Cyclohexane Conformation - Cyclohexane Derivatives

Cyclohexane Derivatives

Substituents found on cyclohexane adopt cis and trans formations and cannot be easily switched by simple single sigma bond rotation as with linear molecules. Cis formation means that both substituents are found on the upper side of the 2 substituent placements on the carbon, while trans would mean that they were on opposing sides. Despite the fact that carbons on cyclohexane are linked by a single bond, the ring remains rigid, in that switching from cis to trans would require breaking the ring. The nomenclature for cis is dubbed (Z) while the name for trans is (E) to be placed in front of the IUPAC name.

For di-substituted cyclohexane rings (i.e. two groups on the ring), the relative orientation of the two substituents affect the energy of the possible conformations. For 1,2- and 1,4-di-substituted cyclohexane, a cis configuration leads to one axial and one equatorial group. This configuration can undergo chair flipping. For 1,2- and 1,4-di-substituted cyclohexane, a trans configuration leads to either both groups axial or both equatorial. In this case, the diaxial conformation is effectively prevented by its high steric strain (four gauche interactions more than the diequatorial). For 1,3-di-substituted cyclohexanes, the cis form is diequatorial and the flipped conformation suffers additional steric interaction between the two axial groups. Trans-1,3-di-substituted cyclohexanes are like cis-1,2- and cis-1,4- and can flip between the two equivalent axial/equatorial forms.

Derivatives of cyclohexane do exist that have a more stable twist-boat conformation. An example is 1,2,4,5-tetrathiane, an organosulfur compound with 4 methylene groups replaced by a sulfide group thus removing unfavorable 1,3-diaxial interactions. In the tetramethyl analogue 3,3,6,6-tetramethyl-1,2,4,5-tetrathiane the twist-boat conformation actually dominates. Also in cyclohexane-1,4-dione with the steric 1,4-hydrogen interaction removed, the actual stable conformation is the twist-boat.

Cis-1,4-di-tert-butylcyclohexane has an axial tert-butyl group in the chair conformation and conversion to the twist-boat conformation places both groups in more favorable equatorial positions. As a result the twist-boat conformation is more stable by 0.47 kcal/mol (1.96 kJ/mol) at 125 K as measured by NMR spectroscopy.

Effect of polar substituent:

• cis-cyclohexane-1,3-diol prefers diaxial conformation "formation of intrahydrogen bond".
• 2,5-di-tert-butyl-1,4-cyclohexanediol present in boat or twist-boat form "also intra-H-bond"
• 2-bromocyclohexanone prefers a-Br "min.dipolar repulsion"
• 2-bromo-4,4-dimethylcyclohexanone prefers e-Br "1,3 diaxial interaction(-ve in e-Br) more than

dipolar repulsion:

• trans-1,2-dibromocyclohexane present in axial form in non-polar solvents "dipoles cancel"

while present in equtorial form in polar solvents "dipoles reinforce".