Protons equatorial

One property of NMR spectroscopy is that it is too slow a technique to see the mdi vidual conformations of cyclohexane What NMR sees is the average environment of the protons Because chair-chair mterconversion m cyclohexane converts each axial pro ton to an equatorial one and vice versa the average environments of all the protons are the same A single peak is observed that has a chemical shift midway between the true chemical shifts of the axial and the equatorial protons... 

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. 

The multiplets and coupling constants of the axial) protons at = 3.15, 3.50 and 4.08 moreover confirm the equatorial positions of all three OH groups, as can be seen in formula B. Here the couplings from 10.0 to 11.5 Hz, respectively, identify vicinal protons in diaxial configurations, whilst values of 4.5 and 5.0 Hz, respectively, are typical for axial-equatorial relationships. As the multiplets show, the protons at 5 = 3.50 and 4.08 couple with two axial and one equatorial proton (triplet of doublets) respectively, whereas the proton at = 3.15 couples with one axial and one equatorial proton (doublet of doublets). 

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 C-2 equatorial proton is selectively removed when 1,3-ditiiianes are deprotonated. Furthermore, if the resulting carbanion is protonated, there is a strong preference for equatorial protonation, even if fliis leads to a less stable axial orientation for the 2-substituent. 

There is no generally reliable rule of thumb to predict whether an axial or equatorial proton will exchange faster. For instance, in the examples discussed above, the slowest rate of exchange is found for the S -axial proton in 5a-androstan-7-one (1) [see(2)-(4)] and the 2fi-equatorial proton in 5a-androstan-l 1-one (5) [see (6)-(9)]. Furthermore, the results of base-catalyzed exchange cannot necessarily be predicted from the corresponding... 

It would be pertinent to point out (25,27) that the trisubstituted isomer of the enamine of 2-aIkylcyclohexanone reacts in a quantitative manner with ethyl azodicarboxylate to give the addition product (35). This reaction in Conjunction with NMR spectroscopy can thus be employed for the determination of the amount of the trisubstituted isomer. According to the authors, hydrolysis of 35 furnishes the corresponding cw-2,6-disubstituted cyclohexanone (36) this seems unlikely since it would involve the stereo-electronically unfavored equatorial protonation of the enamine. 

The formation of the less favored trisubstituted isomer (37) occurs by the usual intramolecular axial proton transfer from the 6 position, whereas that of the tetrasubstituted isomer (38) involves the intramolecular proton transfer of the stereoelectronically less favored equatorial proton either via a four-membered transition state (39) or a six-membered transition state (40). 

The NMR spectrum of d-inositol has a signal at 3.83 p.p.m. for four axial protons, and a signal at 4.10 p.p.m. for two equatorial protons. The NMR spectrum of kasugamycin contains similar signals of d-inositol. 

It is also worth noting that nine times out of ten, equatorial protons absorb at somewhat lower field than the corresponding axial protons. This can be reversed in certain cases where the specific anisotropies of the substituents predominate over the anisotropies of the rings themselves but this is relatively rare. The difference is typically 0.5-1.0ppm, but may be more. 

H N.m.r. Chemical Shifts (p.p.m. from 4,4-Dimethyl-4-silapentane-l-sulfonate at 295 K) of the H-3 Axial and H-3 Equatorial Protons of Oligosaccharides Containing TV-Acetylneuraminic Acid and N-Glycolylneuraminic Acid... 

After having found the peaks due to axial and equatorial protons, the proportions of the conformers can also be established by measuring the area under the peaks. The rate of conversion from one conformer to another has been obtained from the shape of the peaks. 

In saturated cyclic hydrocarbons, diamagnetic currents are induced due to circulating bond electrons. Although, the effect is weak, but this is sufficient to distinguish between axial and equatorial protons is cyclohexane. The axial protons are shielded and they absorb 0.5 ppm upfield as compared to the equatorial. 

From the projection drawings below, it is obvious that the lone pair is anti to a C-C bond in F and a C-H bond in G. Table 50 lists the relative rates for deuterium exchange in some substituted cyclohexanes. In all cases, the equatorial proton exchanges at a much faster rate. 

The same features exist at a reduced level for POCCH couplings in phosphorinanes and these are about 2-6 Hz for an equatorial proton at C-5, while it is less than 1 Hz for an axial proton. Similarly, coupling of phosphorus to an equatorial methyl at C-4 is 2 to 3 Hz while that to axial methyl is less than 1 Hz (1968>83) similar trends have been found in the corresponding phosphonates.<1969,52)... 

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