Conformations of propane

Experiments show that there is a 14 kJ/mol (3.4 kcal/mol) barrier to rotation in propane. The most stable (low energy) conformation is the one in which all of the bonds as far away from each other as possible (staggered when viewed end-on in a Newman projection). The least stable (high energy) conformation is the one in which, for any two adjacent carbon atoms, the six bonds (five C–H and one C–C) are as close as possible (eclipsed in a Newman projection). All other conformations lie between these two limits. The barrier to rotation is the result of one C–C/C–H eclipsing interaction and two equal C–H/C–H eclipsing interactions. We know from ethane that each C–H/C–H eclipsing interaction 'costs' about 4 kJ/mol (1 kcal/mol), so we can assign a value of about 6 kJ/mol (1.4 kcal/mol) to the C–C/C–H eclipsing interaction in propane.

Graphic showing the energetics of C–C bond rotation in propane

The 14 kJ/mol of extra energy in the eclipsed conformation of propane is called torsional strain. The barrier to rotation that results from this strain can be represented in a graph of potential energy versus degree of rotation in which the angle between the bonds on adjacent carbon atoms (the dihedral angle) completes one revolution. Energy minima occur at staggered conformations, and energy maxima occur at eclipsed conformations. The torsional strain is thought to be due to the slight repulsion between electron clouds in the eclipsed bonds.

We can represent conformational isomers in one of two ways. Sawhorse representations view the carbon-carbon bond at an angle so as to show the spatial orientation of all of the bonds. In a Newman projection the carbon-carbon bond is viewed along its axis and the bonded carbon atoms are repesented by a circle. The bonds attached to the front carbon are represented by lines going to the centre of the circle, and bonds attached to the rear carbon are represented by lines going to the edge of the circle. The advantages of Newman projections are that they are easy to draw and they clearly show the relationships among substituents on the different carbon atoms.

Use the interactive model below to explore the conformations of propane.

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