Conformational Analysis – Acyclics
Most single bonds are continually rotating, which leads to different shapes being possible around that bond axis. These shapes have different energies depending on the relative placement of attached groups. Conformational Analysis is the study of these shapes and is important for assessing what conformations (shapes) will predominate in the overall shape of the molecule. We begin simple with ethane and building up to more complex acyclic chains in which the basics still apply.
In Organic Chemistry we use several different devices to describe structures of molecules, each of which brings different value to the discussion. The simple bond-line structure (a) shows how atoms are attached while the older sawhorse idea (b) attempts to show relative disposition of atoms in space. The more modern molecular model picture (c) gives a better idea of how atoms are arranged while the space-filling model (d) helps us remember how big atoms and groups are and how close they can get in space.
Beginning with Ethane, we are able to draw numerous different diagrams that offer information on the molecule’s shape. the first is the simple structure (a), which quickly tells us which atoms are involved but not much about the shape. The side-on “wedge-dash” picture (b) is much more informative because we can assess how each atom is orientated relative to the rest of the molecule. If we turn this structure slightly, to result in (c), we begin to view the “front” carbon relative to the “back” carbon and can assess relative orientations of all atoms involved. Taking this further, to (d), we begin to view what will become a Newman projection, which are invaluable in assessing relative stabilities of shapes that are a consequence of rotation about the central C-C bond.
Newman depictions are drawn “head-on” down a chosen C-C bond axis so that the front carbon of that bond may be seen but the rear carbon cannot. In projection (a) all of the atoms are staggered and none overlap. If substituents other than hydrogen are attached (X) then this shape is known as anti with the two larger X groups being as far away as possible. This will be the most stable conformation (shape) since there is no interaction between the two large X groups. When we rotate around the central C-C bond we keep one carbon in place, in this case at the back, and move the other. Rotating by 60o gives us conformation (b), which is less stable because atoms/groups at the front and back are starting to interact. This is an eclipsed conformation. Rotating further gives us conformation (c), which is a second staggered shape but less stable than (a) because the X groups are getting closer. This is a gauche conformation. When we rotate to (d) we get the least stable anti shape in which both X groups are overlapping. If we rotate further we get the same conformations repeating themselves. While the rotation around the C-C bond is continuous, there will be barriers to rotation and staggered conformations will generally be preferred.
If we plot the angle of rotation around the central C-C bond against the potential energy of the resulting shapes we get the graph below, which conveniently shows favoured and unfavoured conformations. Anti is lowest energy, since the larger CH3 groups are far apart. Rotation of the front carbon of the central C-C bond through 60o then causes destabilizing eclipsing interactions, which are relieved by further rotation to 120o to give the gauche conformation. Through to 180o causes significant interaction of the CH3 groups in the second eclipsed conformer. Carrying on through 360o creates the mirror image conformations on the way back to anti. If we swap out the CH3 groups for anything else larger than H we get essentially the same graph that only differs by the energy increments.
The models below represent the most stable staggered, the gauche staggered, and anti conformations of butane focused along the C2-C3 bond. Move the models around to get an idea of the relationships between the from and back CH3 groups and H atoms.