Equatorial and axial hydrogens

Equatorial halide is gauche to axial and equatorial hydrogens on adjacent carbon cannot undergo anti elimination in this conformation... 

For example, cyclohexane gives a single averaged resonance in the H NMR at room temperature, but separate signals are seen for the axial and equatorial hydrogens when spectra are acquired at very low temperature. 

Correspondingly, the interconversion of the twist-boat intermediate into the other chair form can be viewed as rotation about the opposite ring bond. Overall, two independent bond rotations , pausing at the high-energy (but stable) twist-boat intermediate, effect conversion of one chair structure into another equivalent chair, and at the same time switch axial and equatorial hydrogens. 

A detailed experimental investigation of the THP-HzO complex (with molecular beam Fourier transform (FT) microwave spectroscopy, free-jet millimeter-wave spectroscopy, and ab inito calculations) revealed that the lowest energy species has the water in an axial position. <1998CEJ1974>. Similar studies into the TFIP-FICl complex allowed both the axial and equatorial hydrogen-bond complexes to be characterized it was not possible to unequivocally determine which of the two is more stable <1999AGE1772>. 

Figure 27.2 Axial and equatorial hydrogens in the chair conformation.

Draw a chair cyclohexane. Show the axial and equatorial hydrogens. 

The relative mobilities of the axial and equatorial hydrogen atoms from C(2) and C(6) positions of rigid cyclohexanones have been determined by several authors from the relative rate constants of hydrogen-deuterium exchange. Indeed, according to the principle of microscopic reversibility, the elementary rate constants for axial and equatorial proton detachment correspond to those of proton addition on the two faces of the enol or enolate (26). 

Ring-flipping interconverts axial and equatorial hydrogens in cyclohexane... 

Axial and equatorial hydrogen atoms in chair cyclohexane. The six axial hydrogens (red) are parallel to the ring axis, and the six equatorial hydrogens (blue) are in a band arourtd the ring equator. 

What must happen is an equilibration between structures 1 and 2, very slow at-110° C, but very fast at room temperature, faster than the NMR can differentiate. So the signal which has coalesced is an average of a and a and c and c. (This type of low temperature NMR experiment is also used to differentiate axial and equatorial hydrogens on a cyclohexane.)... 

Make a model of the chair form of cyclohexane. Notice that each carbon-carbon bond is in a staggered conformation. Identify the axial and equatorial hydrogens. 

---