Axial and equatorial

In order to account for axial and equatorial positions of protons bonded to cyclo-hcxanc-likc rings, Eq, (19) was used, where 1 is an atom three non-rotatablc bonds (totally atoms) away from the proton and belonging to a six-membered ring, and is a dihedral angle in radians (Figure 10.2-6c). 

The conformational features of six membered rings are fundamental to organic chemistry so it is essential that you have a clear understanding of the directional prop erties of axial and equatorial bonds and be able to represent them accurately Figure 3 17 offers some guidance on the drawing of chair cyclohexane rings... 

The C—H bonds in the chair conformation of cyclohexane are not all equivalent but are divided into two sets of six each called axial and equatorial... 

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

We know from Chapter 3 that the protons m cyclohexane exist m two different envi ronments axial and equatorial The NMR spectrum of cyclohexane however shows only a single sharp peak at 8 1 4 All the protons of cyclohexane appear to be equivalent m the NMR spectrum Why" ... 

The rate of ring flipping can be slowed down by lowering the temperature At tern peratures on the order of — 100°C separate signals are seen for the axial and equatorial protons of cyclohexane... 

One-bond couplings ( /ch) in saturated systems do not seem to have been investigated extensively. The value for cyclohexane (an average of couplings to axial and equatorial protons) iSk 123 Hz, and is increased by substitution adjacent to the carbon by an electronegative element, as with the aromatic systems discussed above. 

Scheme 3.2 gives some data that illustrate the differences in reactivity between groups in axial and equatorial positions. It should be noted that a group can be either more or less reactive in an axial position as compared to the corresponding equatorial position. 

The rates of removal of axial and equatorial protons from 4-t-butylcyclohexane in NaOD/dioxan have been measured by an NMR technique. The rate of removal of an axial proton is 5.5 times faster than for an equatorial proton. What explanation can you offer for this difference ... 

The relation of rates of reduction with NaBH4 to variations in structure in a wide variety of monocyclic and bridged bicyclic compounds has also been discussed for example, a methyl a to a ketone slows the rate of reduction. Brown ° stated that reactions should not be discussed in terms of axial and equatorial attack, since the rates simply reflect differences in the energies of the possible transition states and not enough is known about the transition state to analyze it. He accepted th concepts of SAC and PDC, but preferred to call them steric strain contrpl and product stability control. ... 

For the equilibrium between the axial and equatorial conformations of a monosubstituted cyclohexane. 

We noted in Section 13.13 that an NMR spectrum is an average spectrum of the conformations populated by a molecule. From the following data, estimate the percentages of axial and equatorial bromine present in bromocyclohexane. 

Figure 12.13 Interchange of axial and equatorial positions by Berry pseudorotation (BPR).

P-F 153 pm). However, the F nmr spectrum, as recorded down to — 100°C, shows only a single fluorine resonance peak (split into a doublet by P- F coupling) implying that on this longer time scale (milliseconds, as distinct from instantaneous for electron diffraction) all 5 F atoms are equivalent. This can be explained if the axial and equatorial F atoms interchange their positions more rapidly than this, a process termed pseudorotation by R. S. Berry (1960) indeed, PF5 was the first compound to show this effect. The proposed mechanism is illustrated in Fig. 12.13 and is discussed more fully in ref. 91 the barrier to notation has been calculated as 16 2kJmol". ( ... 

Figure 12.22 Structures of (aj (Cl PNMe) , and (b) jCl(S)PNMc 2- Note the difference in length of the axial P-N and equatorial P-N bonds (and of the axial and equatorial P-Cl bonds) about the trigonal bipyramidal P atoms in (a).

Fig. 16.13b (Se-Fax 180 pm, Se-Feq 167 pm, with axial and equatorial angles subtended at Se of 169.3° and 96.9°, respectively).However, these pseudo-tbp molecules are arranged in layers by weaker intermolecular interactions to neighbouring molecules so as to form an overall distorted octahedral environment with two further Se F at 266 pm (Fig. 16.13b) somewhat reminiscent of the structure found earlier for Tep4 (see Fig. 16.13c and below).

Ring inversion, leading to interconversion of different ring conformers, is typically as facile a process as single-bond rotation. Particularly important are six-membered rings, where interconversion leads to interchange of axial and equatorial positions. 

It was assumed above that the methyl group adopts an equatorial conformation. Actually, methylcyclohexanone exists as a mixture of axial and equatorial conformations. 

In the conformationally biased cyclohexene ring, the two dioxolane oxygens must assume fixed axial and equatorial positions. Coordination of the zinc carb-... 

Finally, the use of the constant pressure minimization algorithm allows searching for phenomena that can be considered as precursors of pressure-induced transitions. For example, the predicted behaviour of the anatase cell constants as a function of pressure shows that the a(P) and c(P) plots are only linear for P<4 GPa, the value that is close to both the theoretical and experimental transition pressures. At higher pressures the a constant starts to grow under compression, indicating inherent structural instability. In the case of ratile there is a different precursor effect, nami y at 11 GPa the distances between the titanium atom and the two different oxygens, axial and equatorial, become equal. Once again, the pressure corresponds closely to the phase transition point. 

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