3.12: Stability of Substituted Cyclohexanes
This lesson discusses the stability of substituted cyclohexanes with a focus on energies of various conformers and the effect of 1,3-diaxial interactions.
The two chair conformations of cyclohexanes undergo rapid interconversion at room temperature. Both forms have identical energies and stabilities, each comprising equal amounts of the equilibrium mixture. Replacing a hydrogen atom with a functional group makes the two conformations energetically non-equivalent.
For example, in methylcyclohexane, the CH3 group occupies an axial position in one chair conformation and an equatorial position in another. This leads to an increase in energy of the axial conformation to approximately 7.6 kJ mol−1, making the equatorial conformation more stable with an abundance of 95%.
The reason for such variations in energy and stability is that the methyl hydrogens experience repulsive dispersion interactions with the two parallel and closely positioned axial hydrogens on the same side of the ring. Since the steric strain originates between groups on C1 and C3 or C5, it is called a 1,3-diaxial interaction. These interactions, when shown with the Newman projection, exhibit a gauche relationship. However, if the methyl group is positioned equatorially, it is placed anti to C3 and C5, minimizing the steric repulsion.
As the size of a functional group increases, 1,3-diaxial interactions become more pronounced, increasing the energy difference between the two conformations.