Why should there be an energy barrier in the rotation about a single bond? In order to answer this question, we should start with the simplest C–C bond possible—the one in ethane. Ethane has two extreme conformations called the staggered and eclipsed conformations. Three different views of these are shown below.

You can see why the conformations have these names by looking at the end-on views in the diagram. In the eclipsed case the near C–H bonds completely block the view of the far bonds, just as in a solar eclipse the Moon blocks the Sun as seen from the Earth. In the staggered con formation, the far C–H bonds appear in the gaps between the near C–H bonds—the bonds are staggered. Chemists often want to draw these two conformations quickly and two different methods are commonly used, each with its own merits. In the first method, shown on the right, we simply draw the side view of the molecule and use wedged and hashed lines to show bonds not in the plane of the paper. Particular attention must be paid to which of the bonds are in the plane and which go into and out of the plane. In the second method we draw the end-on view, looking along the C–C bond. This view is known as a Newman projection, and Newman projections are subject to a few conventions:

• The carbon atom nearer the viewer is at the junction of the front three bonds.

• The carbon further away (which can’t in fact be seen in the end-on view) is represented by a large circle. This makes the perspective inaccurate, but this doesn’t matter.

• Bonds attached to this further carbon join the edge of the circle and do not meet in the centre.

• Eclipsed bonds are drawn slightly displaced for clarity—as though the bond were rotated by a tiny fraction.

The staggered and eclipsed conformations of ethane are not identical in energy: the staggered conformation is lower in energy than the eclipsed by 12 kJ mol−1, the value of the rotational barrier. Of course, there are other possible conformations too with energies in between these extremes, and we can plot a graph to show the change in energy of the system as the C–C bond rotates. We defi ne the dihedral angle, θ (sometimes called the torsion angle), to be the angle between a C–H bond at the nearer carbon and a C–H bond at the far carbon. In the staggered conformation, θ = 60° whilst in the eclipsed conformation, θ = 0°. The energy level diagram shows the staggered conformation as a potential energy mini mum whilst the eclipsed conformation represents an energy maximum. This means that the eclipsed conformation is not a stable conformation since any slight rotation will lead to a conformation lower in energy. The molecule will actually spend the vast majority of its time in a staggered or nearly staggered conformation and only briefly pass through the eclipsed conformation enroute to another staggered conformation.

But why is the eclipsed conformation higher in energy than the staggered conformation? There are two reasons. The first is that the electrons in the bonds repel each other and this repulsion is at a maximum when the bonds are aligned in the eclipsed conformation. The second is that there may be some stabilizing interaction between the C–H σ bonding orbital on one carbon and the C–H σ* antibonding orbital on the other carbon, which is greatest when the two orbitals are exactly parallel: this only happens in the staggered conformation. The same effects—repulsion between filled orbitals (a form of steric effect) and stabilization by donation into antibonding orbitals—govern the favoured conformations about all rotating bonds.

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Oxidative addition inserts metal atoms into single bonds

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