Kinetic control nitration

The nitration of enol acetates with acetyl nitrate is a regiospecific electrophilic addition to the 3-carbon of the enol acetate, followed by a hydrolytic conversion of the intermediate to the a-nitro ketone. With enol acetates of substituted cyclohexanones the stereochemistry is kinetically established. So, 1-acetoxy-4-methylcyclohexene (22) yields the thermodynamically less stable rrans-4-methyl-2-nitrocylo-hexanone (24) in greater proportion cis. trans = 40 60) (equation 8). This mixture can be equilibrated in favor of the thermodynamically more stable cis diastereomer (23) (cis. trans = 85 15). Nitration of 1-ace-toxy-3-methylcyclohexene (25) leads to frans-3-methyl-2-nitrocyclohexanone (26), which is also the thermodynamically more stable isomer (equation 9). No stereoselection occurs in the kinetically controlled nitration with acetyl nitrate of l-acetoxy-5-methylcyclohexene (27 equation 10), but the 1 1 mixture of the 5-methyl-2-nitrocyclohexanones can be equilibrated in favor of the trcms diastereomer (28) (cis trans = 10 90). 2-Alkyl-2-nitrocyclohexanones cannot be prepared in acceptable yields by nitration of the corresponding enol acetates with acetyl nitrate. 

For some reactions, particularly those in which the reverse reaction is known to occur readily, it is necessary to determine if the product isomer distribution is due to thermodynamic rather than kinetic control. Products which are sterically hindered are particularly susceptible to rearrangement to less hindered isomers. It is also necessary to determine whether reaction occurs on the free base or conjugate acid, though for many of these reactions the conditions are considerably less acidic than those used in nitration or hydrogen exchange. 

Albright, Hanson, et al (1,2,3 ) have reviewed the work of previous investigators and concluded that the criteria sometimes used to determine the rate-limiting step were not adequate. The reaction kinetics measured by some workers (4,5 ) exhibit considerable differences even though they claimed to be measuring intrinsic kinetics. Such differences may result from diffusion effects. Recently, Cox and Strachan (6 ) reported the nitration of chlorobenzene to be kinetically controlled at a nitric acid concentration of 0.032 mole/liter in 70 weight percent sulfuric acid for the nitration of toluene in the same acid, the rates of chemical reaction and diffusion were of comparable magnitude. 

The early workers (1,3) claimed the rates they measured were kinetically controlled, but the evidence in support of their claims was critically examined by later workers (6,T) and found wanting. Albright and Hanson (7 ) went so far as to conclude in 1969 that toluene nitration in the two phase is mass transfer controlled under all conditions so far investigated. Recent results (8,9) with a batch reactor must cast doubts upon this conclusion. This paper therefore seeks to clarify the conditions under which kinetic and mass transfer control occur in the nitration of toluene in CFSTRs and to provide an explanation for the rate correlations shown in Figures 1 and 2. 

Most workers (3,7,10) are agreed that in the two phase system the nitration reaction with toluene occurs almost exclusively in the acid phase. Batch reactor studies (8,9) show that at low acid strengths the rate is kinetically controlled and given by Rg, = k2[HN03] [T]g, where k2 is the rate constant for homogeneous nitration in the acid phase, and CHNO3] and [Tl are the acid phase concentrations of nitric acid and toluene respectively. 

This complication however does not detract from the general, conclusion that in most of McKinley and White s runs at strengths of sulphuric acid of 30 mole % and below the rate of nitration is kinetically controlled. 

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. 

The rate of nitration of toluene in a CFSTR is kinetically controlled when low acid strengths are used and Rg /X is below 10 mol dm" hour". It is mass transfer controlled when high acid strengths are employed and Rg/Xip is above 10 mol dm 3 hour l... 

Figures 6 and 7 show the transformation between mass transfer cind kinetic control during the nitration of benzene and toluene for differing initial concentrations of nitric acid. Where the plots of log(Aw - At) cure linear, chemical kinetics are rate controlling, curved plots indicate the diffusion controlled and a treuisition region. If the initial concentration of nitric acid is low enough, the entire reaction is kinetically controlled.

Investigations on the hydrothermal crystallisation of nitrate enclathrated cancrinite were performed using the alkaline transformation of zeolites A and X at a temperature of 353 K in 2-molar and 16-molar NaOH-solutions. The conversion of the zeolites was followed in the early stage of the reactions for times up to 48 hours by XRD and IR- spectroscopy. A fast and total transformation of zeolite X into cancrinite could be stated in most of the experiments, independent of the alkalinity. In contrast the conversion of zeolite A under low alkaline conditions was slower and accompanied by a sodalite-cancrinite cocrystallisation as well as the formation of an intermediate phase between both structure types. The results indicate a more kinetically controlled reaction mechanism for zeolite A transformation. 

These reactions occur as low as 200°C. The exact temperature depends on the specific hydrocarbon that is nitrated, and reaction 8 is presumably the rate-controlling step. Reaction 9 is of minor importance in nitration with nitric acid, as indicated by kinetic information (32). 

Fe(III) displacement of Al(III), Ga(III), or In(III) from their respective complexes with these tripodal ligands, have been determined. The M(III)-by-Fe(III) displacement processes are controlled by the ease of dissociation of Al(III), Ga(III), or In(III) Fe(III) may in turn be displaced from these complexes by edta (removal from the two non-equivalent sites gives rise to an appropriate kinetic pattern) (343). Kinetics and mechanism of a catalytic chloride ion effect on the dissociation of model siderophore-hydroxamate iron(III) complexes chloride and, to lesser extents, bromide and nitrate, catalyze ligand dissociation through transient coordination of the added anion to the iron (344). A catechol derivative of desferrioxamine has been found to remove iron from transferrin about 100 times faster than desferrioxamine itself it forms a significantly more stable product with Fe3+ (345). 

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