Tautomerism, nitro compounds

The nitro form is much more stable than the aci form in sharp contrast to the parallel case of nitroso-oxime tautomerism, undoubtedly because the nitro form has resonance not found in the nitroso case. Aci forms of nitro compounds are also called nitronic acids and azinic acids. 

The taste of various amino acids, sugars, and aliphatic nitro compounds was studied, and it was concluded that the distance over which this hydrogen atom migrates, to give a second tautomeric form, determines the sweetness. In the case of saccharin, the sweetness was explained as due to two tautomeric forms. 

Tautomerism, strictly defined, could be used to describe the reversible interconversion of isomers, in all cases and under all conditions. In practice, the term has increasingly been restricted to isomers that are fairly readily interconvertible, and that differ from each other only (a) in electron distribution, and (b) in the position of a relatively mobile atom or group. The mobile atom is, in the great majority of examples, hydrogen, and the phenomenon is then referred to as prototropy. Familiar examples are / -ketoesters, e.g. ethyl 2-ketobutano-ate (ethyl acetoacetate, 23), and aliphatic nitro compounds, e.g. nitro-methane (24) ... 

The tautomerism of the aliphatic nitro-compounds is very closely allied to that of the ketones and aldehydes. Here also there are two ... 

At an early stage it was found that the nitro group had the power of activating the hydrogen atom(s) on the carbon to which the NO2 is attached. Victor Meyer found that primary and secondary (but not tertiary) nitro compounds dissolve slowly in alkali and if alcohol is added an alkali metal salt of the nitro compound is precipitated. Thus the activation of CH by NO2 was associated with incipient acidity and the behaviour of the group in this way was similar to that of certain other groups such as CN, COMe and COOEt6. It was more than twenty years, however, before the incipient acidity of CH adjacent to NO2 was correctly formulated in terms of the tautomerism of nitro and isonitro or aci forms. 

Aliphatic nitro compounds exhibit rather different behavior from nitroaromatic compounds. Secondary and primary nitro compounds tend to produce oximes because the intermediate nitroso compound quickly tautomerizes to the oxime (equation 1). Under aprotic conditions the radical anions of primary and secondary nitro compounds are relatively stable those derived from tertiary nitro compounds, on the other hand, eject nitrite ion relatively readily (equation 2)8. 

Aliphatic nitro compounds with the nitro group on a tertiary carbon were reduced to amines with aluminum amalgam [146 or iron [559]. 2-Nitro-2-methylpropane afforded ferf-butylamine in 65-75% yield [146. Even some secondary nitroalkanes were hydrogenated to amines. fra s-l,4-Dinitrocy-clohexane was converted to frans-l,4-diaminocyclohexane with retention of configuration. This may be considered as an evidence that the intermediate nitroso compound is reduced directly and not after tautomerization to the isonitroso compound [560] (see Scheme 54). 

The photoinitiated colour change of aromatic nitro compounds such as the dinitrobenzyl-pyridine (140) is due to a hydrogen transfer from the methylene bridge to the oxygen of the nitro group (aci-nitro tautomerism, Scheme 14) (60JCP(32)1888). 

Aromatic Nitro Compounds. The photoinduced isomerization of crystalline 2(2, 4 -dinitrobenzyl) pyridine was first reported by Tchit-chibabine et al. (29) who proposed the following tautomeric equilibrium ... 

Nitro ac/ -nitro Tautomerism in High-Energetic Nitro Compounds. 

Nitro = ac/-nitro tautomerism occupies a special role as it has been speculated to occur under conditions where high energy explosives detonate. It has been proposed as one of the important initial steps responsible for the cascade of reactions leading to the high energy release during detonation [1], Nitro = aci-nitro tautomerism in nitro compounds RjCH-NO leads to nitronic acids, R2C=N(0)OH. Kinetic studies show similar pKa values for nitronic acids and their carbon counterparts, the carboxylic acids [1]. Nitroalkane = nitronic acid tautomerism has been assumed not to be an acid-catalyzed process because of the poor basicity of N02 group [4]. However, recent work by Erden et al. showed that this tautomerism can, in fact, be achieved under acid catalyzed... 

The first of these was abandoned long ago, since the existence of an unstable three-membered ring in such a stable group as the nitro group can hardly be postulated. Besides, the formula does not explain the tautomerism, that exists in primary nitro compounds, analogous to the tautomerism of the CH2CO system ... 

Tertiary nitro compounds, of course, do not undergo tautomeric transformation, and they might be expected to be resistant to alkalis. Nevertheless aromatic nitro compounds, and polynitro-ones in particular, are very sensitive to alkalis, and undergo transformation when treated with them. For example, sym-trinitrobcnzcnc and also a- trinitrotoluene, when reacted with potassium hydroxide in methyl alcohol solution, form dark addition products (see also p. 202). Under certain conditions the nitro group can break off to form high molecular compounds. 

The influence of solvents on chemical equilibria was discovered in 1896, simultaneously with the discovery of keto-enol tautomerism in 1,3-dicarbonyl compounds (Claisen [14] acetyldibenzoylmethane and tribenzoylmethane Wislicenus [15] methyl and ethyl formylphenylacetate Knorr [16] ethyl dibenzoylsuccinate and ethyl diacetylsuccinate) and the nitro-isonitro tautomerism of primary and secondary nitro compounds (Hantzsch [17] phenylnitromethane). Thus, Claisen wrote Es gibt... 

The naphthylamines may be prepared by reduction of the corresponding nitro compound, but they are readily accessible from naphthois by the Bucherer reaction The naphthol is heated, preferably under pressure in an autoclave, with ammonia and aqueous sodium hydrogen sulfite solution, when an addition-elimination sequence occurs. The detailed mechanism is not completely elucidated, but the Bucherer reaction is restricted to those phenols that show a tendency to tautomerize to the keto form, such as the naphthois and 1,3-dihydroxybenzene (resorcinol). Using 1-naphthol for illustration, the first step is addition of the hydrosulfite across the 3,4-double bond of either the enol or keto tautomer (Scheme 12.9). Nucleophilic attack by ammonia at the carbonyl group... 

Aliphatic nitro compounds show a number of reactions which parallel those of carbonyl chemistry. Primary and secondary nitro compounds exhibit tautomerism paralleling keto-enol tautomerism (Scheme 3.94a). Aliphatic nitro compounds dissolve in aqueous sodium hydroxide with the formation of sodium salts. The resultant anions behave as carban-ions and will condense with aldehydes. An example involves the formation of m-nitrostyrene from nitromethane and benzaldehyde (Scheme 3.94b). 

There are three possible products when NOCl is added to alkenes, a (3-halo nitro-so compound, an oxime, or a (3-halo nitro compound.The initial product is always the (3-halo nitroso compound, but these are stable only if the carbon bearing the nitrogen has no hydrogen. If it has, the nitroso compound tautomerizes to the oxime, H—C-N=0 C=N-OH. With some alkenes, the initial (3-halo nitroso compound is oxidized by the NOCl to a (3-halo nitro compound. Many functional groups can be present without interference (e.g., COOH, COOR, CN, OR). The mechanism in most cases is probably simple electrophilic addition, and the addition is usually anti, although syn addition has been reported in some cases.Markovnikov s mle is followed, the positive NO going to the carbon that has more hydrogens. 

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