Chlorocyclohexane, chair conformation

Recall from Section 4.13 that cyclohexane exists as two chair conformations that rapidly interconvert, and that substituted cyclohexanes are more stable with substituents in the roomier equatorial position. Thus, chlorocyclohexane exists as two chair conformations, but A is preferred because the Cl group is equatorial. 

Draw the most stable chair conformation of l-bromo-2-chlorocyclohexane, if the coupling constant between hydrogens on Cl and C2 was found to be 7.8 Hz... 

Draw both chair conformations of chlorocyclohexane one with an axial chlorine and one with an equatorial chlorine. 

PROBLEM 5.45 Draw a chair conformation of ch-l-bromo-3-chlorocyclohexane. Draw the ring-flipped structure. Which is favored Why Is the molecule chiral ... 

Draw the more stable chair conformation of c/s-Ttert-butyl-4-chlorocyclohexane. By how much is it favored ... 

Anti periplanar geometry for E2 reactions is particularly important in cyclohexane rings, where chair geometry forces a rigid relationship between the substituents on neighboring carbon atoms (Section 4.8). As pointed out by Derek Barton in a landmark 1950 paper, much of the chemical reactivity of substituted cyclohexanes is controlled by their conformation. Let s look at the E2 dehydro-halogenation of chlorocyclohexanes to see an example. 

Exercise 12-4 Using the sawhorse convention, draw the possible conformations of chlorocyclohexane with the ring carbons in the planar, in the chair, and in the extreme boat forms. Arrange these in order of expected stability. Show your reasoning. 

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 mono-substituted 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. This proves the transient existence of the boat or twist form, since in order for the two types of methylcyclohexane to be nonseparable, there must be rapid interconversion of one chair form to another (in which aU 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 lOkcalmoP (42kJmol ) and is very rapid at room temperature. However, by working at low temperatures, Jensen and Bushweller were able to obtain the pure equatorial conformers of chlorocyclohexane and trideuteriomethoxycyclo-hexane as solids and in solution. Equatorial chlorocyclohexane has a half-life of 22 years in solution at — 160°C. 

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