbon-carbon single bond various isomers result, two of them are: staggered or anti conformation
© Prentice Hall 2001
eclipsed or syn conformation.
Chapter 4
7
Conformations of Alkanes: Rotation of C-C Single Bonds
© Prentice Hall 2001
Chapter 4
8
Conformations of Alkanes: Rotation of C-C Single Bonds
The extra energy of the eclipsed conformation is called torsional strain:
© Prentice Hall 2001
Chapter 4
9
Conformations of Butane: Rotation of C2-C3 Single Bond
© Prentice Hall 2001
Chapter 4
10
Conformations of Butane: Rotation of C2-C3 Single Bond
© Prentice Hall 2001
Chapter 4
11
Conformational Isomers Open chain and cyclic alkanes
© Prentice Hall 2001
Chapter 4
12
Conformational Isomers Amine Inversion Rotation of a free electron pair:
if R1 ≠ R2 ≠ R3 chiral Nitrogen
© Prentice Hall 2001
Chapter 4
13
Configuration Isomers: geometrical cis-trans and E-/Z-isomers Nomenclature Rules CIP (Cahn-Ingold-Prelog): - ranking all substituents according to atomic number - in case of identical atoms go to the next / adjacent atom
© Prentice Hall 2001
Chapter 4
14
Configuration Isomers: Isomers with One Chirality Center
A chirality center arises when four different substituents are bonded to a carbon.
© Prentice Hall 2001
Chapter 4
15
Chirality
Two enantiomers are nonsuperimposable mirror images of each other (as right and left hand).
© Prentice Hall 2001
Chapter 4
16
Chirality
Chiral objects don’t have a plane of symmetry.
© Prentice Hall 2001
Objects with a plane of symmetry are achiral.
Chapter 4
17
Drawing Enantiomers Perspective Formulas:
© Prentice Hall 2001
Chapter 4
18
Drawing Enantiomers Fischer Projections: vertical bonds horizontal bonds
© Prentice Hall 2001
substituents directed away from the observer substituents dircted toward the observer
Chapter 4
19
Naming Enantiomers: The R,S System 1.
CIP-rules (Cahn-Ingold-Prelog): ranking according to atomic number -
for enantiomers and E/Z-alkene isomers
© Prentice Hall 2001
Chapter 4
20
Naming Enantiomers: The R,S System 2.
Orient the molecule so that the group (or atom) of lowest priority is directed away from you. Draw a curve from group of highest priority to the group of second priority.
© Prentice Hall 2001
Chapter 4
21
Naming Enantiomers: The R,S System
enantiomers can be named from perspective formulas
substituent of lowest priority directed away (behind the plane)
1.
Rank the groups according to atomic number:
1 4 2
© Prentice Hall 2001
Chapter 4
3 22
Naming Enantiomers: The R,S System 2.
Group of lowest priority directed away (hatched wedge), arrow drawn according to decreasing priority:
counterclockwise means S
1 4 2
3
(S)-2-bromobutane © Prentice Hall 2001
Chapter 4
23
Naming Enantiomers: The R,S System 3.
If group of lowest priority is not bonded by a hatched wedge, switch a pair of groups so that the group of lowest priority is bonded by the hatched wedge:
Note: R for switched compound = S for actual compound.
© Prentice Hall 2001
Chapter 4
24
Naming Enantiomers: The R,S System 4.
In drawing arrow from 1 to 2, you can draw past the group of lowest priority (4), but never draw past the group of priority 3:
© Prentice Hall 2001
Chapter 4
25
Optical Rotation
[α ]
Τ λ
=
α lxc
T = temperature [°C] λ = wavelength [nm] α = measured rotation [°] l = path length [dm] c = concentration [g/mL]
[ α ] Tλ = specific rotation © Prentice Hall 2001
Chapter 4
26
Optical Rotation and Absolute Configuration In general there is no relationship between the R or S configuration of an enantiomer and the direction it rotates polarized light. This must be determined by experiment: CH3
OH
CH3 H COOH
OH
(S)-(+)-lactic acid
H COO-Na+
(S)-(–)-sodium lactate
A mixture of equal amounts of two enantiomers is called a racemic mixture. © Prentice Hall 2001
Chapter 4
27
Optical Purity observed specific rotation optical purity = specific rotation of the pure enantiomer For example, if a sample of 2-bromobutane has an observed specific rotation of +9.2 °, and the specific rotation of pure (S)-(+) enantiomer is +23.1 °, then:
+ 9.2 o optical purity = = 0.40 = 40% o + 23.1 © Prentice Hall 2001
Chapter 4
28
Separation of Enantiomers
Racemic mixtures can be separated by naturally occurring chiral compounds, e.g. morphine or strychnine.
The product of the reaction is a pair of diastereomers (nonsuperimposable, but no mirror images).
Properties of diastereomers are sufficiently different from each other to allow separation.
After separation, each diastereomer can be reacted, e.g. with an acid, yielding optically pure R or S compounds.
© Prentice Hall 2001
Chapter 4
29
© Prentice Hall 2001
Chapter 4
30
Naming Enantiomers: The R,S System
1.
Analogue rules for nomenclature of Fischer Projections: If the group of lowest priority is on a vertical line, just draw the curve according to decreasing priority. Clockwise - configuration is R. Counterclockwise - configuration is S.
(S)-2-bromobutane © Prentice Hall 2001
Chapter 4
31
Naming Enantiomers: The R,S System 2.
If the group of lowest priority is on a horizontal line, follow step 1, but reverse the R & S. Rule of thumb: If lowest priority is on a Vertical bond, configuration is Very true. If lowest priority is on a Horizontal bond, configuration is Horribly wrong:
(R)-2-bromobutane © Prentice Hall 2001
Chapter 4
32
Naming Enantiomers: The R,S System
Working with Fischer Projections, a rotation of 90° or a horizontal flip will switch chirality. Rotation of a Fischer projection by 180° in the plane of the paper does not switch the chirality.
(R)-2-bromobutane (S)-2bromobutane
(R)-2-bromobutane (S)-2bromobutane
(R)-2-bromobutane (R)-2bromobutane © Prentice Hall 2001
Chapter 4
33
R and S configuration Indicate whether the following structure has R or S configuration: 4 H
Cl H3CH2CH2C 2
CH3 3 Cl 1
R © Prentice Hall 2001
Chapter 4
34
R and S configuration Indicate whether the structure has R or S configuration: 2 CH(CH3)2
switch CH3
CH(CH3)2
and CH3CH2 C
4
CH3
CH2CH3 CH2Br 1
3
CH3CH2
C
CH3 CH2Br
R © Prentice Hall 2001
Chapter 4
35
Chiral objects don’t have a plane of symmetry.
Problem 2.2: chiral objects • screw, scissors • glove, shoe, jacket • your hand, foot, ear, nose, yourself
© Prentice Hall 2001
Objects with a plane of symmetry are achiral.
achiral (non chiral) objects nail, knife sock, pullover
Chapter 4
1
Introduction to Stereochemistry
Arrangement of Atoms in Space
© Prentice Hall 2001
Chapter 4
2
Contents
Stereoisomers: Conformation and Configuration
Chirality Centers and Enantiomers
Optical Rotation and Optical Purity
Separation of Racemates into Enantiomers
(lab
experiment)
© Prentice Hall 2001
Chapter 4
3
Kinds of Isomers
© Prentice Hall 2001
Chapter 4
4
Conformation Rotation of C-C Single Bonds
Different spatial arrangements of atoms that result from rotation about carbon-carbon single bonds are known as conformations.
Different conformations also are called conformational isomers or conformers:
© Prentice Hall 2001
Chapter 4
5
Conformation Rotation of C-C Single Bonds
A convenient way to describe conformation isomers is to look at the molecule along the axis of the bond of interest.
A Newman projection is a graphical representation of such a view:
© Prentice Hall 2001
Chapter 4
6
Conformations of Alkanes: Rotation of C-C Single Bonds
When ethane molecules rotate about the carbon-carbon single bond various isomers result, two of them are: staggered or anti conformation
© Prentice Hall 2001
eclipsed or syn conformation.
Chapter 4
7
Conformations of Alkanes: Rotation of C-C Single Bonds
© Prentice Hall 2001
Chapter 4
8
Conformations of Alkanes: Rotation of C-C Single Bonds
The extra energy of the eclipsed conformation is called torsional strain:
© Prentice Hall 2001
Chapter 4
9
Conformations of Butane: Rotation of C2-C3 Single Bond
© Prentice Hall 2001
Chapter 4
10
Conformations of Butane: Rotation of C2-C3 Single Bond
© Prentice Hall 2001
Chapter 4
11
Conformational Isomers Open chain and cyclic alkanes
© Prentice Hall 2001
Chapter 4
12
Conformational Isomers Amine Inversion Rotation of a free electron pair:
if R1 ≠ R2 ≠ R3 chiral Nitrogen
© Prentice Hall 2001
Chapter 4
13
Configuration Isomers: geometrical cis-trans and E-/Z-isomers Nomenclature Rules CIP (Cahn-Ingold-Prelog): - ranking all substituents according to atomic number - in case of identical atoms go to the next / adjacent atom
© Prentice Hall 2001
Chapter 4
14
Configuration Isomers: Isomers with One Chirality Center
A chirality center arises when four different substituents are bonded to a carbon.
© Prentice Hall 2001
Chapter 4
15
Chirality
Two enantiomers are nonsuperimposable mirror images of each other (as right and left hand).
© Prentice Hall 2001
Chapter 4
16
Chirality
Chiral objects don’t have a plane of symmetry.
© Prentice Hall 2001
Objects with a plane of symmetry are achiral.
Chapter 4
17
Drawing Enantiomers Perspective Formulas:
© Prentice Hall 2001
Chapter 4
18
Drawing Enantiomers Fischer Projections: vertical bonds horizontal bonds
© Prentice Hall 2001
substituents directed away from the observer substituents dircted toward the observer
Chapter 4
19
Naming Enantiomers: The R,S System 1.
CIP-rules (Cahn-Ingold-Prelog): ranking according to atomic number -
for enantiomers and E/Z-alkene isomers
© Prentice Hall 2001
Chapter 4
20
Naming Enantiomers: The R,S System 2.
Orient the molecule so that the group (or atom) of lowest priority is directed away from you. Draw a curve from group of highest priority to the group of second priority.
© Prentice Hall 2001
Chapter 4
21
Naming Enantiomers: The R,S System
enantiomers can be named from perspective formulas
substituent of lowest priority directed away (behind the plane)
1.
Rank the groups according to atomic number:
1 4 2
© Prentice Hall 2001
Chapter 4
3 22
Naming Enantiomers: The R,S System 2.
Group of lowest priority directed away (hatched wedge), arrow drawn according to decreasing priority:
counterclockwise means S
1 4 2
3
(S)-2-bromobutane © Prentice Hall 2001
Chapter 4
23
Naming Enantiomers: The R,S System 3.
If group of lowest priority is not bonded by a hatched wedge, switch a pair of groups so that the group of lowest priority is bonded by the hatched wedge:
Note: R for switched compound = S for actual compound.
© Prentice Hall 2001
Chapter 4
24
Naming Enantiomers: The R,S System 4.
In drawing arrow from 1 to 2, you can draw past the group of lowest priority (4), but never draw past the group of priority 3:
© Prentice Hall 2001
Chapter 4
25
Optical Rotation
[α ]
Τ λ
=
α lxc
T = temperature [°C] λ = wavelength [nm] α = measured rotation [°] l = path length [dm] c = concentration [g/mL]
[ α ] Tλ = specific rotation © Prentice Hall 2001
Chapter 4
26
Optical Rotation and Absolute Configuration In general there is no relationship between the R or S configuration of an enantiomer and the direction it rotates polarized light. This must be determined by experiment: CH3
OH
CH3 H COOH
OH
(S)-(+)-lactic acid
H COO-Na+
(S)-(–)-sodium lactate
A mixture of equal amounts of two enantiomers is called a racemic mixture. © Prentice Hall 2001
Chapter 4
27
Optical Purity observed specific rotation optical purity = specific rotation of the pure enantiomer For example, if a sample of 2-bromobutane has an observed specific rotation of +9.2 °, and the specific rotation of pure (S)-(+) enantiomer is +23.1 °, then:
+ 9.2 o optical purity = = 0.40 = 40% o + 23.1 © Prentice Hall 2001
Chapter 4
28
Separation of Enantiomers
Racemic mixtures can be separated by naturally occurring chiral compounds, e.g. morphine or strychnine.
The product of the reaction is a pair of diastereomers (nonsuperimposable, but no mirror images).
Properties of diastereomers are sufficiently different from each other to allow separation.
After separation, each diastereomer can be reacted, e.g. with an acid, yielding optically pure R or S compounds.
© Prentice Hall 2001
Chapter 4
29
© Prentice Hall 2001
Chapter 4
30
Naming Enantiomers: The R,S System
1.
Analogue rules for nomenclature of Fischer Projections: If the group of lowest priority is on a vertical line, just draw the curve according to decreasing priority. Clockwise - configuration is R. Counterclockwise - configuration is S.
(S)-2-bromobutane © Prentice Hall 2001
Chapter 4
31
Naming Enantiomers: The R,S System 2.
If the group of lowest priority is on a horizontal line, follow step 1, but reverse the R & S. Rule of thumb: If lowest priority is on a Vertical bond, configuration is Very true. If lowest priority is on a Horizontal bond, configuration is Horribly wrong:
(R)-2-bromobutane © Prentice Hall 2001
Chapter 4
32
Naming Enantiomers: The R,S System
Working with Fischer Projections, a rotation of 90° or a horizontal flip will switch chirality. Rotation of a Fischer projection by 180° in the plane of the paper does not switch the chirality.
(R)-2-bromobutane (S)-2bromobutane
(R)-2-bromobutane (S)-2bromobutane
(R)-2-bromobutane (R)-2bromobutane © Prentice Hall 2001
Chapter 4
33
R and S configuration Indicate whether the following structure has R or S configuration: 4 H
Cl H3CH2CH2C 2
CH3 3 Cl 1
R © Prentice Hall 2001
Chapter 4
34
R and S configuration Indicate whether the structure has R or S configuration: 2 CH(CH3)2
switch CH3
CH(CH3)2
and CH3CH2 C
4
CH3
CH2CH3 CH2Br 1
3
CH3CH2
C
CH3 CH2Br
R © Prentice Hall 2001
Chapter 4
35
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