Conformational Mobility of Cyclohexane

A ring-flip in chair cyclohexane interconverts axial and equatorial positions. 

A chair cyclohexane can be ring-flipped by keeping the middle four carbon atoms in place while folding the two ends in opposite directions. An axial. substituent in one chair form becomes an equatorial substituent in the ring-flipped chair form, and vice versa. For example, axial bromocyclohexane becomes equatorial bromocydohexane after ring-flip. Since the energy barrier to chair-chair interconversion is only about 45 kJ/mol (10.8 kcal/mol), the process is extremely rapid at room temperature. We therefore see only what appears to be a single structure, rather than distinct axial and equatorial isomers. 

A procedure for drawing axial and equatorial bonds in chair cyclohexane. 

Axial bonds The six axial bonds, one on each carbon, are parallel and alternate up-down. 

Equatorial bonds. The six equatorial bonds, one on each carbon, come in three sets of two parallel lines. Each set is also parallel to two ring bonds. Equatorial bonds alternate between sides around the ring. 

The interconversion of two chair conformations of cyclohexane changes all equatorial hydrogens to axial hydrogens, and all axial hydrogens to equatorial hydrogens. 

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Conformations of Cyclobutane and Cyclopentane Conformations of Cyclohexane 127 Axial and Equatorial Bonds in Cyclohexane 129 Conformational Mobility of Cyclohexane 131 Conformations of Monosubstituted Cyclohexanes Conformational Analysis of Disubstituted Cyclohexanes Boat Cyclohexane 140 Conformations of Polycyclic Molecules 141... 

Disubstituted cyclopropanes exemplify one of the simplest cases involving stabil ity differences between stereoisomers A three membered ring has no conformational mobility so the ring cannot therefore reduce the van der Waals strain between cis sub stituents on adjacent carbons without introducing other strain The situation is different m disubstituted derivatives of cyclohexane... 

Chair cyclohexanes are conformationally mobile and can undergo a ring-flip, which interconverts axial and equatorial positions. Substituents on the ring are more stable in the equatorial position because axial substituents cause 1,3-diaxial interactions. The amount of 1,3-diaxial steric strain caused by an axial substituent depends on its hulk. 

A complete silicon analog of cyclohexane, cyclohexapermethyl silane, exists in a mobile chair conformation (Bock et al. 1979). In the cation-radical, the inversion rate is much more lower than... 

When one looks at the hydrogens in the chair conformation of cyclohexane, one can see that they are of two types. Six of them are parallel to the central rotational axis of the molecule, so are termed axial. The other six are positioned around the outside of the molecule and are termed equatorial. One might imagine, therefore, that these two t)T>es of hydrogen would have some different characteristics, and be detectable by an appropriate spectral technique. Such a technique is NMR spectroscopy but, at room temperature, only one type of proton is detectable. At room temperature, all hydrogens of cyclohexane can be considered equivalent this is a consequence of conformational mobility, and the interconversion of two chair conformations. 

However, in a cyclohexane system we also need to consider the conformational mobility that generates two different chair forms of the ring (see Section 3.3.2). Let us consider 3-methylcyclohexanecarboxylic acid. This has two chiral centres, and thus there are four configurational stereoisomers. These are the enantiomeric forms of the trans and cis isomers. 

The stereochemistry of enolisation has been mostly examined in cyclic systems where the relative positions of the enolisable hydrogen atoms are fixed. Over the last decade, these studies have benefited from important improvements in the experimental methods, namely mass spectrometry and nmr spectroscopy. Of great interest is the comparison of the relative mobilities of diastereoisomeric axial and equatorial protons from ketones in the cyclohexane series. Indeed, since axial a(C—H) bonds of rigid cyclohexanones are closer than equatorial a(C—H) bonds to the desirable conformation in which the breaking C—H bond is perpendicular to the direction of the C=0 bond, it can be expected that the axial a(C—H) bond-breaking is easier than that of the equatorial one. 

In the illustration shown, the transition state involves a six-membered ring, and it is presumed on structural grounds that it has the thermodynamically favored conformation of the chair form of cyclohexane. Since it also has a mobile -electron system, it seems reasonable to picture it as two allylic radicals interacting with each other via their terminal (carbon atoms 1 and 3) 7r-electrons. For such a structure, the cis diolefin will have its bulky methyl group in the structurally unfavorable quasi-axial position. The higher activation energy of 1.36 kcal/mole observed for the reaction of the cis isomer supports such a structure. 

There is an important difference between the cis- and /rani-decalin systems with respect to their conformational flexibility. Owing to the nature of its ring fusion, trans-decalin is incapable of chair-chair inversion c/x-decalin is conformationally mobile and undergoes ring inversion at a rate only slightly slower than cyclohexane (AG = 12.3-12.4 kcal/mol). The/ranx-decalin system is a conformationally locked system and can be used to compare properties and reactivity of groups in axial or equatorial environments. 

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