Aci form, of 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. 

When primary nitro compounds are treated with sulfuric acid without previous conversion to the conjugate bases, they give carboxylic acids. Hydroxamic acids are intermediates and can be isolated, so that this is also a method for preparing them. Both the Nef reaction and the hydroxamic acid process involve the aci form the difference in products arises from higher acidity, for example, a difference in sulfuric acid concentration from 2 to 15.5 M changes the product from the aldehyde to the hydroxamic acid. The mechanism of the hydroxamic acid reaction is not known with certainty, but if higher acidity is required, it may be that the protonated aci form of the nitro compound is further protonated. 

Again V. Meyer was the first to describe the reaction. As in the formation of nitrolic acids, here also it is the aci-form of the secondary nitro compound that takes part in the reaction. It reacts with sodium nitrite to give a compound which, after acidification, forms pseudonitrole. 

A few nitro compounds have been obtained in good yields by the interaction of reactive halogen compounds with aci-nitro alkanes. The reaction is usually complicated in that both C- and 0-alkylation occurs. If the stability of the aci form of the nitro compound is high, then the tendency is toward alkylation on carbon rather than on oxygen. An example is the condensation of p-nitrobenzyl chloride with the sodium salt of nitro-ethane to give an 83% yield of 1-p-nitrobenzylnitroethane, P-OjNC.H CHjCHCNOjXIHj. ... 

First, the base produces the salt of the nitro compound. Then, on addition of the sulphuric acid, the aci form of the original nitro compound is formed, which in turn is hydrolysed to the carbonyl compound. This process is called the Nef reaction. 

Nef rearrangement Primary or secondary aliphatic nitro compounds may be hydrolysed to aldehydes or ketones, respectively, by treatment with a base followed by a mild aqueous acid which yields the aci form of the nitro compound. This can then be hydrolysed by concentrated sulphuric acid to give the carbonyl compound. 

Compounds containing these groups arc acidic, although in tin case of oximes, very feebly acidic. They are often nu i in a different guise, the above formulas n pi o.senting simply the aci form of primary and secondary nitroso and nitro compounds. See page 22 and Problem 4,... 

An equihbrium mixture of the isomers usually contains a much higher proportion of the tme nitro compound. The equiUbrium for each isomeric system is influenced by the dielectric strength and the hydrogen-acceptor characteristics of the solvent medium. The aci form is dissolved and neutralized rapidly by strong bases, and gives characteristic color reactions with ferric chloride. 

Experiment.—Dissolve 1 c.c. of nitromethane in water and test the solution with litmus paper. Then add some phenolphthalein and, drop by drop from a burette, OliV-sodium hydroxide solution. Before a permanent pink colour develops about 2 c.c. of the alkali will be added—a sign that an acid, aci-nitromethane, H2C NOOH, has been formed from the neutral nitromethane. A small sample of this solution gives with ferric chloride a blood-red colour, characteristic of aci-nitro-compounds. The salts of the oci-compound undergo extensive hydrolysis. This is shown by further addition of 0-1 N-alkali which produces a deep red colour. If 10 c.c. of alkali were added and 5 c.c. of 0-1 JV-hydrochloric acid are now run in the solution is decolorised because the liberated oci-compound restricts the hydrolysis of its salt. But the conversion of H2C N02H into H3C.N02 proceeds so rapidly that the red colour reappears in a few moments. 

Almost without exception the numerous transformations which primary and secondary nitroparaffins undergo involve the aci-iorm, i.e. they take place under conditions in which the salt of the oci-form is produced. Qualitatively the nitroparaffins greatly resemble ketones in their mode of action, although the much greater reaction velocity of the nitro-compounds brings about a quantitative difference. 

The mechanism of the coupling reaction has been very exhaustively studied. Summarising first what has already been mentioned, it must be noted that the reaction is not confined to the aromatic series, for diazo-compounds condense also with enols and with the very closely related aliphatic aci-nitro-compounds. The final products of these reactions are not azo-compounds, but the isomeric hydrazones formed from them by rearrangement. 

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. 

In 1896 Hantzsch and Schultze obtained the separate nitro and isonitro forms of phenylni-tromethane and studied their properties and their interconversion24. Hantzsch introduced the term pseudo-acid to describe neutral compounds which form alkali metal salts corresponding to their aci forms. 

In general, C-nitro compounds are more stable than /V-nitro compounds because of the higher bonding energies in the former type. Evidence is offered [1] that decomposition and explosion of many nitro-derivatives proceeds through the aci-form, and that sensitivity corresponds to the proportion of that present. In terms of this work, sensitisation by very small proportions of soluble organic bases is most important this is not limited to nitroalkanes. TNT can apparently be brought to the sensitivity of lead azide by this means. For a physicist s view of this sensitisation,... 

A second and related consequence in aliphatic nitro compounds is the acidification of the directly bonded CH unit through the attendant stabilization of the derived conjugate bases (5,6). As with all delocalized anions, reprotonation gives rise to tautomers, the original C-nitro compound (I) and the oci-nitro or isonitro form (II), Eq. 2.1. The aci-nitro tautomers are typically present in very minor concentrations, with equilibrium constants (A eq) between 10 and 10 (7). Alkylation of the delocalized anion leads to both a-substituted nitro compounds and the regioisomeric nitronic esters (nitronates). Nitronates were described as early as 1894 (8), however, the first isolated nitronic ester was obtained several years later upon the addition of diazomethane to phenylazonitromethane (1), Eq. 2.2 (9). 

In the presence of sodium or potassium ethoxide, nitric esters react with substances containing an active methylene group to form salts of aci-nitro compounds (Wislicenus [64]) as quoted above in eqn. (59). Esters are hydrolysed on this occasion. 

According to the scheme, the formation and further reactions of anion radical SOI take place in an alkaline medium. Therefore, there are some restrictions to the electron-transfer reactions from this anion radical to acceptors. For instance, aliphatic nitro compounds react in alkaline media in aci forms. They add SOI and form new anion radicals Scheme 1-92 (Bradic Wilkins 1984) ... 

It had been established several decades ago that the reaction of 1-chloro-l-ni-troethane with sodium nitrite in aqueous-alcohol medium is second order overall and first order in each reactant (Hawthorne 1956). 1-Deutero-l-chloro-l-nitroethane reacts more slowly than its lighter isotopomer. This means that the kinetic isotopic effect is observed. The reaction proceeds only in moderately alkaline media in strongly alkaline media it does not take place. Only those geminal halo nitro compounds, which carry hydrogen in the geminal position, can undergo the conversion. Based on these facts, Hawthorne (1956) suggested the Sn2 substitution preceded by the isomerization of the initial substrate into the aci-nitro form ... 

In the case of simplest nitro compounds the absorption band produced by this group corresponds to the following values. For aliphatic nitro compounds, for example nitromethane, nitroethane, 1- and 2-nitropropanes, they are 260-270 mu at extinctions s = 40-120. (According to Ungnade said Smiley [1] the values for higher nitroparaffins are 274-278 mp, s = 24-41.) For aromatic nitro compounds, such as nitrobenzene the bands are 250-260 mp at extinctions s = 9000-10,000. When a primary or secondary nitro group takes an aci-form, the maximum disappears, as shown by earlier research work of Hantzsch and Voigt [2]. 

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