Chair conformations cyclohexane

In the chair conformation cyclohexane has two different types of hydrogens. The bonds to one type are parallel to the axis of the ring. These are called axial hydrogens. The axial bonds alternate up and down around the ring. 

The chair form of cyclohexane is considered rigid since it must undergo cycle inversion of relatively high energy to yield other conformers. However, besides the rigid chair conformers, cyclohexane also exists as flexible forms which include the boat conformer LXXIXb and the twist form LXXIXc. In fact, the flexible form gives... 

Using a model of twistane, identify the cyclohexane ring systems held in twist conformations. In adamantane, find the chair conformation cyclohexane systems. How many are present Evaluate the torsional and angle strain in adamantane. Which of the three compounds in this exercise are chiral ... 

Staggered arrangement of bonds in chair conformation of cyclohexane... 

The various conformations of cyclohexane are m rapid equilibrium with one another but at any moment almost all of the molecules exist m the chair conformation Not more than one or two molecules per thousand are present m the skew boat confer matron Thus the discussion of cyclohexane conformational analysis that follows focuses exclusively on the chair conformation... 

We have seen that alkanes are not locked into a single conformation Rotation around the central carbon-carbon bond m butane occurs rapidly mterconvertmg anti and gauche conformations Cyclohexane too is conformationally mobile Through a process known as ring inversion, chair-chair mterconversion, or more simply ring flipping, one chair conformation is converted to another chair... 

A potential energy diagram for nng inversion m cyclohexane is shown m Figure 3 18 In the first step the chair conformation is converted to a skew boat which then proceeds to the inverted chair m the second step The skew boat conformation is an inter mediate in the process of ring inversion Unlike a transition state an intermediate is not a potential energy maximum but is a local minimum on the potential energy profile... 

Ring inversion in methylcyclohexane differs from that of cyclohexane m that the two chair conformations are not equivalent In one chair the methyl group is axial m the other It IS equatorial At room temperature approximately 95% of the molecules of methylcyclohexane are m the chair conformation that has an equatorial methyl group whereas only 5% of the molecules have an axial methyl group... 

The following questions relate to a cyclohexane ring depicted in the chair conformation shown... 

The most stable conformation of trans 1 4 dimethylcyclohexane has both methyl groups in equatorial orientations The two chair conformations of trans 1 4 dimethyl cyclohexane are not equivalent to each other One has two equatorial methyl groups the other two axial methyl groups... 

Section 3 7 Three conformations of cyclohexane have approximately tetrahedral angles at carbon the chair the boat and the skew boat The chair is by far the most stable it is free of torsional strain but the boat and skew boat are not When a cyclohexane ring is present m a compound it almost always adopts a chair conformation... 

The C—H bonds in the chair conformation of cyclohexane are not all equivalent but are divided into two sets of six each called axial and equatorial... 

Make molecular models of the two chair conformations of cis 1 tert butyl 4 phenyl cyclohexane What is the strain energy calculated for each conformation by molecular mechanics Which has a greater preference for the equatorial onentation phenyl or tert butyD... 

Axial bond (Section 3 8) A bond to a carbon in the chair conformation of cyclohexane oriented like the six up and down bonds in the following... 

The 12 hydrogen atoms of cyclohexane do not occupy equivalent positions. In the chair conformation six hydrogen atoms are perpendicular to the average plane of the molecule and six are directed outward from the ring, slightly above or below the molecular plane (see Fig. 1.6). Bonds which are perpendicular to the molecular plane are known as axial bonds, and those which extend outward... 

FIGURE 1.6 The two chair conformations of cyclohexane a = axial hydrogen atom and e = equatorial hydrogen atom. 

The overall picture of the many results which have been obtained with hetero-substituted cyclohexane rings is a very consistent one. Cyclohexane itself in its lowest energy conformation adopts the so-called chair conformation, as depicted in Figure 3 by the two outer formulae (a, b). These are contained in energy wells ca. 42 kJ moP deep. Another conformation, of low abundance in cyclohexane at normal temperatures, but which is important in some substituted derivatives, is the twist form (c, d). This is ca. 22 kJ moP less stable than the chair forms, and it lies on the lowest-energy pathway between them. 

Incorporation of stereogenic centers into cyclic structures produces special stereochemical circumstances. Except in the case of cyclopropane, the lowest-eneigy conformation of the tings is not planar. Most cyclohexane derivatives adopt a chair conformation. For example, the two conformers of cis-l,2-dimethylcyclohexane are both chiral. However, the two conformers are enantiomeric so the conformational change leads to racemization. Because the barrier to this conformational change is low (lOkcal/mol), the two enantiomers arc rapidly interconverted. 

The most stable conformation of cyclohexane is the chair. Electron diffraction studies in the gas phase reveal a slight flattening of the chair compared with the geometry obtained when tetrahedral molecular models are used. The torsion angles are 55.9°, compared with 60° for the ideal chair conformation, and the axial C—H bonds are not perfectly parallel but are oriented outward by about 7°. The length of the C—C bonds is 1.528 A, the length of the C—H bonds is 1.119 A, and the C—C—C angles are 111.05°. ... 

Substitution on a cyclohexane ring does not greatly affect the rate of conformational inversion but does change the equilibrium distribution between alternative chair forms. All substituents that are axial in one chair conformation become equatorial on ring inversion, and vice versa. For methylcyclohexane, AG for the equilibrium... 

Make a molecular model of the chair conformation of cyclohexane, and turn it so that you can look down one of the C—C bonds. 

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