Cyclohexane ring conformation twisted boat

In contrast to the results for monocyclic compounds, in bicyclic compounds the rigidity of bridged structure frequently permits more substantial effects than in acyclic or monocyclic compounds. The bridging locks the cyclohexane ring into the boat and/or twist-bond conformation. For example, bicyclo [2.2.1] heptane and bicyclo [2.2.2] octane have one and two boat cyclohexane rings respectively. The enthalpies of the boat and twist-boat conformers of cyclohexane are 27 and 23 kJ/mol higher than that of the chair conformer. Therefore, bicyclo [2.2.1] heptane and bicyclo [2.2.2] octane have ring strains of 64 and 54 kJ/mol, respectively, as shown in Table 4. 

Examine the geometry of norcarane. What is the conformation of the cyclohexane ring Choose a name (chair, twist boat, half-chair, etc. see Chapter 5, Problem 4) that accurately describes its shape. The bridgehead hydrogens in norcarane are cis. Do you think a trans stereoisomer is possible Explain. 

An interesting case of conformational analysis comes to play when we consider a six-membered ring (cyclohexane). There are many conformations that this compound can adopt. You will see them all in your textbook the chair, the boat, the twist-boat. The most stable conformation of cyclohexane is the chair. We call it a chair, because when you draw it, it looks like a chair ... 

In general, things are simpler than that, much to our advantage. Within the limits set by the precision of the present estimates, structural features like the chair, boat, or twist-boat conformations of cyclohexane rings, as well as the butane-gawc/ze effects or the cis-tmns isomerism of ethylenic compounds leave no recognizable distinctive trace in zero-point plus heat content energies. Indeed, whatever residual, presently... 

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. 

Figure 1.10 The six-membered cyclohexane ring can adopt a series of different conformations. The chair conformation is more stable than the boat or twist because it permits a maximal number of substituents to exist in a staggered conformation relative to their neighbors.

Cyclohexane minimizes its ring strain by being puckered rather than flat. The two extreme conformations are the more stable chair and the less stable boat. The twist-boat conformer is less stable than the chair by about 23 kJ/mol, but is more stable than the boat. It is formed from the boat by moving one flagpole" to the left and the other to the right. See Fig. 9-5. 

The chair conformation of cyclohexane is not rigid. It can convert to a twist boat conformation and then to a new chair conformation in a process termed ring-flipping, as shown Figure 6.14 (not all the hydrogens are shown for clarity). 

These cyclohexane-like rings are characterized by the presence of various non-planar conformations chair, boat, and twist conformations. The starting point when dealing with the stereochemistry of these six-membered rings is therefore conformational analysis. The method of choice is NMR spectroscopy as a result the preferred conformer(s) and the axial/equatorial position of attached substituents at the preferred conformer(s) are obtained. 

A substituent on a cyclohexane ring (in the chair conformation) can occupy either an axial or an equatorial position. In many cases, the reactivity of the substituent depends on whether its position is axial or equatorial. The two possible chair conformations for methylcyclohexane are shown in Figure 3-23. These conformations are in equilibrium because they interconvert at room temperature. The boat (actually the twist boat) serves... 

Cyclohexane sometimes adopts a twist-boat conformation, but never a true boat structure, which represents an energy maximum, But boat structures are important in some bicyclic compounds where the compound simply doesn t have any choice in the conformation it adopts. The simplest compound locked into a boat structure is norbornane. The CH2 bridge has to be diaxial (otherwise it can t reach), which means that the cyclohexane ring part of the structure has no choice but to be a boat,... 

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