What makes a chair conformation stable?

Apr 10, 2016

Physics, thermodynamics, geometry, God?

Explanation:

This is certainly a situation where you should get a set of molecular models, and play around with them. You know that the general formula of an alkane is ${C}_{n} {H}_{2 n + 2}$. If we introduce a ring junction, we remove $2 \times H$ from the formula to give ${C}_{n} {H}_{2 n}$.

The formula for cyclopropane is ${C}_{3} {H}_{6}$, for cyclobutane, ${C}_{4} {H}_{8}$. These rings are quite rigid in that the $\angle C - C$ bond angles are rigidly constrained (to what values?).

Expand the ring to 6 members, i.e. cyclohexane, and the natural bond angles of $C - C$ and $C - H$ add a great deal of flexibility to the ring. It turns out that a chair conformation minimizes what are called transannular interactions, in that each carbon on the ring as an axial hydrogen projecting out of the plane of the ring, or an equatorial hydrogen, more or less in the plane of the ring.

When the ring flips conformation, the axial substitutent swaps positions with the equatorial substituent. If we put bulky substituents on the ring, $B {u}^{t}$, or even $C {H}_{3}$, the unfavourable interaction between axial substituents predisposes the bulky group to lie, almost exclusively, in an equatorial position.

How to remember all this? Well the best advice is to repeat what I said at the start. Get a set of molecular models, and model the interactions. Is a chair conformation easy to flip? Does a planar ring introduce strain? And how do you represent the 3D structure on the 2D page? This is NOT trivial, and requires some practice to do so unambiguously.

Such molecular models are always permitted in examinations. Your prof (who will certainly have such a set of models on his/her desk) will recommend a set to get. You might even find them in the library.