Conformational Analysis Stereochemistry 4/28/2017

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

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

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

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) 4/28/2017

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. 4/28/2017

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 4/28/2017

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. 4/28/2017

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. 4/28/2017

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. 4/28/2017

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. 4/28/2017

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. 4/28/2017

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. 4/28/2017

4/28/2017

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. 4/28/2017

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 4/28/2017

4/28/2017

Dihedral angles of ethanes: Staggered conformation: 60° (gauche), 180° (anti), and 300° (-60°, gauche) Eclipsed conformation: 0°, 120°, and 240° (-120°) 4/28/2017

Energy vs. dihedral angle for ethane http://www2.chem.ucalgary.ca/Flash/ethane.html 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 4/28/2017

4/28/2017

Propane CH3CH2CH3 4/28/2017

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 4/28/2017

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. 4/28/2017

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. 4/28/2017

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. 4/28/2017

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 4/28/2017

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. 4/28/2017

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 4/28/2017

4/28/2017

An energy diagram can be made as a function of dihedral angle for butane 4/28/2017

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 4/28/2017

3.2: Conformational Analysis of Butane Two different staggered and eclipsed conformations Staggered: anti Staggered: gauche 3 KJ/mol 4/28/2017

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 4/28/2017

Energy diagram for the rotation of butane Summary: H - H eclipsed 4.0 KJ/mol torsional strain H - CH3 eclipsed 5.0 KJ/mol mostly torsional strain CH3 - CH3 eclipsed 17 KJ/mol torsional + steric strain CH3 - CH3 gauche 3.0 KJ/mol steric strain 4/28/2017

The Shapes of Cycloalkanes: Planar or Nonplanar? Angle Strain: strain due to deforming a bond angle from its ideal value (Baeyer Strain Theory) 60° 90° 108° 120° 128° 135° Internal angles of polygons 4/28/2017

Small Rings: Cyclopropane and Cyclobutane Total strain for cyclopropane = angle strain + torsional strain 4/28/2017 all adjacent CH2 groups are eclipsed