Aci forms

Nitration in acetic acid, in sulpholan and in carbon tetrachloride showed kinetic phenomena similar to those shown in nitromethane this is significant for it denies nitromethane a chemical involvement in the slow step. (Originally the rate of isomerization of nitromethane to its aci-form was believed to be a factor in the reaction. )... 

Nitroparaffins are classed as primary, RCH2NO2, secondary, R2CHNO2, and tertiary, R2CNO2, by the same convention used for alcohols. Primary and secondary nitroparaffins exist ia tautomeric equiUbtium with the enoHc or aci forms. 

The nitro isomer is weaMy acidic the nitronic acid isomer (aci form) is much more acidic. A comparison of the ionisation constants of the two forms ki water at 25°C is given ki Table 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. 

Aliphatic Nitro Compounds Are in Equilibrium With Aci Forms. 

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. 

Despite this fact, acidification of the isolable sodium salt of the carbanion intermediate (35) yields only the less stable aci-form (346)—a colourless solid. This happens because more rapid protonation takes place at the position of higher electron density, i.e. product formation under these conditions is kinetically controlled. The energy profile for the system has the form (Fig. 10.1),... 

In complex 95, an unusual conformation of an organic molecule is present as shown in Fig. 43 (215). The substituents in cyclohexene rings generally assume equatorial positions. In 95, they are axial. The container molecules can also stabilize resonance extremes. Complex 96, bearing a p-nitrophenolate in the quinoid aci-form is an example (228). Again, these unusual coordination... 

Actually, nitronates are the closest related derivatives of nitronic acids, that is, aci forms of AN, which exist in labile equilibrium with true AN. Some derivatives of nitronic acids, —CH=N(0)0X, where OX is the good leaving group, are evident intermediates in the most well-developed procedures for the synthesis of nitrile oxides from primary AN. In this chapter, special emphasis is given to particular nitronates, which are generated from a-functionalized AN and can also be considered as precursors of a-functionalized nitrile oxides. 

Diazo compounds and oxonium salts are the most efficient alkylating agents in the synthesis of alkyl nitronates. It is assumed that diazo compounds are inserted into the O-H bond in the aci forms of the corresponding AN, whereas oxonium salts generally react with AN anions. 

It is most likely that silylation of AN with silyl derivatives of amides, like the processes considered in Section 3.2.3.2, involve the formation of a-nitro-carbanions as the key step. It is also possible that only aci forms of AN can react with DPSU. This is evidenced by a comparison of the results of entries 8 and 9 in Table 3.3. 

Regardless of the coordination mode of the boron atom, boryl nitronates are rather smoothly subjected to alkaline hydrolysis to give, after acidification, the corresponding nitroalkanes (217, 230, 316). Depending on stability of the aci form, the yields of nitroalkanes obtained vary from moderate to high (Scheme... 

The primary and secondary nitroparaffins are neutral substances, but are transformed by alkalis into salts of isomeric aci- forms (Hantzsch) ... 

The synthesis of the sodium salt of aci-phenylnitroacetonitrile, described above, is an example of the preparative use of this reaction. The CH2-group of the benzyl cyanide becomes reactive as a result of the proximity of C6Hs and CN. Here also it may be assumed that an aci-form C6H6.CH=C=NH is the cause of this behaviour. 

The aci-form of the nitro group is frequently claimed in pure chemistry. However, only ad-nitromethane appears to have been comprehensively studied as an isolated species15. Ionized keto-enol systems are characterized by reversal of the relative stabilities of the single species compared to their neutral counterparts. Thus, the ionized enols are generally the thermodynamically more favoured tautomers by approximately 15-20 kcalmol-1, the... 

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. 

The photodecarboxylation of p-(nitrophenyl) glyoxylic acid 156, which was studied by time-resolved and steady-state methods at room temperature93, leads to p-nitrosobenzoic acid and carbon dioxide in good yields with = 0.28 in aqueous solution at pH 2-12 and excitation at 313, 280 or 254 nm (equation 76). An intermediate (Xmax = 350, r 2 xs) observed by nanosecond laser flash photolysis is assigned to the aci-form of the nitroketene... 

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,... 

The addition of a-chloronitroalkanes to solutions of alkali metal hydroxide has been used for the synthesis of some 1,2-dinitroethylene derivatives (43)." " These reactions involve attack of the nitronate salt (40) on the aci-form (39) of the unreacted em-chloronitroalkane followed by formal loss of hydrogen chloride. 2,3-Dinitro-2-butene and 3,4-dinitro-3-hexene (45) are formed in this way from 1-chloro-l-nitroethane and 1-chloro-l-nitropropane (44) respectively. 

Das Fluorid-Ion ist eines der starksten Wasserstoffbindungs-Donatoren und bildet starkc H—F-Bindungen. Dies fuhrt dazu, daB prim, und sek. Nitro-alkane in waBr. Fluorid-haltiger Losung in der aci-Form vor-liegen ... 

Recently Lee et al (Ref 3) re-examined the behavior of PETN under 10 to 50 kbars of external pressure. They also find a reduction in decomposition rate with increasing applied pressure. HMX behaves similarly to PETN. TNT whose explosion products contain a high proportion of solid carbon, as expected from LeChatelier s Principle, shows little pressure effect on its thermal decomposition. Nitro-methane, however, appears to decompose more rapidly under an external pressure of 50 kbars than 10 kbars. This effect is not completely understood but Lee et al suggest that high pressure may favor the formation of the thermally less stable aci form of Nitromethane ... 

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