Conformations of Monosubstituted Cyclohexanes

Test your knowledge of Key Ideas by using resources in ThomsonNOW or by answering end-of-chapter problems marked with . 

Even though cyclohexane rings rapidly flip between chair conformations at room temperature, the two conformations of a monosubstituted cyclohexane aren t equally stable. In methylcyclohexane, for instance, the equatorial conformation is more stable than the axial conformation by 7.6 kj/mol (1.8 kcal/mol). The same is true of other monosubstituted cyclohexanes a substituent is almost always more stable in an equatorial position than in an axial position. 

Fig ure 4.13 Interconversion of axial and equatorial methylcyclohexane, as represented in several formats. The equatorial conformation is more stable than the axial conformation by 7.6 kJ/mol. 

Problem 4.15 What is the energ difference between the axial and equatorial conformations of cyclohexanol (hydroxycyclohexane)  

Problem 4.16 Why do you suppose an axial cyano (-CN) substituent causes practically no 1,3-diaxial steric strain (0.4 kJ/mol) Use molecular models to help with your answer. 

The energy difference between axial and equatorial conlorniatioiis is due lo steric strain caused by 1,3-diaxiaI interactions. The axial metliyl group on Cl is too clo.se to the axial hydrogens three car x)tis away on C3 and C5, resulting in 7.6 kj/rnol of steric strain (1-igure 4.13). 

Problem 4.16 Why do you suppose an axial cyano (—CN) substituent causes practically no 

3-diaxial steric strain (0.4 kJ/mol) Use molecular models to help with your answer. 

You might recall from your general chemistry course that it s possible to calculate the percentages of two isomers at equilibrium using the equation = -RT 

FIGURE 4.12 A plot of the percentages of two isomers at equilibrium versus the energy difference between them. The curves are calculated using the equation A = —RTIn K. 

CHAPTER 4 ORGANIC COMPOUNDS CYCLOALKANES AN D TH El R STEREOCH EM ISTRY 

Equatorial methylcyclohexane, however, has no such interactions and is therefore more stable. 

Practice Problem 4.2 Draw 1,1-dimethylcyclohexane, indicating which methyl group is axial and which is equatorial. 

Problem 4.11 Draw two different chair conformations of cyciohexanol (hydroxycyclohexane), show- 

Problem 4.12 A cts-l,2-di8ubstituted cyclohexane, such as cis-l,2-dichlorocycIohexane, must have one group axial and one group equatorial. Explain. 

Problem 4.13 A trans-l,2-disubslitutcd cyclohexane must either have both groups axial or both groups equatorial. Explain. 

The cyclohexane chair just drawn has the headrest to the left and the footrest to the right. Draw a cyclohexane chair with its axial and equatorial bonds, having the headrest to the right and the footrest to the left. 

Draw 1,2,3,4,5,6-hexamethylcyclohexane with all the methyl groups (a) in axial positions. (b) in equatorial positions. 

If your cyclohexane rings look awkward or slanted when using the analytical approach just shown, then try the artistic approach Draw a wide M, and draw a wide W below it, displaced about half a bond length to one side or the Other. Connect the second atoms and the fourth atoms to give the cyclohexane ring with four equatorial bonds. 

The other two equatorial bonds are drawn parallel to the ring connections. The axial bonds are then drawn vertically. 

Chair-chair interconversion of methylcyclohexane. The methyl group is axial in one conformation, and equatorial in the other. 

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Ring-flip in chair conformation of monosubstituted cyclohexane... 

Table 6.1 A Values Free-Energy Differences between Axial and Equatorial Conformations of Monosubstituted Cyclohexanes (kcal/mol)...

In opposition to a central principle of alicyclic conformational analysis, Kang and Yin reported that a complex O-cyclohexyl nitronate and the corresponding O-cyclohexyloxime constitute the first stable axial conformer of monosubstituted cyclohexanes at ambient temperature (97JA8562). Snyder and co-workers (99JA11864) reevaluated the corresponding... 

Problem-Solving Strategy Drawing Chair Conformations 116 3-14 Conformations of Monosubstituted Cyclohexanes 117 3-15 Conformations of Disubstituted Cyclohexanes 120... 

Conformations of Cyclobutane and Cyclopentane Conformations of Cyclohexane 127 Axial and Equatorial Bonds in Cyclohexane 129 Conformational Mobility of Cyclohexane 131 Conformations of Monosubstituted Cyclohexanes Conformational Analysis of Disubstituted Cyclohexanes Boat Cyclohexane 140 Conformations of Polycyclic Molecules 141... 

As Table 1.1 shows, fluorine is the second smallest element, with size approximately 20% larger than the smallest element, hydrogen. Table 1.2 summarizes four steric parameters for various elements and groups (i) Taft steric parameters Es [44], (ii) revised Taft steric parameters E [45], (iii) Charton steric parameters o [46], and (iv) A values [47], The steric parameters, Es, E, and u are determined on the basis of relative acid-catalyzed esterification rates, while the A values are derived from the Gibbs free energy difference calculated from the ratios of axial and equatorial conformers of monosubstituted cyclohexanes by NMR. 

We have seen that when a cyclohexane ring flips, all equatorial bonds become axial and all axial bonds become equatorial. Now lets consider the consequences of flipping a substituted cyclohexane ring. The chair—chair interconversion of monosubstituted cyclohexanes occurs very rapidly. However, the two conformations of monosubstituted cyclohexanes, unlike those of cyclohexane, are not equally stable. 

Compare models of the chair conformations of monosubstituted cyclohexanes in which the substituent alkyl groups are methyl, ethyl, isopropyl, and tert-butyl. 

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