Conformers

What are Conformers?

If the groups are large enough to have a significant effect on the energy of rotation they may tend to prefer certain spatial arrangements rather than others. These different spatial arrangements of the groups give rise to conformers.

Stereochemistry covers all important aspects of stereochemistry starting right from the history of stereochemistry and discussing molecular geometry in terms of bond distances and dihedral angles along with the biological importance of chirality and its effect on taste, odour, agrochemicals and pharmaceuticals followed by the basic principles, conformations and configurations, the methods for writing structures in two dimensional and three dimensional projections and their interconversions.

Table of Contents

• • Recommended videos
  • Newman Projection
  • Conformation of Butane
  • Conformational Isomers
  • Energy Contents and Nomenclature of Conformers
  • Frequently Asked Questions – FAQs

Recommended Videos

Newman Projection

One of the best ways to look at different conformations of molecules is to use Newman projections. A Newman projection is a convenient way of sighting down a particular carbon carbon bond.

In a Newmann projection, the three lines in the shape of a Y represent the three bonds of the first carbon that are sighting down; where the three lines connect is where the front carbon is. A circle represents the back carbon; the three lines coming out of the circle represents the three bonds that come off of that carbon. Note that the fourth bond for each of these carbons is the carbon-carbon bond that is looking down. A Newmann projection can help to analyze the rotation around a particular carbon-carbon bond.

Using Newman projections, rotating around a specific bond to reach other conformers is a fairly straightforward task. The best way to reach other conformers is to rotate just one carbon at a time, either the front carbon or the back carbon. For consistency it will always hold the front of the Newmann projection fixed and rotate the back carbon.

Conformation of Butane

The situation with butane is a bit more complicated than the situation with ethane. With butane, not all eclipsed conformations are of the same energy, and not all staggered conformations are of the same energy.

The rotation around the C2-C3 bond in butane. In the first staggered conformation, the two methyl groups are 180^o apart from each other. This conformer is called the anti conformer. In the other staggered conformer, the two methyl groups flank each other. The interaction between two adjacent large groups in a staggered conformation is called a gauche interaction. Gauche interactions act to increase the energy of a conformer. Because the staggered anti conformer had no gauche interactions. It’s lower in energy than the staggered gauche conformed.

Conformational Isomers

• Of the many conformational isomers of ethane that can be generated by rotation about the central carbon carbon bond, one is more probable than the others. This is the staggered conformation.
• The free energy of this conformer is lower than any other because unfavorable nonbonded interactions between the electronic charge clouds of the hydrogen atoms are minimal in this arrangement.
• The energy difference between the staggered conformation of ethane and the isomer of maximum energy is about 3 kcal/mole.
• All conformations of ethane are in equilibrium with each other, but because the free energy of the staggered form is lowest most ethane molecules in the equilibrium mixture are present in this conformation.

Energy Contents and Nomenclature of Conformers

The following diagram shows the full rotation about a C-C single bond, the relative energy contents, and the names of the conformers depending on the dihedral angle.

Eclipsed groups with bonds enclosing dihedral angles of 0o in the Newmann projection suffer from overcrowding in space, in this situation they develop the most intense steric interaction, strongly repulsing each other. Thus conformers with fully and partially eclipsed groups occupy energy maxima and have the lowest stability.

Staggered groups with bonds enclosing dihedral angles of 60o in the Newmann projection do not disturb one another as strongly. As a result, conformers with staggered substituents are more stable and occupy energy minima. In these conformers the staggered substituents are able to adopt two different arrangements denoted as syn and anti.