1.4- Dimethylcyclohexane, conformational isomers

Draw the conformational isomers of cis-1,2-dimethylcyclohexane and cis-3,4-dimetliylcyclohexanone. While the cyclohexane conformers are of equal energy, the cyclohexanone conformers are not. Indicate which con-former is favored and explain why. 

Fig. 2.4 Conformational isomers of n-butane (lower row dimethylcyclohexane (only hydrogen atoms adjacent to...

The equihbrium between the conformational isomers of (Z)-1,2-dimethyl-cyclohexane (Figure 4.23a,b) and ( )-l,2-dimethylcyclohexane (Figure 4.23 c,d) demonstrates the interconversions. 

Figure 4.27. Representations of ( )-l,3-dimethylcyclohexanes. The representations (a) and (bX as well as (c) and (d), are, respectively, pairs of nonsuperposable mirror images (enantiomers). They cannot be interconverted by ring flexing, rotation, and so on. However, the pairs (a) and (c), as well as (b) and (d), are conformational isomers. The hydrogens attached to C2 and C4-Ce, inclusive, have been omitted for clarity.

Note that the two conformational isomers of m-l,2-dimethylcyclohexane are enantiomers. The equilibration between the two conformations produces a racemic mixture of the two enantiomers. Have we seen a similar situation before Indeed we... 

PROBLEM 5.21 Estimate the energy difference between the two conformational isomers of cw-l,3-dimethylcyclohexane. 

Disubstituted cyclohexanes can exist as cis-trans isomers as well as axiaEequatorial conformers. Two isomers are predicted for 1,4-dimethylcyclohexane (see Fig. 1.9). For the trans isomer the diequatorial conformer is the energetically favorable form. Only one cis isomer is observed, since the two conformers of the cis compound are identical. Interconversion takes place between the conformational (equatoriaEaxial) isomers but not configurational (cis-trans) isomers. 

Certain dimethylcycloalkanes contain a plane of symmetry. For example, both chair conformers of crs-l,3-dimethylcyclohexane possess a plane of symmetry bisecting the molecule through C-2 and C-5. The trans isomer does not have any element of symm and is chiral. 

Since compounds with alkyl equatorial substituents are generally more stable, trans-1,2 compounds, which can adopt the ee conformation, are thermodynamically more stable than their cis-1,2 isomers, which must exist in the ae conformation. For the 1,2-dimethylcyclohexanes, the difference in stability is about 2kcal moP (8 kJ mol" ). Similarly, trans-1,4 and cis-1,3 compounds are more stable than their stereoisomers. 

Although analysis of the consequences of ring flip in a monosubstituted cyclohexane is pretty straightforward, the presence of two or more substituents requires careful consideration to decide which conformer, if any, is the more favoured. Let us illustrate the approach using 1,4-dimethylcyclohexane. Now, two configurational isomers of this structure can exist, namely trans and... 

Conformational mobihty, such as we get in cyclohexane rings, makes the analysis more difficult, and manipulating molecular models provides the clearest vision of the relationships. Let us look at 1,2-dimethylcyclohexane as an example. Again, we have met the cis and trans isomers when we looked at conformational aspects (see Section 3.3.2). Here, we need to consider both configuration and conformation. 

Problem 9.17 Give your reasons for selecting the isomers of dimethylcyclohexane shown in Figs. 9-8 to 9-12 that exist as (a) a pair of configurational enantiomers, each of which exists in one conformation (ft) a pair of conformational diastereomers (c) a pair of configurational enantiomers, each of which exists as a pair of conformational diastereomers (d) a single conformation (c) a pair of conformational enantiomers. ... 

Problem 9.23 The planar structure of c -l,2-dimethylcyclohexane, which is meso, shows a plane of symmetry [Problem 9.4(e)). (a) Is the chair conformer achiral (f>) Why is this isomer optically inactive -4... 

Construct cis- 1,2-dimethylcyclohexane by placing one —CH3 group axial and the other equatorial. Do ring flips and examine the two chair conformations. Which is the more stable conformation Explain your answer (11a). Given the two isomers, trans- 1,2-dimethylcyclohexane and cis- 1,2-dimethylcyclohexane, which is the more stable isomer Explain your answer (lib). 

For /rani,-l,3-dimethylcyclohexane, one methyl is axial and one methyl is equatorial in either conformation. Both conformations have 1.7 kcal/mol (7.1 kJ/mol) of strain energy, and the equilibrium constant for their interconversion is 1.0. The /rans-isomer is less stable than the civ-isomer by 1.7 kcal/mol (7.1 kJ/mol) because of this axial methyl group. 

One classic example is an experiment reported by Doering and Roth in 196217 (Scheme 1.XIII). Upon heating, racemic 3,4-dimethylhexa-l,5-diene (13) rearranged to a mixture of (2 ,6 )-octa-2,6-diene (90%), (2Z,6Z)-octa-2,6-diene (9%), and a trace amount of (2 , 6Z)-isomer. The experimental results are explained in terms of a six-membered transition state17 (Scheme 1.XIV). Chairlike transition state A is favored over transition state B based on the conformational analysis of 1,2-dimethylcyclohexane, in which the methyl substituents prefer to be in an equatorial position. The observation that 14 Z was formed in only trace amounts indicates that boatlike transition state C is of significantly higher energy than transition state A or B. 

Now we can compare the relative stabilities of the cis and trans isomers of 1,3-dimethylcyclohexane. The most stable conformation of the cis isomer has both methyl groups in equatorial positions. Either conformation of the trans isomer places one methyl group in an axial position. The trans isomer is therefore higher in energy than the cis isomer by about 7.6 kJ/mol (1.8 kcal/mol), the energy difference between axial and equatorial methyl groups. Remember that the cis and trans isomers cannot interconvert, and there is no equilibrium between these isomers. 

The cis isomer of 1,4-dimethylcyclohexane also has two conformations, as shown in Figure 4.17. Because each conformation has one CH3 group axial and one equatorial, they are identical in energy. At room temperature, therefore, the two conformations exist in a 50 50 mixture at equilibrium. 

Let s look at 1,2-dimethyicyclohexane as an example. There are two isomers, m-l,2-dimethylcyclohexane and irons-1,2-dimethylcyclohexane, which must be considered separately. In the cis isomer, both methyl groups are on the same side of the ring, and the compound can exist in either of the two chair conformations shown at the top of Figure 4.22. (It s often easier to see... 

These are represented by 25 and 26, which are the two chair conformations of ra-l,4-dimethylcyclohexane, so called (1) because the bonds between the ring carbons and the attached methyl groups in 25 and 26 are both inclined toward the plane defined above, or (2) because 25 (or 26) is derived from the trans diaxial isomer 17 by a change in configuration at one carbon. 

The stable chair-chair conformation of gauche interactions compared to one in the chair form of frans-1,2-dimethyl-cyclohexane and three in the chair conformations of cis-decalin and cis-l,2-dimethylcyclohexane. The various factors affecting cis-trans equilibrium of the decalins are given in Table III. Both isomers have an... 

Problem 9.14 (a) Draw the possible chair conformational structures for the following pairs of dimethylcyclohex-anes (i) cis- and tram-1,2- (ii) < is- and trims-1,3- (iii) cis- and trans-1,4-. (b) Compare the stabilities of the more stable conformers for each pair of geometric isomers, (r) Determine which of the isomers of dimethylcyclohexane are chiral. ... 

---