Beginning Organic Chemistry (BOC)
8. Isomerism.
a. Conformational Isomers.
1. Open Chain Compounds.

Return to BOC Index. | Ring Conformations.

Conformational Isomers.

One property of single (2 electron) carbon to carbon bonds is free rotation. Free rotation about C,C single bonds leads to different conformations of the molecule which usually interchange quickly at room temperature. At room temperature most conformational isomers cannot be isolated, rotation easily occurring to give other conformations. Rotation about C,C single bonds is not wholely without an energy requirement, as some conformations have a slightly lower energy than others.

Non-Bonding Interactions (Repulsions) between Atoms.

The VSEPR approach to molecular geometry suggests that the most stable arrangement of four atoms bonded to a central atom is the tetrahedron, as found for example in methane.

When adjacent atoms are both surrounded by four atoms the problem arises of two atoms which are not bonded to each other being physically close, they may eclipse each other.

The increased energy due to eclipsing is termed tortional strain.

If the central bond between the two tetrahedral groups allows rotation of those groups, then the energy of the molecule will vary during the rotation as the non-bonded atom distance varies.

Non-Bonding Hydrogens.

The following two models show the positions of highest energy (eclipsed form) and lowest energy (staggered form) for ethane. The energy difference between the two forms is 12.1 kJ/mole. At room temperature there is ample energy to allow the free rotation past this barrier.

Note that since there are three pairs of overlapping hydrogens, the energy to eclipse two hydrogens on adjacent carbons is about 4.0 kJ/mole.

Rotate the following models to look down the C,C bond.

ethane: staggered conformation,
lowest energy conformation.
ethane: eclipsed conformation,
highest energy conformation.

Non-Bonding Methyl Groups.

Hydrogen and fluorine atoms are the smallest of the groups which can physically interact. Other groups will have larger interaction energies. The case of butane illustrates the energies involved when a methyl group eclipses either a hydrogen atom or another methyl group.

To see the structures involved, rotate the model so that you are looking down the centre C,C bond.

Butane: staggered with the two largest groups anti (opposite) to each other.

This is the conformation with lowest energy.

Change this model, and the ones following to the spacefilling view to see the interactions more clearly

Butane: staggered with the two largest groups gauche (next) to each other.

This conformation has 3.8 kJ/mole more energy than the anti conformation.

Butane: eclipsed with a small group (hydrogen) and a large group (methyl) eclipsing.

This conformation has 15.9 kJ/mole more energy than the anti form.

Since the eclipsing pair of H atoms contribute 4.0 kJ/mole excess energy, the two H - CH3 eclipses must contribute 11.9 kJ/mole, or about 6 kJ/mole each.

Butane: eclipsed with the two large groups (methyl) eclipsing.

This conformation has 18.8 kJ/mol more energy than the anti conformation.

Since the eclipsing pairs of H atoms contribute 8.0 kJ/mole excess energy, the methyl - methyl eclipse must contribute 11 kJ/mole

Date created: 2005 06 12.