Bases, nitration species

Co-, Mn-, and Fe-based perovskites, NO peaks appearing at similar temperatures as those in NO-TPD experiments (apart from a minor desorption at medium temperature and an intense one at high temperature) were also found and aseribed to the desorptions of physically adsorbed NO, nitrosyl species and nitrate species in the order of their thermal stability. The NO peaks obtained at low (80-110 °C), medium (200-210 °C), and high (310-390 °C) temperatures in the present NO-TPD analyses were thus correlated to physically adsorbed NO, nitrosyl and nitrate species. 

The rate coefficient (k2 enc) for the collision of two species is given by 8RT/3Z (where Z is the viscosity of the medium at the reaction temperature), the Smoluchowski equation. This is the maximum possible rate of reaction, which is controlled by the rate at which the two reacting species diffuse together. For nitration in >90% H2S04, where nitric acid is completely ionized, if exclusively the free base nitrates the rate coefficient (k2 fb) would equal k2 obs KJhx (where Ka is the ionization constant of the base, and hx the acidity function that it follows). Thus, if k2 fb> k2 enc free base nitration is precluded, but if... 

Fig. 3.2. (a) Relation of (A) majority and (B) free base minority species nitration rates to (C) the decreasing amount of free base, (b) As before but with the minority species rate corrected to constant concentration. 

A further analysis of rate profiles obtained for free base nitrations is possible. If allowance is made for the decrease in the concentration of the reactive species, the resulting rate profile should have a slope similar to that for a conjugate acid. Thefree base rate coefficient 2(fb) is defined by Eq. (3.9) and can be calculated by Eq. (3.10), in which m is the slope of the substrate protonation correlation. 

The species involved in nitration of 2-methylindole with excess concentrated nitric acid is not known, but is probably the free base after the entry of the first nitro group. The final product is 2-methyl-3,4,6-trinitroin-dole and a variety of intermediate mono- and dinitro compounds can be obtained nitrodeacylation can also be used (65JOC3457) (Scheme 8.4). These results again show that electron-supplying substituents in the 2-position preferentially activate the 6-position. Evidence that these results relate to free base nitration comes from nitration of compounds 8.39-8.42 in nitric acid/sulfuric acid (65T1923). The conjugate acid is most probably... 

Corrected for isomer distribution and statistical factor. b 0, free base is reacting species. u +, conjugate acid is reacting species. d Free base nitration is assumed. 

The nitration data of this investigation were first used to discriminate between the kinetic models developed for the slow, fast, and instantaneous reaction systems (or regimes) and second to pick the best kinetic model. In order to do this, values had to be estimated for the concentration of the nitrating species (or entity) in the acid phase, the interfacial concentration of benzene, the kinetic rate constants for acid-phase nitrations, interfacial area (between two liquid phases), and diffusivity values or mass transfer coefficients for benzene. In analyzing the data, the concentration of the nitrating species was assumed to be that of the nitric acid in the acid phase it is appreciated that this assumption is open to question as will be discussed later but based on available information is probably the best assumption possible. The interfacial concentration of benzene (hC., .) was estimated based in part on the results obtained in making distribution measurements h is the distribution coefficient for benzene and C. - is the benzene concentration in the hydrocarbon phase. KinetTc rate constants (for the second-order nitration reactions occurring in the acid phase) were estimated based on the results of Deno and Stein (10),... 

If the concentration of effective aromatic species does vary with acidity, as sometimes happens if the compound is substantially proto-nated, then the acidity-dependence of the rate will be less steep than usual, because the concentration of the active free base diminishes significantly with increasing acidity. This situation has been observed in certain cases ( 8.2). The fall in the concentration of the active species can be allowed for from a knowledge of its pK and the acidity function which, for the particular compound, gives the best measure of the acidity of the medium. Then the corrected acidity-dependence of the rate resembles that observed with compounds the concentration of which does not change significantly with acidity. The nitration of minor species is discussed later ( 8.2). 

Nitration is almost always carried out under acidic conditions. If the compound being nitrated is basic, the problem arises of deciding whether the free base or its conjugate acid is being nitrated, or if both of these species are reacting. 

It is found in practice that for a number of compounds reacting ma the predominant species an almost horizontal plot is obtained. For compounds presumed to be nitrated via the free bases, such as 2,6-lutidine i-oxide and 3- and 5-methyl-2-pyridone, slopes of approximately unity are obtained. Since this type of plot allows for the incomplete ionisation of nitric acid, it can be used at higher acidities than plots using — ( H + logio Hjo) which break down when the condition is no longer true. 

An alternative approach is to assume, in the light of the experimental evidence just mentioned, that the reactions of cations and neutral molecules have similar values of (or, equivalently, of log ( /l mol and to try to calculate the difference which would arise from the fact that the observed entropy of activation for a minority free base includes a contribution from the acidic dissociation of the conjugate acid in the medium in question (see (5) above). Consider the two following reaction schemes one (primed symbols) represents nitration via the free base, the other the normal nitration of a non-basic majority species (unprimed symbols) ... 

When activating substituents are present in the benzenoid ring, substitution usually becomes more facile and occurs in accordance with predictions based on simple valence bond theory. When activating substituents are present in the heterocyclic ring the situation varies depending upon reaction conditions thus, nitration of 2(177)-quinoxalinone in acetic acid yields 7-nitro-2(177)-quinoxalinone (21) whereas nitration with mixed acid yields the 6-nitro derivative (22). The difference in products probably reflects a difference in the species being nitrated neutral 2(177)-quinoxalinone in acetic acid and the diprotonated species (23) in mixed acids. 

Nitration of 3-phenyl-1,2-benzisoxazole with fuming nitric acid has been shown to give dinitro products of undetermined substitution pattern (67AHC(8)277, p. 290>. However, more satisfactory studies have now been described, especially on the kinetics and mechanism of nitration of 3-methyl-l,2-benzisoxazole (77JCS(P2)47). Nitration in cold, concentrated mixed acids yields the 5-nitro derivative exclusively, nitration in 80-90% sulfuric acid occurring on the free base whereas at higher acidities the conjugate acid is the species involved in the nitration. 

Complex [(CXI )Ir(/j,-pz)(/i,-SBu )(/j,-Ph2PCH2PPh2)Ir(CO)] reacts with iodine to form 202 (X = I) as the typical iridium(II)-iridium(II) symmetrical species [90ICA(178)179]. The terminal iodide ligands can be readily displaced in reactions with silversalts. Thus, 202 (X = I), upon reaction with silver nitrate, produces 202 (X = ONO2). Complex [(OC)Ir(/i,-pz )(/z-SBu )(/i-Ph2PCH2PPh2)Ir(CO)] reacts with mercury dichloride to form 203, traditionally interpreted as the product of oxidative addition to one iridium atom and simultaneous Lewis acid-base interaction with the other. The rhodium /i-pyrazolato derivative is prepared in a similar way. Unexpectedly, the iridium /z-pyrazolato analog in similar conditions produces mercury(I) chloride and forms the dinuclear complex 204. 

Quinoxalin-2-one is a very weak base (pK — 1.37) and so the different orientation of substitution in acetic and sulfuric acids may mean that in acetic acid the principal species undergoing nitration is the neutral molecule, and in sulfuric acid, the mono-cation. Treatment of quinoxaline-2,3-dione, or its iViV -dimethyl derivative in sulfuric acid, with 1 equivalent of potassium nitrate, results in nitration at position 6 with 2 equivalents of potassium nitrate, 6,7-dinitro compounds are formed. When quinoxaline is boiled with aqueous nitric acid, 6-... 

---