Cyclohexane monosubstituted, conformers

In monosubstituted cyclohexanes, two conformers can be discriminated by the equatorial (LXXXa) and axial position (LXXXb) of the substituent. While the energy... 

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... 

In a monosubstituted cyclohexane, the substituent can be either in an equatorial or axial position. Equatorial positions are more spacious and in substituted cyclohexanes they are preferred. Cyclohexane rings flip between chair forms and establish an equilibrium. In the process of flipping, all equatorial positions become axial and all axial positions become equatorial. The equilibrium favors the chair in which substituents are equatorial. In monosubstituted cyclohexanes, the conformation in which the substituent is equatorial is favored. In disubstituted cyclohexanes where one group is axial and one equatorial, the equilibrium favors the chair form where the larger group occupies the more spacious equatorial position. 

Conformational inversion (ring flipping) is rapid in cyclohexane and causes all axial bonds to become equatorial and vice versa As a result a monosubstituted derivative of cyclohexane adopts the chair conforma tion in which the substituent is equatorial (see next section) No bonds are made or broken in this process... 

For the equilibrium between the axial and equatorial conformations of a monosubstituted cyclohexane. 

Because chair cyclohexane has two kinds of positions, axial and equatorial, we might expect to find two isomeric forms of a monosubstituted cyclohexane. In fact, we don t. There is only one methylcyclohexane, one bromocydohexane, one cycJohexanol (hydroxycyclohexane), and so on, because cyclohexane rings are confbnnationally mobile at room temperature. Different chair conformations readily interconvert, exchanging axial and equatorial positions. This interconversion, usually called a ring-flip, is shown in Figure 4.11. 

Even though cyclohexane rings rapidly flip between chair conformations at room temperature, the two conformations of a monosubstituted cyclohexane aren t equally stable. In methylcyclohexane, for instance, the equatorial conformation is more stable than the axial conformation by 7.6 kj/mol (1.8 kcal/mol). The same is true of other monosubstituted cyclohexanes a substituent is almost always more stable in an equatorial position than in an axial position. 

Monosubstituted cyclohexanes are more stable with their substituent in an equatorial position, but the situation in disubstituted cyclohexanes is more complex because the steric effects of both substituents must be taken into account. All steric interactions in both possible chair conformations must be analyzed before deciding which conformation is favored. 

Such interconversions with monosubstituted cyclohexanes and also with disubstituted ones do not involve any rearrangement i.e., no chemical bonds are broken nor reformed, only their conformation changes and this has been confirmed by NMR studies e.g., methyl cyclohexane at -110°C gives separate signals for equatorially or axially oriented methyl groups. 

In a completely different interpretation Zefirov (242) proposed a new concept of frontier-orbital mixing (243) to explain how conformational and electronic effects in monosubstituted cyclohexanes are transmitted to remote 8-carbon atoms (Scheme 36). The orbitals at C(l) and C(4) in 112 are considered to be equatorial (242). A perturbation at C(l) (H is replaced by X) produces an electron-density shift from H(4) toward C(4) (242), which is associated with an upheld shift of the latter s signal. Although this approach appears to be quite crude and does not account for axial substituents, it deserves fiirther attention. 

Although analysis of the consequences of ring flip in a monosubstituted cyclohexane is pretty straightforward, the presence of two or more substituents requires careful consideration to decide which conformer, if any, is the more favoured. Let us illustrate the approach using 1,4-dimethylcyclohexane. Now, two configurational isomers of this structure can exist, namely trans and... 

In the trans isomer, one methyl is written down (dotted bond) whilst the other is written up (wedged bond). If we transform this to a chair conformation, as shown in the left-hand structure, the down methyl will be equatorial and the up methyl will also be equatorial. With ring flip, both of these substituents then become axial as in the right-hand conformer. From what we have learned about monosubstituted cyclohexanes, it is now easily predicted that the diequatorial conformer will be very much favoured over the diaxial conformer. 

Hence A (fra .s) = 4117.24kcal/mol for fra .s-l,4-di- -butylcyclohexane and A /(m) = 4113.72kcal/mol for the cis form. These two results are indicative of ring conformation since cA-l,4-di-t-butylcyclohexane is undoubtedly in a twist-boat form while the other is in chair conformation. The spectra of t-butylcyclohexane (in chair conformation) and of rrani-l,4-di- -butylcyclohexane are indeed very similar, except, of course, for carbon 4, which is the same as carbon 1 in the disubstituted molecule, whereas it is similar to the unsubstimted carbons in the monosubstituted cyclohexane. 

Ring-flip in chair conformation of monosubstituted cyclohexane... 

On each carbon, one bond is directed up or down and the other more or less in the plane of the ring. The up or down bonds are called axial and the others equatorial. The axial bonds point alternately up and down. If a molecule were frozen into a chair form, there would be isomerism in monosubstituted cyclohexanes. For example, there would be an equatorial methylcyclohexane and an axial isomer. However, it has never been possible to isolate isomers of this type at room temperature.219 This proves the transient existence of the boat or twist form, since in order for the two types of methylcyclohexane to be non-separable, there must be rapid interconversion of one chair form to another (in which all axial bonds become equatorial and vice versa) and this is possible only through a boat or twist conformation. Conversion of one chair form to another requires an activation energy of about 10 kcal/mol (42 kJ/mol)220 and is very rapid at room temperature.221 However, by... 

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... 

Table 6.1 A Values Free-Energy Differences between Axial and Equatorial Conformations of Monosubstituted Cyclohexanes (kcal/mol)...

Identify the more stable conformation of a monosubstituted cyclohexane also, identify substituents as axial or equatorial when the structure is flipped from one chair conformation to another. 

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