Hydrogens axial

Van der Waals strain between hydrogen of axial CH3 and axial hydrogens at C 3 and C 5... 

Axial halide is in proper orientation for anti elimination with respect to axial hydrogens on adjacent carbon atoms Dehydrobromination is rapid... 

FIGURE 1.6 The two chair conformations of cyclohexane a = axial hydrogen atom and e = equatorial hydrogen atom. 

More bulky nucleophiles usually approach the cyclohexanone carbonyl from the equatorial direction. This is called steric approach control and is the result of van der Waals type repulsions. Larger nucleophiles encounter the 3,5-axial hydrogens on the axial approach trajectory. 

No extensive investigation of mechanism has been undertaken for any of the methods of dehydrohalogenation described. 17-Bromo-20-ketones appear to undergo preferential /ran -elimination. 2-Halo-3-ketones suffer predominant loss of the la (axial) hydrogen, but the geometry of bromine loss is not known. 7>fl -diaxial elimination has sometimes been assumed in configurational assignments, but this is not necessarily correct (see ref. 6). 

Agnello and Laubach suggested that the dehydrogenation of A" -3-ketones to A -3-ketones by chloranil proceeds through the A -enol, which suffers hydride loss from C-7. The failure of 7a-methyl-A -3-ketones to undergo dehydrogenation while the 7/5-isomers do so readily indicates that specific removal of the 7a (axial) hydrogen probably occurs in unsubstituted compounds. ... 

A reaction time of one hour at —7° to — 10°C was found to give maximum yields of 7a-methyl compounds. In some cases it is necessary to subject the reaction mixture to chloranil dehydrogenation this transforms (32) to the A -compound, thereby facilitating separation of the 7a-methyl isomer (31). The latter isomer is not attacked by the quinone since it lacks an axial hydrogen at C-7. 

The presence of 1,3-diaxial interaction between the C-2 alkyl group and the C-4 axial hydrogen atom is reflected in the rate of enamine formation of 2-substituted cyclohexanone. It has been shown by Hunig and Salzwedel (20) that even under forcing conditions, the yield of pyrrolidine and morpholine enamines of 2-methylcyclohexanone does not exceed 58%, whereas the C-2 unsubstituted ketones underwent enamine formation under rather milder conditions in better than 80 % yield. 

Figure 4.8 Axial (red) and equatorial (blue) positions in chair cyclohexane. The six axial hydrogens are parallel to the ring axis, and the six equatorial hydrogens are in a band around the ring equator.

The energy difference between axial and equatorial conformations is due to steric strain caused by 1,3-diaxial interactions. The axial methyl group on Cl is too close to the axial hydrogens three carbons away on C3 and C5, resulting in 7.6 kj/mol of steric strain (Figure 4.13). 

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.

Due to the shielding effect exerted by the carbohydrate framework (C-4 axial hydrogen at C-3), cyanide can more easily attack from the Si-face. 

With the radical 29, even though loss of an equatorial hydrogen should be sterically less hindered and is favored thermodynamically (by relief of 1,3 interactions of the axial methyl), there is an 8-fold preference for loss of the axial hydrogen (at 100 ( i. The selectivity observed in the disproportionation of this and other substituted cyclohexyl radicals led Beckwith18 to propose that disproportionation is subject to stereoelectronic control which results in preferential breaking of the C-H bond which has best overlap with the orbital bearing the unpaired spin. 

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