Given that this chair conformer is the preferred conformation for cyclohexane, what would you expect its 13C NMR spectrum to look like? All six carbon atoms are the same so there should only be one signal (and indeed there is, at 25.2 ppm). But what about the ¹H NMR spec trum? The two different sorts of protons (axial and equatorial) ought to resonate at different frequencies, so two signals should be seen (each with coupling to neighbouring protons). In fact, there is only one resonance in the proton spectrum, at 1.40 ppm. In a monosubstituted cyclohexane there should be two isomers detectable—one with the substituent axial, the other with the substituent equatorial. But again, at room temperature only one set of signals is seen.

This changes when the NMR spectrum is run at low temperature. Now two isomers are visible, and this gives us a clue as to what is happening: the two isomers are conformers that inter convert—rapidly at room temperature, but more slowly when the temperature is lowered. Recall that NMR does not distinguish between the three different stable conformers of butane (two synclinal and one anti-periplanar) because they are all rapidly interconverting so fast that only an average is seen. The same happens with cyclohexane—just by rotating bonds (that is, without breaking any!) cyclohexane can ring invert or ‘flip’. After ring inversion has taken place, all the bonds that were axial are now equatorial and vice versa. The whole inversion process can be broken down into the conformations shown below. The green arrows show the direction in which the individual carbon atoms should move in order to get to the next conformation.

The energy profile (below) for this ring inversion shows that the half-chair conformation is the energy maximum on going from a chair to a twist-boat. The true boat conformation is the energy maximum on interchanging between two mirror-image twist-boat conformers, the second of which is converted to the other chair conformation through another half-chair.

مواضيع ذات صلة

E1 reactions can be stereoselective

The role of the leaving group

Substrate structure may allow E1

E1 and E2 mechanisms

Size

Elimination happens when the nucleophile attacks hydrogen instead of carbon

Axially and equatorially substituted rings react differently

Steroid

Decalins

What happens with more than one substituent on the ring

Substituted cyclohexanes

A closer look at cyclohexane