Cyclohexane monosubstituted cyclohexanes, structure

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... 

In the trans isomer, one methyl is written down (dotted bond) whilst the other is written up (wedged bond). If we transform this to a chair conformation, as shown in the left-hand structure, the down methyl will be equatorial and the up methyl will also be equatorial. With ring flip, both of these substituents then become axial as in the right-hand conformer. From what we have learned about monosubstituted cyclohexanes, it is now easily predicted that the diequatorial conformer will be very much favoured over the diaxial conformer. 

The six axial bonds are directed upward or downward from the plane of the ring, while the other six equatorial bonds are more within the plane. Conversion of one chair form into another converts all axial bonds into equatorial bonds and vice versa. In monosubstituted cyclohexanes, for electronic reasons, the more stable form is usually the one with the substituent in the equatorial position. If there is more than one substituent, the situation is more complicated since we have to consider more combinations of substituents which may interact. Often the more stable form is the one with more substituents in the equatorial positions. For example, in ct-1,2,3,4,5,6-hexachlorocyclohexane (see above) four chlorines are equatorial (aaeeee), and in the /Tisomer all substituents are equatorial. The structural arrangement of the /3-isomer also greatly inhibits degradation reactions [the steric arrangement of the chlorine atoms is unfavorable for dehydrochlorination (see Chapter 13) or reductive dechlorination see Bachmann et al. 1988]. 

Identify the more stable conformation of a monosubstituted cyclohexane also, identify substituents as axial or equatorial when the structure is flipped from one chair conformation to another. 

Two MW investigations of iodine-substituted cyclohexane and adamantane apply several geometric constraints in the structure analyses which result in large values for the estimated experimental uncertainties for the C—I bond distances. The bond lengths of ca 216-217 pm correspond to that in ICHMe2 [216.7 (2) pm] and similar observations were made for C—Cl and C—Br bond lengths in monosubstituted unstrained saturated rings. 

In Kekule s time, cyclohexane was unknown, and there was no proof that benzene must be a six-membered ring. Determination of the structure relied largely on the known numbers of monosubstituted and disubstituted benzenes, together with the knowledge that benzene did not react like a normal alkene. The following 0,110 structures were the likely candidates ... 

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