Nitrogen pentoxide reaction mechanism

In the case of relatively simple reaction mechanisms, the net or overall effect of the elementary reactions can be determined by adding them together. For example, the stoichiometric equation for the decomposition of nitrogen pentoxide is... 

The most satisfactory explanation is that the rate is increased beyond the maximum rate of activation by collision through the operation of a chain mechanism. The observations of Sprenger on the peculiar behaviour of nitrogen pentoxide at low pressures suggest strongly that chains are propagated. Moreover, if the rate of the azoisopropane reaction at the lowest pressures should prove to be greater than can be accounted for on the basis of the simple collision mechanism, a chain mechanism can be assumed without difficulty since the reaction is quite markedly exothermic. 

Decomposition of Nitrogen Pentoxide. The principal facts about this reaction have already been stated. The observation of Sprenger that below a certain pressure the reaction ceases entirely indicates a chain mechanism, but its nature is not clear. 

There are very few cases where one can compare directly the same reaction taking place by the same mechanism in both gas phase and solution. If at all temperatures the reactions have equal velocities in the two phases the values of 5 and of E are the same, and it may be safely assumed that the reaction mechanisms are identical and the solvent has no effect. Undoubtedly the simplest comparison exists in the unimolecular decomposition of nitrogen pentoxide and in this reaction the solvent has little effect. The unimolecular racemization of pinene at 200° proceeds at the same rate in the gas phase, in liquid pinene and in a solution of petrolatum. 

An alternative mechanism has been proposed,18 according to which the activated nitrogen dioxide molecule collides with a molecule of nitrogen pentoxide and causes dissociation of the latter. It is easier to decompose a molecule of nitrogen pentoxide than a molecule of nitrogen dioxide. The over-all reaction is exactly the same and one cannot distinguish between the two mechanisms by stoichiometry, but the facts just cited favor the mechanism proposed by Norrish and none of the experimental data are in opposition to it. 

Regardless of the particular mechanism, if the activation process is accomplished through the absorption of single or multiple frequencies, then a marked increase in the radiation density of these frequencies should cause a change in reaction velocity. The decomposition of ozone and nitrous oxide have been studied from this point of view and are reported here. Since the inception of this work, it has been found elsewhere3 that the oxidation of alcohol vapor and the decomposition of nitrogen pentoxide and hydrochloric acid were not affected by infra-red radiation. The present work confirms these results for two additional reactions. 

Summary.—The mechanism of the activation process in gaseous systems has been investigated from the point of view of (1) activation by radiation (2) activation by collision. An increase in the radiation density of possible activating frequencies has resulted in no increased reaction velocity. The study of the bimolecular decomposition of nitrous oxide at low pressures has led to the conclusion that the reaction is entirely heterogeneous at these pressures. A study of the unimolecular decomposition of nitrogen pentoxide between pressures of 7io mm. Hg and 2 X 10 3 mm. Hg shows no alteration in the rate of reaction such as was found by Hirst and Rideal but follows exactly the rate determined by Daniels and Johnson at high pressures. No diminution of the reaction velocity as might be ex-expected from Lindemann s theory was observed. 

The reaction mechanism for the decomposition of nitrogen pentoxide is complex, as described in Sec. 2-2. However, a satisfactory rate equation can be developed by considering the two reactions... 

In other systems, the solvent only acts as an inert medium where reaction occurs without affecting the mechanism. This appears to be the case of the decomposition of nitrogen pentoxide, in which the pre-exponential factor and the activation energy are very similar in the gas phase and in solvents of various types, as shown in Table 9.1. 

In a three-necked flask equipped with a mechanical stirrer, the tetraamine and solvent PPA were added. The solution was stirred at 110°C under nitrogen atmosphere for about 1.5 h to get a homogeneous solution. An equimolar amount of diacid was added to the solution, and the reaction was continued for 12h at 140 °C. Then a catalytic amount of phosphorus pentoxide and triphenyl phosphite (TPP) were added into the system. The solution became brownish and viscous. It was heated to about 230 °C for another 24 h and then poured into the water, washed with deionized water several times, and neutralized by alkaline solution. Finally, the polymer was dried under vacuum for 24h [15,16]. 

The thermal decomposition of dinitrogen pentoxide in nitrogen dioxide and oxygen (reaction [2.R3]) is a nnique seqnence reaction with a mechanism that consists of the three steps [2.R3a], [2.R3b] and [2.R3c] ... 

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