Cyclohexane structure with substituents

To rationalize the effect of structure and substituents on the acid- or base-catalyzed loss of water from a,/S-dihydroxy-substituted radicals, the complementary techniques of pulse-radiolysis and ESR spectroscopy have been employed in a kinetic study of the dehydration of a variety of a,/9-dihydroxyalkyl radicals [ CR (0H)CR R 0H] into the corresponding carbonyl-conjugated radicals [ CR R C(0)R ]. The overall rates of proton-catalyzed dehydration, as revealed by steady-state (ESR) and time-resolved (pulse-radiolysis) experiments, indicate the importance of the electronic effects of substituents (contrast values of 1.2 x 10 and 9.8 X 10 s for the radicals from cyclohexane-1,2-diol and butane-2,3-diol, respectively, with that for the radicals from erythritol of 4.2 x 10 m s ). Time-resolved experiments enable information to be obtained about the generation of the protonated species [ CR (0H)CR R 0H2 ] and the loss of water from this intermediate. 

The structures of cyclohexane derivatives were also determined by X-ray diffraction techniques. Measurements on the / -isomers of hexachloro-cyclohexane and hexabromocyclohexane verified the ideas of Sachse regarding cyclohexane. The determination of the locations of the halogen atoms in the unit cell showed that these molecules exist in the chair conformation with substituents in so-called equatorial positions as shown in Figure 1.10. 

The crystal structures of cyclic hexoses show a cyclohexane chair with the primary alcohol group in an equatorial position. As an example for a glucose derivative, the crystal structure of 1-decyl—D-glucopyranoside is given here. Five substituents are in equatorial positions, but the long alcohol side chain on the glycosidic carbon atom is axial (a). The cyclohexane unit has about double the width of the alkyl chain and the latter therefore interdigitate in the crystal (Fig. 4.2.10). 

Finally, cyclohexane derivatives show different coupling constants for protons that are axial or equatorial, as indicated in Figure 14.36. Analysis of structure using coupling constants is a powerful tool, when such data are available. As indicated previously, coupling constant data will be provided occasionally with homework problems, but only for relatively simple cases (cis-, mns-alkenes, cis-, ra/ s-cyclohexanes, and some substituent patterns on benzene rings). 

As shown in Fig. 5, the reagent may approach perpendicularly from above or perpendicularly from below the plane of the unsaturated system undergoing attack. The preference for one direction or another depends on the accessibility of the reaction center which, in its turn, is determined by steric and polar factors connected both with the cyclic system containing the unsaturated function and with the attacking reagent. In cyclohexane structures an important role in this is played by axial substituents present in the meta-position to the reaction center. The factor of accessibility favors the formation of products of kinetic control and therefore affects the stereochemistry of the reaction only in the case of nonequilibrium conditions. 

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]. 

When comparing two stereoisomeric cyclohexane derivatives, the more stable stereoisomer is the one with the greater number of its substituents in equatorial orientations. Rewrite the structures as chair conformations to see which substituents are axial and which are equatorial. 

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