Ch 3 - Simple alkanes C1 to C3

Chapter 3: Conformations of Alkanes and Cycloalkanes

Alkanes

Let's start with simple alkanes so we can prepare ourselves for the more complex world that lies beyond.....

Methane
Ethane
Propane

Methane: CH₄

Although σ bonds are able to rotate about the internuclear axis, the spherical symmetry of H atoms means that methane has a single unique conformation.

methane

Ethane: CH₃-CH₃

Rotation about the C-C bond in ethane (see the JMOL animation to the right) produces different *conformations. Although an infinite number of conformations are possible, the staggeredeclipsed* conformation which represent the most and least stable respectively are the two most important. Try rotating the 3D model of the animation so that you are looking directly along the C-C** bond to see the interconversion of the two conformations. conformation and the
The differences between these two conformations are most apparent when viewed directly down the C-C bond, as in a Newman projection, see below:
		STAGGERED
Look at how the each H-C-H bond angle is bisected by a C-H bond on the adjacent C atom. This is the most stable conformation for ethane since the torsional strain is minimised. The staggered conformation is 12kJ/mol (2.9 kcal/mol) more stable than the eclipsed conformation (below).

		ECLIPSED
Think like an astronomer....each of the C-H bonds is aligned with a C-H bond on the adjacent C atom so that the H attached to the front C obscures (eclipses) those on the rear C

The animation below shows how the potential energy of the ethane molecule varies for a full rotation about the central C-C bond.
Use the controls to show the energies of the important conformations.

Propane: CH₃-CH₂-CH₃

Although there are 2 C-C bonds in propane, they are equivalent and rotation produces conformations that are similar to those of ethane except that the "extra" methyl group is interacting with the H atoms. These can be seen by in the 3D model of propane to the right.