Cyclohexane conformation
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Cyclohexane conformation is a much studied topic in organic chemistry because of the complex interrelationship between the different conformers of cyclohexane and its derivatives.
Due to the inherent need of the sp³ hybrid orbitals (and therefore the carbon-hydrogen bonds) on tetravalent carbons to reach 109.5°, cyclohexane is not a planar molecule. The chair conformation is a term used for the most stable chemical conformation of a six membered single bonded carbon ring like cyclohexane. Odd Hassel received the Nobel Prize for work on the conformations of cyclohexane.
In the lowest-energy chair conformation, half of the 12 hydrogens are in axial positions, which means their C-H bonds are parallel and appear to stick up and down from the structure, the other half are in equatorial positions; meaning that they are splayed out. In the chair form, a process known as ring flipping is possible, and leads to the axial hydrogens exchanging positions with the equatorial hydrogens . However, the relative direction of the hydrogens to the ring remains the same, so that an 'up' axial hydrogen, when flipped, remains an 'up' equatorial hydrogen.
In stepping round the ring, it can be seen that the up-axial positions alternate with up-equatorial positions, so that for trans-1,2-cyclohexane, the substituents must either both be axial or both be equatorial to remain on opposite sides of the ring. Similarly, for cis-1,2-cyclohexane, the substituent at 1 must be equatorial and the 2 substituent axial, or vice versa. Each conformation will differ in its stability depending upon the identity of the functional groups. Generally, substituents are most stable when in equatorial positions, as in this case there are no 1,3-diaxial interaction between the axial substituent group and any other axial groups on the ring. For example, if there is a methyl group on carbon 1 in an axial position, it will interact with the axial hydrogens on carbon 3 and carbon 5. However, when there are electronegative heteroatoms involved, the opposite may be observed; this is called the anomeric effect.
In addition to the chair conformation (1) with D3d symmetry cyclohexane can also exist in the half-chair (2), twist or twist-boat (3) with D2 symmetry and boat (4) conformers. Only the twist form is isolable as - like the chair form - it represents an energy minima. The boat conformation does not suffer from angle strain but has a higher energy than the chair form due to steric strain resulting from the two axial 1,4-hydrogen atoms. The torsional strain in the boat conformation has a maximum value because all the carbon bonds are eclipsed. Compare this to the chair with all bonds staggered and complete absence of torsional strain and the twist-boat with 2 out 6 bonds partially eclipsed. In the half-chair conformation 4 carbon atoms are located on a plane in which two bonds are fully eclipsed.
The boat and half-chair forms are transition states between the twist forms and the twist and chair forms respectively, and are impossible to isolate. The twist-boat conformation is 5.5 kcal/mol (23 kJ/mol) less stable than the chair conformation. The energies of the two transition states are 6.6 kcal/mol (28 kJ/mol) (boat) and 10.8 kcal/mol (45 kJ/mol) (half chair) higher than that of the chair [#endnote_Gill]. The ring flipping process can now be described with more precision as taking place through a twist-boat conformation and through two half-chair transition states.
The difference in energy between the chair and the twist-boat conformation of cyclohexane can be measured indirectly by taking the difference in activation energy for the conversion of the chair to the twist-boat conformation and that of the reverse isomerization. The concentration of the twist-boat conformation at room temperature is very low (less than 0.1%) but at 1073 kelvins this concentration can reach 30%. The reverse reaction is measured by IR spectroscopy after rapidly cooling cyclohexane from 1073 K to 40 K, freezing in the large concentration of twist-boat conformation.
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.
See also
External links
References
- ↑ Conformational Study of cis-1,4-Di-tert-butylcyclohexane by Dynamic NMR Spectroscopy and Computational Methods. Observation of Chair and Twist-Boat Conformations Gill, G.; Pawar, D. M.; Noe, E. A J. Org. Chem. (Article); 2005; 70(26); 10726-10731. DOI: 10.1021/jo051654z [Abstract]
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