Chapter 3: Conformations of Alkanes and Cycloalkanes

Energetics

• Fundamentals of electrostatics
• Types of strain

The force between two charged particles q1 and q2 is inversely proportional to the distance, r, between them. If the particles are of opposite polarity, then the force is attractive. As an example, the attraction of electrons to an atomic nucleus.   If the particles are of the same polarity, then the force is repulsive. As an example, electron pair repulsion used in VSEPR

IMPORTANT IMPLICATIONS :  Electron pairs repel each other. As a result, molecules are most stable when pairs of electrons are as far apart from each other as possible. When the pairs of electrons are too close together, then the molecule is destabilized and it is at higher energy. Remember, the most stable states are those of lowest energy.

Conformational analyzis is essentially an investigation of forces and energies associated with the interactions of pairs of electrons (these could be pairs of electrons in bonds or lone pairs). Strain is the term used to for the energy associated with a system due to its geometry.
There are various types of strain that we need to be familar with. These and associated terms are described below:
 

Angle strain	If the angle between a pair of bonds in an X-C-Y system is less than the optimal value(e.g. 109.5o at a tetrahedral center), then there is a destabilization due to the electrostatic repulsion of the electrons in the bonds. Note that the two bonds share a common atom, here C.
Steric strain	If the electron clouds of a pair of atoms or group of atoms (such as a methyl group) are too close to each other, then there is a destabilization due to the electrostatic repulsion of the electrons clouds. Note that these groups don't always have to be parts of the same molecule. This is also known as Van der Waals strain or repulsion.
Torsional strain	The electrons in a C-X bond are replled by those in a C-Y bond within a  X-C-C-Y system. When these pairs of bonds are too close to each other, then there is a destabilization due to the electrostatic repulsion of the electrons in the bonds. (note students often have difficulty with this type of strain since they confuse it with angle strain)
Torsional angle	Angle between C-X and C-Y bonds in a X-C-C-Y system. Also known as a dihedral angle.
Ring strain	If we compare the energy of a cyclic structure compared to a related non-cyclic structure (e.g. cyclopropane and cyclopropane), then typically the cyclic structure is less stable, mainly due to angle and torsional strain. This extra energy is released when the ring is broken and is called ring strain.