Chapter 4: Stereochemistry of Alkanes and Cycloalkanes
Chapter 4: Stereochemistry of Alkanes and Cycloalkanes Coverage: 1. Conformations of Ethane, Propane, Butane 2. Stabilities of Cycloalkanes 3. Conformations of Cyclohexane and Substituted Cyclohexanes. 4. Molecular Mechanics End of Chapter Problems: 4.24,-4.27, 4.31-4.35, 4.39, 4.4, 4.43. Goals: 1. Be able to draw Newman projections of ethane, propane, butane and others. 2. Know the meaning of gauche, anti, eclipsed, staggered in conformations. 3. Be able to estimate the relative energies of the different conformations of alkanes. 4. Be able to draw chair conformations of cyclohexanes, showing equatorial and axial positions. 5. Be able to estimate the relative energies of the conformations of substituted cyclohexanes. 6. Understand the meaning of torsional, steric, angle and ring strain and be able to apply them to the conformations of alkanes and cycloalkanes. 7. Understand how the stabilities of cycloalkanes are measured. 8. Understand why cyclohexane is the most stable cycloalkane. 4-1
1. Conformation of Ethane CH3CH3 H
H
H C
H
Rotate 60o
H
C
2
H C
1
H
H
H H
H
H
H
Sawhorse
C
H
H
H H
H H
Newman projection H H
HH
H
Staggered Conformation Lower Energy More Stable
Eclipsed Conformation Higher Energy Less Stable
The staggered conformation is more stable by 3.0 kcal/mol 4-2
Dihedral Angle – the angle defined by the C-H bond on the front carbon and the C-H bond on the back carbon in a Newman projection. 600
0 H 0 H
H H
H H
H
H
HH
H
H
3.0 kcal/mol
0.0 kcal/mol
Eclipsed
Staggered
Eclipsed
Staggered
Eclipsed
Staggered
Eclipsed
4-3
Why is the staggered conformation more stable than the eclipsed conformation? In the eclipsed conformation, the C-H bonds are closer, resulting in a repulsion of the electron clouds. This effect is referred to torsional strain. Torsional strain exists anytime C-H bonds are eclipsed.
1.0 kcal/mol
H 1.0 kcal/mol H
H H
Total Energy: 3.0 kcal/mol
HH
1.0 kcal/mol
4-4
2. Conformations of Propane H
H
H C1
H
H
Rotate
600
C
1
CH3
H
H
H
C 2
H
H
CH3
H C
2
H
H
H
Rotate
600
H
H H
CH3
H H
HCH3
More stable
Less stable
0.0 kcal/mol
3.3 kcal/mol
1.0 kcal/mole H H
H 1.0 kcal/mol H
HCH3
1.3 kcal/mol
The additional 0.3 kcal/mol energy is due to steric strain. Steric strain – electronic repulsion that occurs when two atoms or groups are forced together. 4-5
3. Conformations of Butane
H
H3C
H C2
H
H
60o
H3C
C
C2
CH3
H
CH3
H CH3
H
H CH3
H H
HCH3
Anti Conformation Staggered 180o dihedral H
H H
3
CH3
H
H
C
H
60o
H
H
3
H
H
Eclipsed
H H
H H
H
H3C H
H H
Anti
H CH3
CH3
Gauche Conformation Staggered 60o dihedral
H3CCH3
H3C
CH3
H
HCH3
4-6
There are two staggered conformations of different energy. There are two eclipsed conformations of different energy.
Remember, staggered conformations are more stable than eclipsed conformatiions Quicktime Movie
4-7
Conformations and Stabilites of Cycloalkanes Cycloalkanes possess types of strain that do not exist in noncyclic alkanes Angle Strain – the amount of strain due to deviation from normal bond angle. What is the normal bond angle for an sp3 carbon? 109.5o 600
Deviation 109.5 – 60 = 49.9o Cyclopropane
Ring Strain – total strain (sum of torsional, steric and angle strain) in a cycloalkane compared to a open, noncyclic reference compound.
4-8
Cyclopropane also possesses torsional strain because of eclipsed bonds.
Planar molecule
How much torsional strain (kcal/mol) does it possess? Answer: You figure it out.
4-9
Ring Strain in Cycloalkanes Baeyer Strain Theory
Johann Friedrich Wilhelm Adolf von Baeyer
• First graduate student of Kekule’ • Developed theory that cycloalkanes possess different amounts of strain or stability, depending on the size of the ring. • Awarded Nobel Prize in 1905 for work with organic dyes. • Total synthesis of indigo. "I have never set up an experiment to see whether I was right, but to see how the materials behave".
4-10
Ring Strain in Cycloalkanes The relative stabilites of cycloalkanes are determined by measuring their heats of combustion.
[ -CH2- ]n +
3/2nO2
Æ
nCO2 + nH2O + Heat
The more heat per CH2, the less stable the alkane. Ring Size
Heat per CH2 kcal/mol
Ring Strain per CH2, kcal/mol
Total Ring Strain, kcal/mol
Long-Chain Alkane
157.4
0.0
0.0
3
166.6
9.2
27.6
4
164.0
6.6
26.4
5
158.7
1.3
6.5
6
157.4
0.0
0.0
7
158.3
0.9
6.3
8
158,6
1.2
9.6 4-11
4-12
Cyclohexane is free of ring strain? Why? 1. All Bonds are staggered and therefore no torsional strain. 2. All bond angles are near 109.50 so no angle strain. 3. It does possess steric strain (internal gauche interaction) but no more than a noncyclic alkane. Cyclohexane – the most stable conformation of cyclohexane is the chair.
2
4 5
1
Chair conformation showing only carbon atoms 1
Chair conformations showing positions of hydrogens
5
Newman projection shows that all bonds are staggered. Bond angles are about 109.5o
4-13
Equatorial and Axial Hydrogens of Cyclohexane
Six axial hydrogens point straight up (3) and straight down (3). Six equatorial hydrogens point out away from the ring. The axial and equatorial hydrogens are exchangable by a process called “ring flipping”.
4-14
Interconversion of Chair Conformations or “Ring Flipping”
The axial and equatorial positions exchange during this process Quicktime Movie
4-15
You need to be able to draw a cyclohexane ring!
4-16
You also need to be able to draw in the equatorial and axial positions.
4-17
Other Conformations of Cyclohexane Chair 1
Half Chair 1
Twist Boat 1
Boat
Twist Boat 2
Half Chair 2
Chair 2
What are the shapes and energies of these other conformations?
4-18
Monosubstituted Cyclohexanes Methylcyclohexane Axial Conformer
Equatorial Conformer
4-19
The equatorial conformer is more stable by 1.8 kcal/mol due to a steric interaction between the axial methyl and two axial hydrogens. These interactions are termed 1,3-diaxial interactions. 0.9 kcal/mol
0.9 kcal/mol
2 x 0.9 = 1.8 kcal/mol 5%
0.0 kcal/mol 95%
These 1,3-diaxial interactions are really gauche interactions in disguise! Remember that a gauche interaction for butane is worth 0.9 kcal/mol.
0.9 kcal/mol
1.8 kcal/mol
4-20
Other Substituents
X X X
Eaxial – Eequatorial kcal/mol
-H
0.0
-F
0.24
-C≡N
0.2
-Cl
0.50
-Br
0.50
-CH3
1.8
-CH2CH3
1.9
-CH(CH3)2
2.2
-C(CH3)3
5.4 4-21
Disubstituted Cyclohexanes
0.9 + 1.8 = 2.7 kcal/mol
0.9 + 1.8 = 2.7 kcal/mol
4-22
Disubstituted Cyclohexanes
0.9 kcal/mol
Relative 0.0 kcal/mol
3.6 kcal/mol
Relative 2.7 kcal/mol
4-23
Can you name these dimethylcyclohexanes?
H3C
cis-1,2-dimethylcyclohexane equatorial - axial
CH3 CH3 H3C
trans-1,3-dimethylcyclhexane equatorial - axial
CH3 trans-1,4-dimethylcyclohexane axial-axial or diaxial
CH3 Which of these represent the most stable conformation possible?
4-24
Molecular Mechanics Molecular Mechanics consist of computation methods for determining the shapes and strain energies of molecules. The methods are based on the following principles: •Nuclei and electrons are lumped into atom-like particles. •Atom-like particles are spherical (radii obtained from measurements or theory) and have a net charge (obtained from theory). •Interactions are based on springs and classical potentials. •Interactions must be preassigned to specific sets of atoms. •Interactions determine the spatial distribution of atom-like particles and their energies
Etotal = Ebond stretching + Eangle strain + E torsional strain + Esteric strain
4-25
Where is Molecular Mechanics used? • Chemistry • Biology • Pharmaceutical Industry • Material Science
4-26
1. Conformation of Ethane CH3CH3 H
H
H C
H
Rotate 60o
H
C
2
H C
1
H
H
H H
H
H
H
Sawhorse
C
H
H
H H
H H
Newman projection H H
HH
H
Staggered Conformation Lower Energy More Stable
Eclipsed Conformation Higher Energy Less Stable
The staggered conformation is more stable by 3.0 kcal/mol 4-2
Dihedral Angle – the angle defined by the C-H bond on the front carbon and the C-H bond on the back carbon in a Newman projection. 600
0 H 0 H
H H
H H
H
H
HH
H
H
3.0 kcal/mol
0.0 kcal/mol
Eclipsed
Staggered
Eclipsed
Staggered
Eclipsed
Staggered
Eclipsed
4-3
Why is the staggered conformation more stable than the eclipsed conformation? In the eclipsed conformation, the C-H bonds are closer, resulting in a repulsion of the electron clouds. This effect is referred to torsional strain. Torsional strain exists anytime C-H bonds are eclipsed.
1.0 kcal/mol
H 1.0 kcal/mol H
H H
Total Energy: 3.0 kcal/mol
HH
1.0 kcal/mol
4-4
2. Conformations of Propane H
H
H C1
H
H
Rotate
600
C
1
CH3
H
H
H
C 2
H
H
CH3
H C
2
H
H
H
Rotate
600
H
H H
CH3
H H
HCH3
More stable
Less stable
0.0 kcal/mol
3.3 kcal/mol
1.0 kcal/mole H H
H 1.0 kcal/mol H
HCH3
1.3 kcal/mol
The additional 0.3 kcal/mol energy is due to steric strain. Steric strain – electronic repulsion that occurs when two atoms or groups are forced together. 4-5
3. Conformations of Butane
H
H3C
H C2
H
H
60o
H3C
C
C2
CH3
H
CH3
H CH3
H
H CH3
H H
HCH3
Anti Conformation Staggered 180o dihedral H
H H
3
CH3
H
H
C
H
60o
H
H
3
H
H
Eclipsed
H H
H H
H
H3C H
H H
Anti
H CH3
CH3
Gauche Conformation Staggered 60o dihedral
H3CCH3
H3C
CH3
H
HCH3
4-6
There are two staggered conformations of different energy. There are two eclipsed conformations of different energy.
Remember, staggered conformations are more stable than eclipsed conformatiions Quicktime Movie
4-7
Conformations and Stabilites of Cycloalkanes Cycloalkanes possess types of strain that do not exist in noncyclic alkanes Angle Strain – the amount of strain due to deviation from normal bond angle. What is the normal bond angle for an sp3 carbon? 109.5o 600
Deviation 109.5 – 60 = 49.9o Cyclopropane
Ring Strain – total strain (sum of torsional, steric and angle strain) in a cycloalkane compared to a open, noncyclic reference compound.
4-8
Cyclopropane also possesses torsional strain because of eclipsed bonds.
Planar molecule
How much torsional strain (kcal/mol) does it possess? Answer: You figure it out.
4-9
Ring Strain in Cycloalkanes Baeyer Strain Theory
Johann Friedrich Wilhelm Adolf von Baeyer
• First graduate student of Kekule’ • Developed theory that cycloalkanes possess different amounts of strain or stability, depending on the size of the ring. • Awarded Nobel Prize in 1905 for work with organic dyes. • Total synthesis of indigo. "I have never set up an experiment to see whether I was right, but to see how the materials behave".
4-10
Ring Strain in Cycloalkanes The relative stabilites of cycloalkanes are determined by measuring their heats of combustion.
[ -CH2- ]n +
3/2nO2
Æ
nCO2 + nH2O + Heat
The more heat per CH2, the less stable the alkane. Ring Size
Heat per CH2 kcal/mol
Ring Strain per CH2, kcal/mol
Total Ring Strain, kcal/mol
Long-Chain Alkane
157.4
0.0
0.0
3
166.6
9.2
27.6
4
164.0
6.6
26.4
5
158.7
1.3
6.5
6
157.4
0.0
0.0
7
158.3
0.9
6.3
8
158,6
1.2
9.6 4-11
4-12
Cyclohexane is free of ring strain? Why? 1. All Bonds are staggered and therefore no torsional strain. 2. All bond angles are near 109.50 so no angle strain. 3. It does possess steric strain (internal gauche interaction) but no more than a noncyclic alkane. Cyclohexane – the most stable conformation of cyclohexane is the chair.
2
4 5
1
Chair conformation showing only carbon atoms 1
Chair conformations showing positions of hydrogens
5
Newman projection shows that all bonds are staggered. Bond angles are about 109.5o
4-13
Equatorial and Axial Hydrogens of Cyclohexane
Six axial hydrogens point straight up (3) and straight down (3). Six equatorial hydrogens point out away from the ring. The axial and equatorial hydrogens are exchangable by a process called “ring flipping”.
4-14
Interconversion of Chair Conformations or “Ring Flipping”
The axial and equatorial positions exchange during this process Quicktime Movie
4-15
You need to be able to draw a cyclohexane ring!
4-16
You also need to be able to draw in the equatorial and axial positions.
4-17
Other Conformations of Cyclohexane Chair 1
Half Chair 1
Twist Boat 1
Boat
Twist Boat 2
Half Chair 2
Chair 2
What are the shapes and energies of these other conformations?
4-18
Monosubstituted Cyclohexanes Methylcyclohexane Axial Conformer
Equatorial Conformer
4-19
The equatorial conformer is more stable by 1.8 kcal/mol due to a steric interaction between the axial methyl and two axial hydrogens. These interactions are termed 1,3-diaxial interactions. 0.9 kcal/mol
0.9 kcal/mol
2 x 0.9 = 1.8 kcal/mol 5%
0.0 kcal/mol 95%
These 1,3-diaxial interactions are really gauche interactions in disguise! Remember that a gauche interaction for butane is worth 0.9 kcal/mol.
0.9 kcal/mol
1.8 kcal/mol
4-20
Other Substituents
X X X
Eaxial – Eequatorial kcal/mol
-H
0.0
-F
0.24
-C≡N
0.2
-Cl
0.50
-Br
0.50
-CH3
1.8
-CH2CH3
1.9
-CH(CH3)2
2.2
-C(CH3)3
5.4 4-21
Disubstituted Cyclohexanes
0.9 + 1.8 = 2.7 kcal/mol
0.9 + 1.8 = 2.7 kcal/mol
4-22
Disubstituted Cyclohexanes
0.9 kcal/mol
Relative 0.0 kcal/mol
3.6 kcal/mol
Relative 2.7 kcal/mol
4-23
Can you name these dimethylcyclohexanes?
H3C
cis-1,2-dimethylcyclohexane equatorial - axial
CH3 CH3 H3C
trans-1,3-dimethylcyclhexane equatorial - axial
CH3 trans-1,4-dimethylcyclohexane axial-axial or diaxial
CH3 Which of these represent the most stable conformation possible?
4-24
Molecular Mechanics Molecular Mechanics consist of computation methods for determining the shapes and strain energies of molecules. The methods are based on the following principles: •Nuclei and electrons are lumped into atom-like particles. •Atom-like particles are spherical (radii obtained from measurements or theory) and have a net charge (obtained from theory). •Interactions are based on springs and classical potentials. •Interactions must be preassigned to specific sets of atoms. •Interactions determine the spatial distribution of atom-like particles and their energies
Etotal = Ebond stretching + Eangle strain + E torsional strain + Esteric strain
4-25
Where is Molecular Mechanics used? • Chemistry • Biology • Pharmaceutical Industry • Material Science
4-26
