Chair conformation, conformational

FIGURE 3 12 The (a) planar (b) envelope and (c) half chair conformations of cyclopentane... 

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

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... 

FIGURE 3 17 A guide to representing the orienta tions of the bonds in the chair conformation of cycio hexane... 

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... 

The most important result of ring inversion is that any substituent that is axial in the original chair conformation becomes equatorial in the ring flipped form and vice versa... 

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... 

Make a molecular model of each chair conformation of methylcyclohexane and compare their energies... 

Wedge and dash drawings fail to show conformation and it s important to remember that the rings of cis and trans 1 2 dimethylcyclohexane exist m a chair conformation This... 

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... 

Both methyl groups are axial less stable chair conformation)... 

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... 

The following are representations of two forms of glucose The six membered ring is known to exist in a chair conformation in each form Draw clear representations of the most stable con formation of each Are they two different conformations of the same molecule or are they stereoisomers Which substituents (if any) occupy axial sites ... 

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... 

Typical carbon-oxygen bond distances m ethers are similar to those of alcohols (—142 pm) and are shorter than carbon-carbon bond distances m alkanes (—153 pm) An ether oxygen affects the conformation of a molecule m much the same way that a CH2 unit does The most stable conformation of diethyl ether is the all staggered anti conformation Tetrahydropyran is most stable m the chair conformation—a fact that has an important bearing on the structures of many carbohydrates... 

Make a molecular model of the chair conformation of 3 D glucopyranose... 

Haworth formulas are satisfactory for representing configurational relationships in pyranose forms but are uninformative as to carbohydrate conformations X ray crystal lographic studies of a large number of carbohydrates reveal that the six membered pyra nose ring of D glucose adopts a chair conformation... 

All the ring substituents m p D glucopyranose are equatorial m the most stable chair conformation Only the anomenc hydroxyl group is axial m the a isomer all the other substituents are equatorial... 

Other aldohexoses behave similarly m adopting chair conformations that permit the CH2OH substituent to occupy an equatorial orientation Normally the CH2OH group is the bulkiest most conformationally demanding substituent m the pyranose form of a hexose... 

Next convert the Haworth formula to the chair conformation that has the CH2OH group equatorial... 

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... 

Half chair (Section 3 6) One of the two most stable conforma tions of cyclopentane Three consecutive carbons in the half chair conformation are coplanar The fourth and fifth carbon he respectively above and below the plane... 

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 bicyclic compound decahydronaphthalene, or bicyclo[4.4.0]decane, has two fused six-mem-bered rings. It exists in cis and trans forms (see Fig. 1.10), as determined by the configurations at the bridgehead carbon atoms. Both cis- and rran -decahydronaphthaiene can be constructed with two chair conformations. 

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