50. Conformational Analysis
Stereochemistry
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51. Ch. 3: Alkanes and Cycloalkanes: Conformations and
cis-trans Stereoisomers
Stereochemistry: three-dimensional aspects of molecules
Conformation: different spatial arrangements of atoms that
result from rotations about single (σ) bonds
Conformer: a specific conformation of a molecule
Conformational Analysis of Ethane
Sawhorse
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52. Ch. 3: Alkanes and Cycloalkanes: Conformations and
cis-trans Stereoisomers
Stereochemistry: three-dimensional aspects of molecules
Conformation: different spatial arrangements of atoms that
result from rotations about single (σ) bonds
Conformer: a specific conformation of a molecule
Conformational Analysis of Ethane
Sawhorse
4/28/2017

53. Ch. 3: Alkanes and Cycloalkanes: Conformations and
cis-trans Stereoisomers
Stereochemistry: three-dimensional aspects of molecules
Conformation: different spatial arrangements of atoms that
result from rotations about single (σ) bonds
Conformer: a specific conformation of a molecule
Conformational Analysis of Ethane
Sawhorse
4/28/2017

54. Different spatial arrangements (structures) of a molecule which are generated by the relatively easy rotation around a single bond (usually a C-C single bond)
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55. Single bonds are sigma bonds
Which by definition are symmetrical with respect to the bond axis.
Thus rotation does not change the extent of overlap and thus does not change the strength of the bond.
Consequently rotation around a sigma bond is relatively easy , in contrast to a pi bond, where rotation around the bond completely destroys the pi bond.
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56. The rotation we're speaking of is "internal rotation", that is the rotation of one part of the molecule relative to the other.
For example in ethane, the rotation of one methyl group relative to the other.
The angle of rotation is called a dihedral angle , and it is represented by a Greek theta symbol. θ
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57. Since the different structures which are generated by this rotation are all very similar (they have the same number and kind of bonds) and are very easily interconverted, they are not referred to as isomers but as different Conformations of the same molecule.
These conformations have similar energies, but not precisely the same energy.
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58. The highest energy structure, called the Eclipsed conformation.
Since rotation around the C-C bond of ethane is continuous, there are an infinite number of conformations, but only two are of major interest.
These are the lowest energy structure, called the Staggered conformation.
The highest energy structure, called the Eclipsed conformation.
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59. It defines the Shape of ethane molecules.
The Staggered structure is important because, being the structure of lowest energy, it is the structure actually adopted by ethane (the ground state structure).
It defines the Shape of ethane molecules.
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60. The eclipsed structure is important not because it is highly populated , but because its energy relative to the staggered structure defines how much energy is required to complete a rotation in ethane.
This is called the Energy barrier to rotation, and it is about 3 kcal/mol.
That is, the eclipsed structure is 3 kcal higher in energy than the staggered structure.
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61. Important Torsional Strain
An energy increase caused by the eclipsing of bonds.
Strain
Any increase in energy of a molecule, particularly an increase relative to that which would normally be expected.
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62. The relationship between two bonds when the dihedral angle is zero.
Eclipsing
The relationship between two bonds when the dihedral angle is zero.
Conformational Energy Diagram
A plot of energy, E (vertically) vs. dihedral angle , theta (horizontally).
It should include the structures, energies, and names of the energy minima and maxima.
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63. 4/28/2017

64. Basis for Torsional Strain
Torsional strain is considered to be caused by the repulsions between electrons in the two bonds which are eclipsed.
These electrons are closer together in the eclipsed form than in the staggered form, and so the repulsions are greater.
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65. There are two conformations of ethane:
Newman projection
Staggered
Eclipsed
Dihedral (torsion) angle:
Angle between an atom (group) on the
front atom of a Newman Projection and an atom (group)
on the back atom
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66. 4/28/2017

67. Dihedral angles of ethanes:
Staggered conformation: 60° (gauche), 180° (anti), and 300° (-60°, gauche)
Eclipsed conformation: 0°, 120°, and 240° (-120°)
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68. Energy vs. dihedral angle for ethane
The barrier (Eact) for a 120° rotation of ethane (from one staggered
conformer to another) is 12 KJ/mol. The eclipsed conformer is
the barrier to the rotation. An H-H eclipsing interaction = 4 KJ/mol
Torsional Strain: strain (increase in energy) due to eclipsing
interactions
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69. 4/28/2017

70. Propane
CH3CH2CH3
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71. Groups larger than hydrogen cause larger energy barrier.
In propane the barrier to rotation is due to methyl-hydrogen eclipsed interactions which
In addition to the torsional strain also involves nonbonded vander waals repulsive forces i.e. steric strain
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72. Torsional strain
It can be thought as the repulsion due to electrostatic forces between electrons in adjacent MOs.
Meanwhile steric strain (also known as van der Waals strain) can be thought as the repulsion when two bulky groups which are not directly bonded to each other become too close to each other and hence there isn't enough space for them.
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73. Conformation of Ethane
The C-C sigma bond is free to rotate and in principle there are an infinite number of possible conformations.
However only 2 are significant, these are staggered and eclipse conformations.
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74. The staggered conformer is the most stable conformer while the eclipsed conformer is the least stable conformer. The staggered conformer is approximately 12 kJ mol−1 more stable than the eclipsed conformer.
This energy difference between this maxima and minima is known as the torsional barrier.
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75. The barrier to rotation in a propane molecule is about 14.2 kj/mole.
The difference in energy between the eclipsed and the staggered conformations of propane is therefore expected to be more than that in the case of ethane.
The barrier to rotation in a propane molecule is about 14.2 kj/mole.
But the conformational energy diagram for propane is similar to that for ethane.
It also has 3 equal maxima and 3 equal minima during a rotation
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76. Conformational analysis of butane
In butane, two of the substituents, one on each carbon atom being viewed, is a methyl group.
Methyl groups are much larger than hydrogen atoms. Thus, when eclipsed conformations occur in butane, the interactions are especially unfavorable.
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77. There are four possible "extreme" conformations of butane:
1) Fully eclipsed, when the methyl groups eclipse each other;
2) Gauche, when the methyl groups are staggered but next to each other;
3) Eclipsed, when the methyl groups eclipse hydrogen atoms; and
4) Anti, when the methyl groups are staggered and as far away from each other as possible
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78. 4/28/2017

79. An energy diagram can be made as a function of dihedral angle for butane
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80. Conformations of Propane
staggered
The barrier to C-C rotation for
propane is 13 KJ/mol = 1 (CH3-H)
+ 2 (H-H) eclipsing Interactions.
A CH3-H eclipsing interaction
is 5 KJ/mol
5.0 KJ/mol
eclipsed
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81. 3.2: Conformational Analysis of Butane
Two different staggered and eclipsed conformations
Staggered: anti
Staggered: gauche
3 KJ/mol
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82. Steric Strain: repulsive interaction that occurs when two groups
are closer than their atomic radii allow
3 KJ/mol
Eclipsed conformations of butane: rotational barrier of butane is
25 KJ/mol. A CH3-CH3 eclipsing interaction is 17 KJ/mol.
CH3 - H
CH3 - CH3
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83. Energy diagram for the rotation of butane
Summary:
H - H eclipsed 4.0 KJ/mol torsional strain
H - CH3 eclipsed KJ/mol mostly torsional strain
CH3 - CH3 eclipsed 17 KJ/mol torsional + steric strain
CH3 - CH3 gauche 3.0 KJ/mol steric strain
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84. The Shapes of Cycloalkanes: Planar or Nonplanar?
Angle Strain: strain due to deforming a bond angle from its ideal value (Baeyer Strain Theory)
60° ° ° ° ° °
Internal angles of polygons
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85. Small Rings: Cyclopropane and Cyclobutane
Total strain for cyclopropane
= angle strain +
torsional strain
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all adjacent CH2 groups are eclipsed