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Cis-Trans Isomerism and Conformational Equilibria for Cyclohexane Derivatives

The cis-trans isomerism of cyclohexane derivatives  is complicated by conformational isomerism. For example, 4-terf-butylcyclohexyl chloride theoretically could exist in four stereoisomeric chair forms, 1, 2, 3, and 4.


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Conformations 1 and 2 have the substituents trans to one another, but in 1 they both are equatorial, whereas in 2 they both are axial. Conformations 3 and 4 have the substituents in the cis relationship, with the tert-butyl and chlorine equatorial and axial, respectively, in 3, and the reverse in 4. A tert-butyl group is large and bulky compared to chlorine and considerable steric hindrance results when tert-butyl is axial (Figure 12-10). For this reason, 1 and 3 with tert-butyl equatorial are much more favorable than 2 and 4. The properties of a substituent located in an axial or an equatorial position on a cyclohexane ring can be studied by synthesizing the cis- or trans-4-tert-butyl derivative analogous to 3 or 1. The tert-butyl is characterized as a “holding group” because its own tendency to be in the equatorial position holds a smaller substituent group axial or equatorial, depending on whether it is cis or trans. However, when there are two large substituents in the cis-1,4 arrangement on a cyclohexane ring, neither of which will go easily into an axial position, then it appears that the twist-boat conformation (Section 12-3 A) is most favorable (Figure 12-11).

Figure 12-10 1,3-Interactions in a cyclohexane ring with an axial tert-butyl group

Figure 12-11 Twist-boat conformation of c/'s-1,4-di-fert-butylcyciohexane

 

Figure 12-12 Proton-decoupled, 63.1 MHz, 13C spectrum of methylcy-clohexane at —110°. The upper right curve was taken with the signal sensitivity control turned up by a factor of 64. (Courtesy of Dr. F. A. L. Anet.)

Use of 13C nmr spectroscopy to determine whether a substituent is in an axial or equatorial position is well illustrated with cis- and trcins-4-tert-butylcyclohexanols, 5 and 6:

In this case, the tert-butyl group acts as a “holding group” so that in the cis isomer the OH is axial and in the trans isomer it is equatorial. The 13C resonance of Cl of the axial isomer, 5, is 5.4 ppm upheld of Cl in 6, and the resonances of C3 and C5 are 4.7 ppm upheld of those of the corresponding carbons of 6. Similar large upheld shifts of the ring carbons Cl, C3, and C5 also are produced by axial methyl groups. In addition, the 13C resonance of an axial methyl carbon is shifted upheld 5-7 ppm compared to the resonance of an equatorial methyl. These effects are clearly evident in the 13 C spectrum of methylcyclohexane at —110°, shown in Figure 12-12. At —110° the equatorial form is 99% of the mixture and is interconverted only very slowly with the 1% of axial form. Despite the strong 13C nmr signals from the equatorial form, the chemical shifts of C3, C5, and CH3 carbons of the axial form are sufficiently different that they can be seen upheld of the methyl resonance of the equatorial form.

Figure 12-13 Changes in the 19F nmr spectrum of 1,1-difluorocyclohexane with temperature at 56.4 MHz (see Exercise 12-12). Generally, H—C—C—F spin-spin splittings are on the order of 5-15 Hz and change with rotational angles in much the same way as for H—C—C—H couplings.

 

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