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  Oxidation States of Nitrogen Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Oxidation States of Nitrogen

In comparing the chemistry of the amines with ethers and alcohols, we find out several classes of related compounds in which nitrogen assumes higher oxidation states, in difference to limited oxidation states of oxygen. In this context, remember that the oxidation state of elemental nitrogen (N2) and oxygen (O2) is defined as zero.
The most prevalent state of covalently bonded oxygen is -2. This is the example for alcohols, water, ethers and carbonyl compounds. The single general higher oxidation state (-1) is found in the peroxides, R-O-O-R, where R=hydrogen, alkyl, acyl or aryl. Due to the low covalent bond energy of the peroxide bond (ca.35 kcal/mole), these compounds are extensively employed as free radical initiators and are sometimes severely explosive in their reactivity (example- triacetone triperoxide employed by terrorist bombers). 
Alternatively, Nitrogen compounds, include oxidation states of nitrogen ranging from -3, like in amines and ammonia, to +5, as in nitric acid. The table shown below lists some of the known organic compounds of nitrogen, having distinct oxidation states of that element. A number of these classes of compounds have been explained here.

 

Oxidation State

_3

_2

_1

0

+1

+3

Formulas
(names)

R3N (amines)
R4N(+) (ammonium)
C=N-R (imines)
C≡N (nitriles)

R2N-NR2 (hydrazines)
C=N-NR2 (hydrazones)

RN=NR (azo cpd.)
R2NOH (hydroxyl amine)
R3NO (amine oxide)

N2 (nitrogen)
R-N2(+) (diazonium)

R-N=O (nitroso)

R-NO2 (nitro)
RO-N=O (nitrite ester)

Amine Oxides

By oxidizing 3º-amines or pyridines, amine oxides are prepared with hydrogen peroxide or peracids (example ZOOH, where Z=H or acyl).


R3N:   +   ZOOH      R3N(+)-O(-)   +   ZOH

Amine oxides are comparatively weak bases, pKa ca. 4.5, compared with the parent amine. Coordinate covalent N-O function is polar, by the oxygen being a powerful hydrogen bond acceptor. If one of the alkyl substituents contain a long chain, like C12H25, the resulting amine oxide is an amphoteric surfactant and locates use in shampoos and other mild cleaning agents.

An elimination reaction, complementary to the Hofmann elimination, takes place when 3º-amine oxides are heated at temperatures of 150 to 200 ºC. This reaction is termed as the Cope Elimination. It is generally carried out by dropwise addition of an amine oxide solution to a heated tube packed with small glass beads. A stream of nitrogen gas flowing via the column carries the volatile alkene products to a chilled receiver. The nitrogen-containing result is a hydroxyl amine. Different from the Hofmann elimination, this reaction occurs by a concerted cyclic reorganization, as displayed in the diagram. For such type of a technique, the beta-hydrogen and amine oxide moieties essentially have a syn-relationship.

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Cope elimination of diastereomeric amine oxides, like those displayed in instances #2 & 3 above, provide evidence of the syn-relationship of the beta-hydrogen and amine oxide groups. These instances also illustrate a strong regioselectivity favoring the more stable double bond.

Nitroxide Radicals

2º-Amines lacking α-hydrogens are oxidized by peroxides (ZOOH) to nitroxide radicals of surprising stability. In the instance displayed at the top of the following diagram it should be noticed that resonance delocalization of the unpaired electron contributes to a polar N-O bond. The R=H compound, recognized by the acronym TEMPO, is a comparatively stable red solid. Several other nitroxides have been prepared, three of which are displayed at the lower right. If one or more hydrogens are exist on an adjacent carbon, nitroxide decomposes to mixtures including amine oxides and nitrones, as displayed at the lower left. Nitroxides are oxidized to unstable oxammonium cations by halogens.

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The spin of the nitroxyl unpaired electron may be studied by a method called electron paramagnetic resonance (epr or esr). Experiments of this type have illustrated that the epr spectra are sensitive to substituents on the radical like its immediate environment. This has led to a spin labeling approach for investigating the conformational structures of macromolecules such as proteins. So, site-directed spin labeling (SDSL) has emerged as a valuable method for mapping elements of secondary structure, at the level of backbone fold, in a wide variety of proteins, including those not amenable to structural characterization using classical structural methods, like nuclear magnetic resonance and X-ray crystallography.

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