Methyl groups in methylcyclohexanes

We can relate the confonnational preference for an equatorial methyl group in methylcyclohexane to the confonnation of a noncyclic hydrocarbon we discussed earlier, butane. The red bonds in the following structural formulas trace paths through four-carbons, beginning at an equatorial methyl group. The zigzag anangernent described by each path mimics the anti confonnation of butane. 

Fig. 1.9 Different representations of a methyl group in methylcyclohexane (From Hoffmann and Laszlo, 1991)...

Which position is more stable for the methyl group in methylcyclohexane an equatorial position or an axial position Explain your answer. 

With the stereogenic center in the -position the induced diastereoselectivity is controlled by the preferential equatorial orientation of the substituent in a chair-like transition structure. However, the selectivity is higher (96.3 3.7) than could be anticipated from a simple comparison with the ratio of the equatorial-ly and axially orientated methyl group in methylcyclohexane (95 5). 

Route 1 involves the substitution of a hydrogen atom on the methyl group in methylcyclohexane (Structure 2.9) by bromine. However, there are 14 hydrogen atoms in the molecule (Figure 2.2), and other brominated products are possible, such as the four shown below. In practice, the direct bromination reaction shown in Route 1 would probably give a mixture of all of the possible brominated products. It would then be difficult to separate the products, and chemicals would be wasted. 

We can relate the conformational preference for an equatorial methyl group in methylcyclohexane to the conformation of butane. The red bonds in the following structural... 

Additivity Rule for Estimating C-Chemical Shifts for Methyl Groups in Methylcyclohexanes <6 in ppm relative to TMS)... 

Figure 4.14 The origin of 1,3-diaxial interactions in methylcyclohexane. The steric strain between an axial methyl group and an axial hydrogen atom three carbons away is identical to the steric strain in gauche butane. Note that the -CH3 group in methylcyclohexane moves slightly away from a true axial position to minimize the strain.

Even using the same style, chemists can represent a simple methyl group in different ways, as demonstrated in the example of methylcyclohexane (Fig. 1.9). A chemist will interpret these diagrams, usually subconsciously, as all being equivalent in that they all represent the same molecule, methylcyclohexane. 

The two chair conformations of methylcyclohexane interconvert at room temperature, so the one that is lower in energy predominates. Careful measurements have shown that the chair with the methyl group in an equatorial position is the most stable conformation. It is about 7.6 kJ/mol (1.8 kcal/mol) lower in energy than the conformation with the methyl group in an axial position. Both of these chair conformations are lower in energy than any boat conformation. We can show how the 7.6 kJ energy difference between the axial and equatorial positions arises by examining molecular models and Newman projections of the two conformations. First, make a model of methylcyclohexane and use it to follow this discussion. 

In methylcyclohexane, 95% of molecules have the methyl group in the equatorial position. 

A nickel-on-kieselguhr catalyst containing about 70% of nickel in the reduced form, similar to the catalyst used by Haensel, was employed in the initial work. Since the removal of the methyl group from methylcyclohexane is considerably more difficult than removal of an unbranched methyl from a paraffin, the catalyst was pretreated for maximum activity by reducing in a stream of hydrogen for 15 hours at 700°F. 

In Section 2.10, we saw that the gauche interaction between the methyl groups of butane caused the gauche conformer to be 0.9 kcal/mol (3.8 kJ/mol) less stable than the anti conformer. Because there are two such gauche interactions in the chair conformer of methylcyclohexane when the methyl group is in an axial position, this chair conformer is 1.8 kcal/mol (7.5 kJ/mol) less stable than the chair conformer with the methyl group in the equatorial position. 

A eq) for the conformers of methylcyclohexane indicates that 95% of methylcyclo-hexane molecules have the methyl group in the equatorial position at 25 °C ... 

Also interesting is the following combination of two conformational effects. The Ph group in phenylcy-clohexane has a larger A-value (2.9) than the Me group in methylcyclohexane (1.8). However, in an apparent paradox, the Ph group is forced into the axial position in 1-methyl-1-phenylcyclohexane (Figure 6.16). These observations further illustrate that simple steric models based on effective volume of substituents cannot explain the full diversity of conformational profiles of substituted cyclohexanes. 

If we use a Newman projection that lets us view down two carbon-carbon bonds at the same time to compare the two conformations of methylcyclohexane (Figure 3.22), we see that the axial methyl and a carbon-carbon bond on an adjacent carbon atom are gauche with respect to each other. We can also describe the interaction of the axial methyl with the third carbon atom away from the point of attachment of the methyl group in a different way. 

FIGURE 4.19 (a) The conformations of methylcyclohexane with the methyl group axial (I) and equatorial (II). (b) 1,3-Diaxial interactions between the two axial hydrogen atoms and the axial methyl group in the axial conformation of methylcyclohexane are shown with dashed arrows. Less crowding occurs in the equatorial conformation. 

The strain caused by a 1,3-diaxial interaction in methylcyclohexane is the same as the strain caused by the close proximity of the hydrogen atoms of methyl groups in the gauche form of butane (Section 4.9). Recall that the interaction m gauche-h xtda t (called. 

Draw chair methylcyclohexane, with the methyl group in the axial position. 

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