Methyl group conformation

An interesting sidelight is the information revealed about the methyl group conformation there are two anionic possibilities which have C2V and Cg point symmetries. Viewed down the nitrogen atom chain they appear as ... 

Vibrational overtone spectroscopy has been applied to methyl-group conformational analysis in aromatic molecules (Henry, 1987). In addition to conformational data, very accurate information on the C—H bond lengths are obtained, showing that the methyl C—H bond eclipsing the ring plane is slightly shorter than the other methyl C—H bonds, in excellent agreement with ah initio calculations. 

Somewhat special is the position of polyisobutylene, which is due to the crowding of the two bulky methyl groups on the a carbon. Stereoregularity considerations are irrelevant in this case. Because of the crowding of the methyl groups, conformational states are separated by low barriers, and thus under normal conditions, at room temperature without stress, the polymer behaves like a rubber. When oriented by drawing, the chain shows a tendency to crystallize, with a chain conformation of an 83 helix, which is near to the tg conformation [41]. This behavior is a result of the regularity of the chain. 

The log(k/ko) value for 4-isopropyl-2,5-dimethylthiazole is twice that expected if the curve were linear, which implies a rate constant 6.5 times smaller than expected. This result can be explained by the existence of a privileged conformation, induced by the presence of the methyl group in the 5-position and that has a lower reactivity (258). This result also leads to a limitation in the use of Tafts Eg parameter to cases where the environment of a substituent does not induce particular conformation for this latter (258). 

FIGURE 3 6 The gauche and anti conformations of butane shown as ball and spoke mod els left) and as Newman projections right) The gauche conformation is less stable than the anti because of the van der Waals strain between the methyl groups... 

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... 

We can relate the conformational preference for an equatorial methyl group m methylcyclohexane to the conformation of a noncyclic hydrocarbon we discussed ear her butane The red bonds m the following structural formulas trace paths through four carbons beginning at an equatorial methyl group The zigzag arrangement described by each path mimics the anti conformation of butane... 

When the methyl group is axial each path mimics the gauche conformation of butane... 

Their heats of combustion (Table 3 2) reveal that trans 1 4 dimethylcyclohexane is 7 kJ/mol (17 kcal/mol) more stable than the cis stereoisomer It is unrealistic to believe that van der Waals strain between cis substituents is responsible because the methyl groups are too far away from each other To understand why trans 1 4 dimethylcyclo hexane is more stable than cis 1 4 dimethylcyclohexane we need to examine each stereoisomer m its most stable conformation... 

The methyl groups are described as cis because both are up relative to the hydrogen present at each carbon If both methyl groups were down they would still be cis to each other Notice that ring flipping does not alter the cis relationship between the methyl groups Nor does it alter their up versus down quality substituents that are up m one conformation remain up m the ring flipped form... 

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... 

Orientation of methyl groups m most stable conformation Heat of combustion heat of combustion More stable stereoisomer... 

Both methyl groups are equatorial m the most stable conformation of trans 1 2 dimethyl cyclohexane... 

Recall from Section 3 10 that the equatorial conformation of methylcyclohexane is 7 kJ/mol (1 7 kcal/mol) lower in energy than the confer mation with an axial methyl group... 

The most stable conformation of cis 1 3 dimethylcyclohexane has both methyl groups equatorial... 

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

If a disubstituted cyclohexane has two different substituents then the most stable conformation is the chair that has the larger substituent m an equatorial orientation This IS most apparent when one of the substituents is a bulky group such as tert butyl Thus the most stable conformation of cis 1 tert butyl 2 methylcyclohexane has an equatorial tert butyl group and an axial methyl group... 

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