Return to Contents Chapter 3: Conformations of Alkanes and Cycloalkanes  Ch 3 contents

Energetics

The predominant forces involved in chemistry are electrical in origin based on the physics associated with Coulomb's Law of electrostatics. The basics are reviewed below:
 
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 replusive.

  • As an example, electron pair repulsion used in VSEPR
basic electrostatics

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 destabilised and it is at higher energy. Remember, the most stable states are those of lowest energy.

(You should already know this from General Chemistry and Chapter 1, VSEPR.)

Conformational analysis 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:
 

Steric strain The overall strain in a molecule due to the non-bonded interactions of atoms or groups of atoms that are in close proximity so that their electrons repel each other.  It can be broken down into other types of strain as desecribed 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 destabilisation due to the electrostatic repulsion of the electrons in the bonds. Note that the two bonds share a common atom, here C. angle strain



Van der Waals strain
 

 

If the electron clouds of a pairs of atoms or group of atoms (such as a methyl group) are too close to each other, then there is a destabilisation due to the electrostatic repulsion of the electron clouds. Note that these groups don't even have to be parts of the same molecule. This is also known as Van der Waals repulsion. steric strain



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 this pair of bonds are too close to each other, then there is a destabilisation 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 strain
Torsional angle For example, the angle between C-X and C-Y bonds in a X-C-C-Y system. Also known as a dihedral angle. Rotation of the single bond C-C results in a change in the torsional angle.  torsional or dihedral angle



Ring strain
 

 

If we compare the energy of a cyclic structure compared to a related non-cyclic structure (e.g. cyclopropane and propane), 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.

 


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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary