Kinetics nitration

Kinetic measurements were performed on a Hitachi 150-20 UV/VIS spectrophotometer. Dehydrobrominations were studied in DMF solution using cyclohexyl amine (CHA) as the base. Applied CHA concentrations were 2, 2.5, 3, 3.5, 4 and 5 10 3 mole.dm-3, initial concentration of 1 was 5 10 5 mole.dm-3 in every case (pseudo-first-order conditions). Ionic strength was adjusted to lO l mole.dm 3 with potassium nitrate. Kinetic curves / D(t) / were recorded at fix wavelength, X = 290 ran and the temperature was maintained at 30, 35.5, 40°C. Stock solutions were made daily for la and freshly for every measurement of Ih. The reaction was started by injection of solution of 1 to the thermostated solution of CHA. 

The first attempts to study nitration kinetics were made by L. Meyer and coworkers [81,82], It was in their publication that for the first time a statement appeared that the nitration reaction is irreversible and that its rate decreases with time. 

J. A. Hetherington and Masson [84], basing their views on the studies of benzene nitration kinetics, stated that the reaction of nitrobenzene nitrated with a mixture of HN03, H2S04 and H20 would stop before all the nitric acid was used if the amount of sulphuric acid in the mixture was insufficient to form the H2S04.H20... 

The problem of the orientation of nitro groups is connected with that of nitration kinetics. Nitration kinetics have already been discussed in connection with the composition of nitrating mixtures, especially with regard to the presence and the role of nitronium and other ions in the nitration process. Now nitration... 

Martinsen [64] in his studies on nitration kinetics determined the influence of substituents on the rate of the reaction. He classified them according to their influence on the reaction rate, comparing it with that of chlorine, which in some cases (as compared with hydrogen) can lower, while in others raise the rate ... 

In his work on the nitration kinetics, which has already been referred to, Mar-tinsen [145] found that in the nitration of phenol with nitric acid, nitrous acid, if present, acted as a catalyst on the nitration process. The presence of nitrous acid was necessary for initiating the reaction, further quantities being formed in the reaction of oxidation of phenol with nitric acid, which accompanied the nitration reaction. Thus the reaction of nitrating phenol is an autocatalytic one. 

Nitration kinetics are usually followed by measuring the absorption of the nitro product by UV spectroscopy [67JCS(B)1204 68JCS(B)800 76JCS(P2)1135]. The wavelength chosen for measuring the optical density is usually close to Xmax for the nitro product. 

Garside, C. (1991). Shift up and the nitrate kinetics of phytoplankton in upwelling systems. Limnol. 

Liquid phase aromatic mononitration under normal industrial conditions is an example of mass transfer with simultcuieous chemical reaction. The problem of determining the magnitude cind nature of the resistance to interphase transfer has been avoided in much research on nitration kinetics by the simple expedient of working in a solvent with vdiich all reactcints are miscible. 

The rate of the uncatalysed reaction in all four solvents is rather slow. (The half-life at [2.5] = 1.00 mM is at least 28 hours). However, upon complexation of Cu ion to 2.4a-g the rate of the Diels-Alder reaction between these compounds and 2.5 increases dramatically. Figure 2.2 shows the apparent rate of the Diels-Alder reaction of 2.4a with 2.5 in water as a lunction of the concentration of copper(II)nitrate. At higher catalyst concentrations the rate of the reaction clearly levels off, most likely due to complete binding of the dienophile to the catalyst. Note that in the kinetic experiments... 

Nitration can be effected under a wide variety of conditions, as already indicated. The characteristics and kinetics exhibited by the reactions depend on the reagents used, but, as the mechanisms have been elucidated, the surprising fact has emerged that the nitronium ion is preeminently effective as the electrophilic species. The evidence for the operation of other electrophiles will be discussed, but it can be said that the supremacy of one electrophile is uncharacteristic of electrophilic substitutions, and bestows on nitration great utility as a model reaction. 

Nitration in aqueous solutions of nitric acid Added water retards nitration in concentrated nitric acid without disturbing the kinetic order of the reaction. The rate of nitration of nitrobenzene was depressed sixfold by the addition of 5 % of water, (c. 3 2 mol 1 ), but because of the complexity of the equilibria involving water, which exist in these media, no simple relationship could be found between the concentration of water and its effect on the rate. 

Nitration at a rate independent of the concentration of the compound being nitrated had previously been observed in reactions in organic solvents ( 3.2.1). Such kinetics would be observed if the bulk reactivity of the aromatic towards the nitrating species exceeded that of water, and the measured rate would then be the rate of production of the nitrating species. The identification of the slow reaction with the formation of the nitronium ion followed from the fact that the initial rate under zeroth-order conditions was the same, to within experimental error, as the rate of 0-exchange in a similar solution. It was inferred that the exchange of oxygen occurred via heterolysis to the nitronium ion, and that it was the rate of this heterolysis which limited the rates of nitration of reactive aromatic compounds. 

A simple kinetic order for the nitration of aromatic compounds was first established by Martinsen for nitration in sulphuric acid (Martin-sen also first observed the occurrence of a maximum in the rate of nitration, occurrii for nitration in sulphuric acid of 89-90 % concentration). The rate of nitration of nitrobenzene was found to obey a second-order rate law, first order in the concentration of the aromatic and of nitric acid. The same law certainly holds (and in many cases was explicitly demonstrated) for the compounds listed in table 2.3. 

TABLE 2.3 Nitration in concentrated sulphuric acid compounds which have been studied kinetically... 

Although the nitronium ion cannot be detected by physical methods in these media, kinetic studies using these solutions have provided compelling evidence for the formation and effectiveness of this species in nitration. 

Under conditions in which benzene and its homologues were nitrated at the zeroth-order rate, the reactions of the halogenobenzenes ([aromatic] = c. o-1 mol 1 ) obeyed no simple kinetic law. The reactions of fluorobenzene and iodobenzene initially followed the same rates as that of benzene but, as the concentration of the aromatic was depleted by the progress of the reaction, the rate deviated to a dependence on the first power of the concentration of aromatic. The same situation was observed with chloro- andjbromo-benzene, but these compounds could not maintain a zeroth-order dependence as easily as the other halogenobenzenes, and the first-order character of the reaction was more marked. 

The observation of nitration in nitromethane fully dependent on the first power of the concentration of aromatic was made later. The rate of reaction of /)-dichlorobenzene ([aromatic] = 0-2 mol [HNO3] = 8-5 mol 1 ) obeyed such a law. The fact that in a similar solution 1,2,4-trichlorobenzene underwent reaction according to the same kinetic law, but about ten times slower, shows that under first-order conditions the rate of reaction depends on the reactivity of the compound. 

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. )... 

For nitration carried out in sulpholan ([HNO3] = 4-91 mol 1" ), zeroth-order nitration was observed with mesitylene. With toluene and benzene the kinetics were of mixed-order and first-order, respectively. ... 

Data for zeroth-order nitration in these various solvents are given in table 3.1. Fig. 3.1 shows how zeroth-order rate constants depend on the concentration of nitric acid, and table 3.2 shows how the kinetic forms of nitration in organic solvents depend on the reactivities of the compounds being nitrated. 

The influence of added species upon the rates and kinetic forms of nitration in organic solvents were of the greatest importance in elucidating details of the processes involved, particularly of the steps leading to the nitronium ion. These influences will first be described, and then in the following section explained. The species to be considered are sulphuric acid, nitrate ions, urea and water. The effect of nitrous acid is considered later ( 4.3). 

Nitration in organic solvents is strongly catalysed by small concentrations of strong acids typically a concentration of io mol 1 of sulphuric acid doubles the rate of reaction. Reaction under zeroth-order conditions is accelerated without disturbing the kinetic form, even under the influence of very strong catalysis. The effect of sulphuric acid on the nitration of benzene in nitromethane is tabulated in table 3.3. The catalysis is linear in the concentration of sulphuric acid. 

The addition of water depresses zeroth-order rates of nitration, although the effect is very weak compared with that of nitrate ions concentrations of 6x io mol 1 of water, and 4X io mol 1 of potassium nitrate halve the rates of reaction under similar conditions. In moderate concentrations water anticatalyses nitration under zeroth-order conditions without changing the kinetic form. This effect is shown below (table 3.5) for the nitration of toluene in nitromethane. More strikingly, the addition of larger proportions of water modifies the kinetic... 

The most crucial observation concerning the effects of added species is that nitrate ion anticatalyses nitration without changing the kinetic form of the reaction. This shows that nitrate does not exert its effect by consuming a proportion of the nitronium ion, for, as outlined above, this would tend to bring about a kinetically first-order reaction. Nitrate ions must be affecting the concentration of a precursor of the nitronium... 

When large concentrations of water are added to the solutions, nitration according to a zeroth-order law is no longer observed. Under these circumstances, water competes successfully with the aromatic for the nitronium ions, and the necessary condition for zeroth-order reaction, namely that all the nitronium ions should react with the aromatic as quickly as they are formed, no longer holds. In these strongly aqueous solutions the rates depend on the concentrations and reactivities of the aromatic compound. This situation is reminiscent of nitration in aqueous nitric acid in which partial zeroth-order kinetics could be observed only in the reactions of some extremely reactive compounds, capable of being introduced into the solution in high concentrations ( 2.2.4). 

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