Cyclohexanes conformation isomerisms chair

This boat-chair difference is not immediately obtainable from experiment, e.g. the high-temperature conformational isomerization—and then quenching—of cyclohexane does not result in a chair — boat interconversion, but rather chair — twist-boat M. Squillacote, R. S. Sheridan, O. L. Chapman and F. A. L. Anet, 7. Am. Chem. Soc., 97, 3244 (1975). 

The cis-trans isomerism of cyclohexane derivatives (Section 5-1 A) is complicated by conformational isomerism. For example, 4-tert-butylcyclohexyl chloride theoretically could exist in four stereoisomeric chair forms, 1, 2, 3, and 4. 

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. 

Conformational isomerism in cycloalkyl and cycloheteroalkyl structures is characterized by several different conformational extremes. For example, cyclohexane systems can exist in three distinct conformations boat, twist boat, and chair. Of these, the chair form is the most stable conformation because steric... 

A further common example of conformational isomerism is the chair and boat forms of cyclohexane (Figure 20.58). The two forms are able to flip between each other but the boat form is under more strain. The bond angles are close to tetrahedral, so there is little angle strain, but the boat form of cyclohexane does have eclipsed bonds on four of its carbon atoms. This eclipsing produces a significant amount of torsional strain. More importantly, the close contact of the flagpole hydrogens at either end of the molecule destabilizes the boat conformation. 

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. 

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... 

These considerations apply to all cycloalkane derivatives, including steroids. However, the chair form of a ring is inherently more stable than the boat form. Moreover, the fnsed-ring natnre of the system lends it a very considerable rigidity, and cis-trans isomerization wonld necessitate the breaking and formation of covalent bonds. Therefore, steroid snbstitnents maintain their conformation at room temperature, whereas cyclohexane substituents usually do not. Steroids are classified according to their substituents in addition to their occurrence. 

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... 

Know the meaning of chair conformation of cyclohexane, equatorial, axial, geometric or cis-trans isomerism, conformational and configurational isomerism. 

The loss of rotational freedom when the cyclic transition state is formed from the acyclic starting material accounts for the observed negative entropies and volumes of activation. It has been established that the transition state of the closely related Cope rearrangement (see below) resembles the chair conformation of cyclohexane. An analogous transition state structure for the Claisen rearrangement would account for the fact that isomerization of vinyl a-methylallyl ether yields 97% trans 4-hexQm and 3% cw-4-hexenal, since there should be a preference for an equatorial orientation of the methyl group in a cyclohexane chair-like transition state . 

C(5) and C(10) are displaced by —0.25 and —0.88 A from the plane of the lactone group. The y-lactone ring, which is rraoj-fused to the cyclohexane ring, adopts a non-planar half-chair conformation, with C(Q and C(7) respectively 0.31 and —0.36 A distant from the plane of the other four atoms. The analysis of isocollybolide, which has additionally defined the structure of the isomeric compound cxrllybolide (74), reveals that in this... 

Conformations of Alkanes and Cycloalkanes HOW TO Draw Alternative Chair Conformations of Cyclohexane 2- Cis,Trans Isomerism in Cycloalkanes and Bicycloalkanes HOW TO Convert Planar Cyclohexanes to Chair Cyclohexanes 2-7 Physical Properties of Alkanes and Cycloalkanes 2-8 Reactions of Alkanes... 

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