Nitration encounter control

An example of the application of this test to a compound that nitrates as its free base is provided by pyridine 1-oxide. Under an identical set of conditions, nitration of this N-oxide had a half life of 20 min, whilst 1-methoxypyridinium gave no nitro compound in 144 h. Two further criteria have been used to provide confirmatory evidence, namely comparison of the rate of nitration for the reactive species with the encounter controlled rate, and by determination of the Arrhenius parameters. 

The previously described reactivity data for the five-membered heterocycles are gathered (in terms of o+ values) in Table 6.12 no data are given for nitration because the rates are encounter controlled and meaningless in terms of electronic effects. Among the other data, those for mercuriation, protiodemercuriation, and protiodeboronation are doubtful, and other qualifying aspects are noted in the table footnotes. The following main features are noteworthy. 

Relative reactivity data for nitration must be treated with special caution because of the possibility of encounter control. An example of this can be seen in Part A of Table 9.7, where no difference in reactivity between mesitylene and xylene is found in H2SO4-HNO3 nitration, whereas in HNO3-CH3NO2 the rates differ by a factor of more than 2. Encounter-control prevails in the former case. In general, nitration is a relatively unselective reaction with toluene being about 50-60, as shown in... 

Complex kinetic behavior Is often observed and considerable effort is required to define the reaction path. Thus, Coombes in a recent investigation of the nitration reaction in carbon tetrachloride pointed out that the reaction with mesitylene approached the encounter controlled diffusion limit.However, he noted that the work was Incomplete. 

Olah and Lin studied the boron trlfluoride-catalyzed reaction of methyl nitrate with aromatic compounds in nltromethane.2 Using competition methods they found that the toluene to benzene rate ratio was 25 and that the rate ratios for the methylben-zenes. Table IX, reached a limiting value, about 1000. Unfortunately, kinetic rate data were not obtained. However, the leveled reactivity of the polymethylbenzenes suggests that these compounds react at the encounter controlled diffusion limit. 

The nitration reactions also exhibit high selectivity when the reactions proceed at the encounter controlled diffusion limit. To Illustrate, the nitration of anisole yields little meta product the ortho and para isomers are produced almost exclusively. Similarly, the substitution products of m-xylene... 

Nitration in sulphuric acid is a reaction for which the nature and concentrations of the electrophile, the nitronium ion, are well established. In these solutions compounds reacting one or two orders of magnitude faster than benzene do so at the rate of encounter of the aromatic molecules and the nitronium ion ( 2.5). If there were a connection between selectivity and reactivity in electrophilic aromatic substitutions, then electrophiles such as those operating in mercuration and Friedel-Crafts alkylation should be subject to control by encounter at a lower threshold of substrate reactivity than in nitration this does not appear to occur. 

Another type of electrophilic substitution subject to microscopic diffusion control occurs when a highly reactive form of the substrate is produced in a pre-equilibrium step (e.g. by proton loss) and when this form reacts on encounter with the electrophile. The nitration of p-nitroaniline in 90% sulphuric acid appears to be a reaction of this type (Hartshorn and Ridd, 1968), although the short lifetime of the free amine complicates the mechanistic interpretation. The formulation in Scheme 1 fits this type of reaction provided A is taken to represent the protonated amine, X the free amine, and B the nitronium ion. In 90% sulphuric acid, the nitronium ion is the bulk component of the NOJ—HN03 equilibrium mixture. Many of the reactions in this review can be represented by Scheme f with some reservations concerning the lifetime of the intermediate X. 

The term macroscopic diffusion control has been used to describe processes in which the rate of reaction is determined essentially by the rate of mixing of the reactant solutions. The nitration of toluene in sulpholane by the addition of a solution of nitronium fluoroborate in sulpholane appears to fall into this class (Ridd, 1971a). Obviously, if a reaction is subject to microscopic diffusion control when the reactants meet in a homogeneous solution, it must also be subject to macroscopic diffusion control when preformed solutions of the same reactants are mixed. However, the converse is not true. The difficulty of obtaining complete mixing of solutions in very short time intervals implies that a reaction may still be subject to macroscopic diffusion control when the rate coefficient is considerably below that for reaction on encounter. The mathematical treatment and macroscopic diffusion control has been discussed by Rys (Ott and Rys, 1975 Rys, 1976), and has been further developed recently (Rys, 1977 Nabholtz et al, 1977 Nabholtz and Rys, 1977 Bourne et al., 1977). It will not be considered further in this chapter. 

The limiting rate of such a reaction can exceed the diffusion-controlled limit (Sections 2 and 6). For the nitration of a species such as benzene, this distinction between the formation of the encounter pair by diffusion and pre-association is irrelevant, since the encounter pair formation is not rate-determining. Hence, it may be significant that the relative reactivity of mesitylene to benzene is greater in acetic anhydride (a factor of 650) than in the other solvents (Table 6). However, this can also be explained by the greater viscosity of most of the other media. 

It is interesting to note that the direct method of nitration was attempted in Europe as well as in the U.S.A. (Barksdale Plant), long before the Canadian and Keystone Ordnance Works trials indicated the great benefits derived from this process. However, the early European experimentation indicated that the method was unsafe and that it was difficult to control the important temperature and time factors. It seems highly probable that the difficulties encountered were due lo improper feeding of toluene or MNT or DNT to the acid in the nitrator, poor agitation, inefficient cooling and the use of incorrect acid compositions. 

Hydroxylamine is prepared by reduction of nitrates or nitrites either elec-trolytically or with S02, under very closely controlled conditions. It is also made, in 70% yield, by H2 reduction of N02 in HC1 solution with platinized active charcoal as catalyst. Free hydroxylamine is a white solid (m.p. 33°) which must be kept at 0°C to avoid decomposition. It is normally encountered as an aqueous solution and as salts, e.g., [NH3OH]Cl, [NH30H]N03 and [NH30H]2S04, which are stable, water-soluble, white solids. Although... 

In summary, the kinetic observations for aromatic nitration provide a consistent pattern of results. Conventional behavior Is observed in the aqueous acid solvents for the compounds which are less reactive than the xylenes. However, the aromatic compounds which are approximately 50-fold more reactive than benzene exhibit a limiting reaction rate In these solvents. The rate limit can be identified with the diffusion controlled formation of the encounter complex. 

By analogy with literature discussions ( ) of diffusion-controlled processes it was implied that there was no interaction within the encounter pair. If this were the case, as k i can be estimated ( 7) as about 10 -10 ° s and k2 can in the limit approach 10 - 10 s , it would be possible for considerable intramolecular selectivity to remain between positions in a substrate which are sufficiently reactive to be nitrated at the limiting rate.( )... 

For any aromatic A the transition from kinetic to mass transfer control will occur when Rg /X - a k[A] , being the mole fraction of A in the organic phase and [A] the solubility of A in the acid phase. The value of a k is unlikely to vary widely from one aromatic to another. Hence the probability of encountering kinetic or mass transfer control depends largely on the solubility of the aromatic in the acid phase. The more soluble the aromatic, the more likely is its nitration rate to be kinetically controlled. Thus, since nitroaromatics are much more soluble in siilphuric acid than their parent compounds, it is highly probable that the rates of industrial di- and trinitration in CFSTRs are kinetically controlled. 

In liquid-liquid systems, a chemical reaction is encountered for three distinct purposes. Firstly, the reaction may be a part of the process, such as nitration and sulphonation of aromatic substances, alkylation, hydrolysis of esters, oximation of cyclohexanone, extraction of metals and pyrometallurgical operations involving melts and molten slag. Secondly, a chemical reaction is deliberately introduced for separation purposes (e.g. removal of dissolved acidic solutes from a variety of hydrocarbons). Finally, the yield and the rate of formation of many single phase reactions are affected and often can be favourably increased by the deliberately controlled addition to the reaction system of an immiscible extractive phase, whose major purpose is to extract the product from the reactive phase. Such operations are sometimes referred to as "extractive reactions" and have been discussed previously in some detail (14-17). 

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