Protons Bonded To Oxygen and Nitrogen

Chemical shifts, and the observation or otherwise of couplings, for protons attached to oxygen (and nitrogen) depend on the extent of proton exchange and hydrogen bonding, which in turn depend on temperature, solvent, pH, and pK of the proton in question. 

Acid and base catalysis of a chemical reaction involves the assistance by acid or base of a particular proton-transfer step in the reaction. Many enzyme catalysed reactions involve proton transfer from an oxygen or nitrogen centre at some stage in the mechanism, and often the role of the enzyme is to facilitate a proton transfer by acid or base catalysis. Proton transfer at one site in the substrate assists formation and/or rupture of chemical bonds at another site in the substrate. To understand these complex processes, it is necessary to understand the individual proton-transfer steps. The fundamental theory of simple proton transfers between oxygen and nitrogen acids and... 

A more specific Coulomb mediated interaction is the hydrogen bond that occurs when a proton resonates between two ions, e.g., in a polypeptide the proton links an oxygen and nitrogen ion on different repeat units (generally separated by 5-7 other monomers) leading to a helical structure. 

Most organic reactions are done in solution, and it is therefore important to recognize some of the ways in which solvent can affect the course and rates of reactions. Some of the more common solvents can be roughly classified as in Table 4.10 on the basis of their structure and dielectric constant. There are important differences between protic solvents—solvents fliat contain relatively mobile protons such as those bonded to oxygen, nitrogen, or sulfur—and aprotic solvents, in which all hydrogens are bound to carbon. Similarly, polar solvents, those fliat have high dielectric constants, have effects on reaction rates that are different from those of nonpolar solvent media. 

Thus, l,6-methano[10]annulene (77) and its oxygen and nitrogen analogs 78 and 79 have been prepared and are stable compounds that undergo aromatic substitution and are diatropic. For example, the perimeter protons of 77 are found at 6.9-7.3 5, while the bridge protons are at —0.5 5. The crystal structure of 77 shows that the perimeter is nonplanar, but the bond distances are in the range 1.37-1.42A. It has therefore been amply demonstrated that a closed loop of 10 electrons is an aromatic system, although some molecules that could conceivably have such a system are too distorted from planarity to be aromatic. A small distortion from planarity (as in 77) does not prevent aromaticity, at least in part because the s orbitals so distort themselves as to maximize the favorable (parallel) overlap of p... 

In many eliminations to form C=0 and C=N bonds the initial step is loss of a positive group (normally a proton) from the oxygen or nitrogen. These may also be regarded as ElcB processes. 

The transfer of a proton between an acidic and a basic group within the same molecule is often more complex than the process shown in (1). The proton may be transferred along hydrogen-bonded solvent molecules between the acidic and basic groups if these are too remote to permit formation of an intramolecular hydrogen bond. Alternatively, two inter-molecular proton transfers with an external acid or base may be necessary. Tautomerisation of oxygen and nitrogen acids and bases (3) will be described in Section 6. The reactions are usually quite rapid and fast reaction... 

The really important aspect to all of this is that nuclei in similar chemical environments exhibit similar chemical shifts. Thus protons ( H nuclei) attached to a carbon atom bonded to oxygen, H-C-O, show a characteristic chemical shift (3.5-A.5 ppm), while protons attached to a carbon atom bonded to nitrogen, H-C-N, have a different chemical shift range (2.5-3.5 ppm) and, since the carbon is attached to the less electronegative N atom, resonate at lower frequency. We can therefore use chemical shifts to our great advantage when interpreting NMR spectra. 

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