Cyclohexane conformational isomers

How to draw conformational isomers and to flip cyclohexane rings... 

Small molecules may also form condis crystals, provided they posses suitable conformational isomers, It is of interest to note that several of the organic molecules normally identified as plastic crystals are probably better described as condis crystals. Their motion was, as already shown in Sect. 5.2.2, not the complete reorientation of the presumed rigid molecule, but rather an exchange between a limited number of conformational isomers. The examples treated in Sect. 5.2.2 are 2,3-dimethyl-butane, cyclohexanol and cyclohexane. 

The chair conformer can undergo conformational isomerism to a second chair conformer which is degenerate in energy with the first. Cyclohexane is thus a dynamic molecule which exists largely in one of two chair isomers. These are the lowest energy conformations. Other higher energy conformations of cyclohexane include the boat form, which is 10.1 kcal/mol (42.3 kJ/mol) above the chair form, and the twist boat form, which lies 3.8 kcal/mol (15.9 kJ/mol) above the chair form. 

Next relative configurations (R,S) are possible for 1,2- or 1,3-disubstituted isomers. (The 1,4 isomer has a plane of symmetry.) The relative stereochemistry can be denoted as cis or trans, depending on whedter the substituents point toward the same side or opposite sides of die ring. Finally, die cyclohexane ring can undergo chair-chair interconversion leading to different conformational isomers. These possibilities are shown for metiiylcyclohexanol in (6.6). 

Viewed another way, if the axial-equatorial energy difference is mainly a function of steric bulk, then it might be used to assess the relative size of various groups. That is, if the energy difference between the two chair conformational isomers of a monosubstituted cyclohexane were measured, it might serve as a... 

Draw the conformational isomers of cis-1,2-dimethylcyclohexane and cis-3,4-dimetliylcyclohexanone. While the cyclohexane conformers are of equal energy, the cyclohexanone conformers are not. Indicate which con-former is favored and explain why. 

The conformation is thus not only free of angle strain but free of torsional strain as well. It lies at an energy minimum, and is therefore a conformational isomer. The chair form is the most stable conformation of cyclohexane, and, indeed, of nearly every derivative of cyclohexane. 

The cyclohexane ring, either alone or as part of a more complex structural unit, occurs in certain natural products and, accordingly, cyclohexane is the most important saturated cyclic hydrocarbon. Two principal conformational isomers exist. The more stable is called the chair conformation 14, and the less stable the boat conformation (see Section 1.9). In both these conformations the C-C-C bond angles are close to the tetrahedral value of 109°28 consequently, cyclohexane has little angle strain Angle strain becomes significant in saturated hydrocarbons if there are meaningful departures from the above value. 

If two conformational isomers are cooled to a temperature low enough to prevent their interconversion by bond rotation or other motion, then they can be described as stereoisomers structures with the same connectivity but with different atomic arrangements in three dimensions. For example, the conformers in (d) are nonsuperimposable mirror images (enantiomers) if bond rotation is not allowed to occur (make a model ). Those in (g) are not enantiomers, but they are still stereoisomers. The temperatures required to freeze out conformational interconversion are very low on the order of -200°C for substituted ethanes and — 100°C for cyclohexanes. Conformers of these types are occasionally referred to as interconvertible stereoisomers. 

CY Dg cyclohexane modelled by the lowest energy conformational isomer of cyclohexane. 

Gunther and Aydin (1981) have investigated the conformational equilibrium (59) in di-cyclohexane. The two conformational isomers [65a] and [65b] with deuterium in the axial and in the equatorial position can be frozen out in the 100 MHz C spectrum below — 80 C. The deuteriated carbons in the two isomers have different intrinsic isotope shifts which are upheld compared to the nondeuteriated carbons. The triplet with the smaller coupling constant was assigned to the carbon with axial deuterium and was shifted 0.0482 ppm to higher held than the triplet caused by the carbon with equatorial deuterium. 

Figure 4.19. Representations of the chair form of cyclohexane in which all but 1 of the 12 protons ( H) have been replaced by deuterium ( H abbreviated here as D ). Both conformational isomers are shown (a) with the equatorial and (b) with the H axial.

The equihbrium between the conformational isomers of (Z)-1,2-dimethyl-cyclohexane (Figure 4.23a,b) and ( )-l,2-dimethylcyclohexane (Figure 4.23 c,d) demonstrates the interconversions. 

Disubstituted cyclohexanes can exist as cis-trans isomers as well as axiaEequatorial conformers. Two isomers are predicted for 1,4-dimethylcyclohexane (see Fig. 1.9). For the trans isomer the diequatorial conformer is the energetically favorable form. Only one cis isomer is observed, since the two conformers of the cis compound are identical. Interconversion takes place between the conformational (equatoriaEaxial) isomers but not configurational (cis-trans) isomers. 

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