Chair conformations placing groups

D i me thy I eye I ohex anc does not have a conformation with a 1,3-diaxial interaction between two methyl groups. Either of its chair conformations places one methyl group in an axial position and one in an equatorial position. These conformations have equal energies, and they are present in equal amounts. 

There is another aspect to the question of the reactivity of the carbonyl group in r ck)hexanone. This has to do with the preference for approach of reactants from the axial ir equatorial direction. The chair conformation of cyclohexanone places the carbonyl coup in an unsynunetrical environment. It is observed that small nucleophiles prefer to roach the carbonyl group of cyclohexanone from the axial direction even though this is 1 more sterically restricted approach than from the equatorial side." How do the ctfcnaices in the C—C bonds (on the axial side) as opposed to the C—H bonds (on the equatorial side) influence the reactivity of cyclohexanone ... 

Optimization of the valence and dihedral angles yields planar cyclic structures for the 3- to 5-ring intermediates in contrast to a chair conformation for that of the 6-ring. In the cases of n = 4, 5, 6 the oxygen atom is placed almost in the plane of the three C-atoms directly bonded to it. Therefore, an intramolecular solvation of the cationic chain end by methoxy groups which are bonded to the polymer backbone is preferred in the gas phase. The calculations show that for a non-polar solvent such as CH2C12 a decrease in stability of the cyclic intermediates exists. But this decrease does not result in a total break of the intramolecular solvation (Table 13). An equilibrium between open chain and cyclic intermediates must only be taken into account in more polar solvents, due to the competition of intra- and intermolecular solvation. 

SAMPLE SOLUTION (a) The most stable conformation is the one that has the larger substituent, the fe/t-butyl group, equatorial. Draw a chair conformation of cyclohexane, and place an equatorial tert-butyl group at one of its carbons. Add a methyl group at C-3 so that it is trans to the tert-butyl group. 

A more striking example of the influence of conformation on the reaction outcome is seen in the nitrous acid deamination of 2-aminocyclohexanols which takes place by rearrangement of a group on the carbinol carbon that is anti to the developing carbocation. The deamination reaction is very fast and the products reflect the population of the chair conformers. The trans isomer exists mainly in the diequatorial conformer thus the only group anti to the amino... 

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). 

Construct trrms-1,4-dimethylcyclohexane by placing both —CH3 groups axial. Do ring flips and examine the two chair conformations. Which is the more stable conformation Explain your answer (14). 

Most molecules tend to favor one conformer over the others based on the stereochemistry of the particular monosaccharide and the steric bulk of the groups that are appended to it. For example, most aldohexoses prefer the chair conformation that places the bulky C5 hydroxymethyl group in the equatorial position. Having said that, the energy barrier between the two possible chair conformations is... 

An E2 elimination can take place on this chair conformation only if the proton and the leaving group can get into a trans-diaxial arrangement. Figure 7-9 shows the E2 dehy-drohalogenation of bromocyclohexane. The molecule must flip into the chair conformation with the bromine atom axial before elimination can occur. 

In a chair conformation of a cyclohexane ring, a trans-diaxial arrangement places the two groups anti and coplanar. 

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