Rate of Spontaneous Decomposition

Though we cannot hope to give a precise picture of the mode of decomposition of a polyatomic molecule, we can represent schematically, as in Fig. X.4, the general behavior of the potential energy of a molecule as a function of configuration. In Fig. X.4 is shown a schematic hyper- 

The erratic curve (enclosed within the surface) represents a possible 

Wc note that P E,t) must have the property that i Efi) = 0 and P(Eyt) — 1 as oo. H. A. Kramers, Physics 7,284 (1940), has given an approximate solution to this problem based on a simplified analogy to a model for brownian motion in one dimension. 

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A few details have been reported on the slow reductions of 8303 by As(ni) and T1(I) . In the anaerobic reductions by As(III) the reaction is first-order in 820 and although As(III) certainly catalyses decomposition, the dependence of the rate on [As(III)] is small. Aeration leaves the rate of spontaneous decomposition of 820g unaffected, but the As(III)-catalysed route is accelerated by a factor of ten, the kinetic law remaining unchanged. The oxygen effect is interpreted in terms of the chain reaction... 

The enzymic study of the catabolism of tyrosine has been hindered by the chemical instability and the high rate of spontaneous decomposition of many of the metabolic intermediates and other auxiliaiy compounds required fw the oxidation. This has made it difficult to establish dependable balances of the oxygen utilized and the reaction products formed with the... 

The rate at which initiator molecules decompose, and hence the rate of cross-linking, is a function of their chemical stability and the temperature to which they are subjected. The decomposition of organic peroxides is an approximately first-order reaction, its rate increasing exponentially as a function of temperature. The rate of spontaneous decomposition of an initiator at any temperature is typically characterized in terms of its half-life. The approximate half-lives of some commonly used peroxide intiators are listed in Table 1. Initiator molecules for cross-linking polyethylene are selected with respect to the temperature at which the resin must be processed. The goal is to homogenize and mold the resin into the desired shape below the temperature at which peroxide decomposition becomes significant premature decomposition of the peroxide is known as... 

A distinction must be made between chemical and physical stability. While physical stability is important, particularly in the evaluation of solid propellants, the chemical stability is of prime importance in the estimation of the course of decomposition of nitrate esters. The nitrate esters which are processed for use as propellants - unlike nitro compounds, which are relatively stable under these conditions - undergo a steady decomposition, which is due to imperfect purification of the starting materials and to the effect of other parameters such as temperature and air humidity. The rate of this decomposition is auto-catalyzed by the acidic decomposition products and may in certain cases produce spontaneous ignition. In order to reduce the decomposition rate as much as possible, suitable stabilizers are added to the powders, which are capable of accepting the acid cleavage products with formation of the corresponding nitro compounds (- Stabilizers). The stability is controlled by means of several tests (- Hot Storage Tests). 

Numerous lists are available in the literature that give the decomposition temperatures or the half-lives at certain elevated temperatures of many initiators. Decompositions of peroxides may proceed via concerted mechanisms and the rates are structure-dependent. This can be illustrated on benzoyl peroxide. The benzoyl groups, the two halves of the molecule, are dipoles. They are attached, yet they repel each other. Rupture of the peroxide link releases the electrostatic repulsion between the two dipoles. Presence of electron donating groups in the para position increases the repulsion, lowers the decomposition temperature, and increases the decomposition rate. The opposite can be expected from electron attracting groups in the same position. The effect of substituents on the rate of spontaneous cleavage of dibenzoyl peroxide was expressed in terms of the Hammett equation, log K/Ko) = pa. This is shown in Table 2.3. 

Pressure tends to increase the chemical reactivity of nitromethane as well as the rate of thermal decomposition. It was observed, quite accidentally, that a pressure-induced spontaneous explosion of single crystals of nitromethane at room temperature can occur. Further study revealed that single crystals grown from the liquid with the (111) and either the (001) or the (100) crystal faces perpendicular to the applied load direction in the DAG, if pressed rapidly to over 3 GPa, explode instantaneously accompanied by an audible snapping sound. The normally transparent sample becomes opaque instantly. Visual examination of the residue revealed a dark brown solid which was stable when heated to over 300 C. Subsequent x-ray analysis showed the material to be amorphous. Mass spectral analysis of the residue was inconclusive because no well defined spectra were observed. Because most of the sample is recovered as solid residue after the explosion and is stable to over 300°C, the material may be amorphous carbon. This stress-induced explosion occurs only in protonated nitromethane because similar attempts on the deuterated form did not result in explosion. Shock experiments on oriented pentaerythritol (PETN) crystals have shown similar type behavior [25]. In this case it was suggested that the sensitivity of shock pressures to crystal orientation is the result of the availability of slip planes or system of planes in the crystal to absorb the shock, thereby increasing the threshold to explosion. A similar explanation may be applicable to the nitromethane crystals as well. The deuteration effect must play a role in the initiation chemistry. An isotope effect has been observed previously in the sensitivity of HMX and RDX to shock and thermal conditions [23]. 

The solvents listed in Table 2.1 were chosen to cover a broad range in solvent properties. In fact hexane was initially also among them, but unfortunately the rate of the reaction in this solvent is extremely low. It turned out that in this solvent spontaneous decomposition of 2.4a competes with the Diels-Alder reaction. 

Although thermodynamics can be used to predict the direction and extent of chemical change, it does not tell us how the reaction takes place or how fast. We have seen that some spontaneous reactions—such as the decomposition of benzene into carbon and hydrogen—do not seem to proceed at all, whereas other reactions—such as proton transfer reactions—reach equilibrium very rapidly. In this chapter, we examine the intimate details of how reactions proceed, what determines their rates, and how to control those rates. The study of the rates of chemical reactions is called chemical kinetics. When studying thermodynamics, we consider only the initial and final states of a chemical process (its origin and destination) and ignore what happens between them (the journey itself, with all its obstacles). In chemical kinetics, we are interested only in the journey—the changes that take place in the course of reactions. 

Organic peroxides and hydroperoxides decompose in part by a self-induced radical chain mechanism whereby radicals released in spontaneous decomposition attack other molecules of the peroxide.The attacking radical combines with one part of the peroxide molecule and simultaneously releases another radical. The net result is the wastage of a molecule of peroxide since the number of primary radicals available for initiation is unchanged. The velocity constant ka we require refers to the spontaneous decomposition only and not to the total decomposition rate which includes the contribution of the chain, or induced, decomposition. Induced decomposition usually is indicated by deviation of the decomposition process from first-order kinetics and by a dependence of the rate on the solvent, especially when it consists of a polymerizable monomer. The constant kd may be separately evaluated through kinetic measurements carried out in the presence of inhibitors which destroy the radical chain carriers. The aliphatic azo-bis-nitriles offer a real advantage over benzoyl peroxide in that they are not susceptible to induced decomposition. 

If the rate constant kd for the spontaneous decomposition of the inhibitor is known, and its efficiency has been established, the important ratio kl/kt may then be evaluated. With greater generality, Eq. (8) may be combined with (12) to give... 

If the rate constant kd for spontaneous decomposition of the initiator is known, the efficiency / of initiation may be determined. (This is a refinement of the molecular weight method set forth in Sec. Ic) The spontaneous decomposition rate of benzoyl peroxide in styrene, according to the work of Swain, Stockmayer, and Clarke is 3.2X10- sec. at 60°C. Hence the efficiency of initiation of the polymerization of styrene by benzoyl peroxide at 60°C is indicated to be about 0.60. 

The rate of copolymerization in a binary system depends not only on the rates of the four propagation steps but also on the rates of initiation and termination reactions. To simplify matters the rate of initiation may be made independent of the monomer composition by choosing an initiator which releases primary radicals that combine efficiently with either monomer. The spontaneous decomposition rate of the initiator should be substantially independent of the reaction medium, as otherwise the rate of initiation may vary with the monomer composition. 2-Azo-bis-isobutyronitrile meets these requirements satisfactorily. The rate Ri of initiation of chain radicals of both types Ml and M2 is then fixed and equal to 2//Cd[7], or twice the rate of decomposition of the initiator I if the efficiency / is equal to unity (see Chap. IV). The relative proportion of the two types of chain radicals created at the initiation step is of no real importance, for they wall be converted one into the other by the two cross-propagation reactions of the set (1). Melville, Noble, and Watson presented the first complete theory of copolymerization suitable for handling the problem of the rate. The theory was reduced to a more concise form by Walling, whose procedure is followed here. 

Yet another situation is observed in the 2,4-dinitrophenyl phosphate dianion. A significant effect of amines on the rate of decomposition is admittedly observed however, typical 2nd order kinetics, lower enthalpy of activation compared with spontaneous hydrolysis, and strongly negative AS values (see Table 3) indicate an Sn2(P) reaction. Surprisingly, the reaction rate remains unaffected by the basicity of the amine, even when its pKa value changes by 8 units. 

A lire disaster costing 67 million occurred in Texas City, Texas, on the SS Grandcamp (16 April 1987) due to spontaneous ignition of stored fertilizer in the ship s hold. A release of steam from an engine leak caused the atmosphere of the ammonium nitrate fertilizer to be exposed to temperatures of 100 °C. Ammonium nitrate (NH4NO3) decomposes exothermically releasing 378 kJ/g mol. Its rate of decomposition can be described by the Arrhenius equation ... 

The kinetics of coupling a diazonium salt (D) with naphthol (N) to form a dye (A) is complicated to a minor extent by spontaneous decomposition of D to form inert products P. Bata of A, but not of other participants, are in the table (Hanna et al, JACS 96 7222, 1974). Temperature was 0 C, N0 = 0.01 mols/liter, D0 - 0.0001. Find the specific rates. 

Capable of polymerization, decomposition or rearrangement. Initiation of the reaction can be spontaneous, by energy input such as thermal or mechanical energy, or by catalytic action increasing the reaction rate. 

These catalytic effects are usually signaled by irreproducible behavior. If it is suspected that traces of metal ions may be causing peculiar rate effects, a strong ligand may be added to sequester the metal ion. The spontaneous decomposition of H2O2 has been reported as 4.7 X 10 M s at pH 11.6 and 35°C. This is the lowest recorded value and is obtained in the presence of strong chelators. In a similar way the decomposition of permanganate in alkaline solution (3.6) is markedly slowed when the reactants are extensively purified... 

A critical size for "spontaneous detonation thru self-heating may be a general phenomenon, even though widely varying for different explosives. The view of this as a possibility arises from the indication, in the Arrhenius equation, that there is some decomposition constantly taking place even at room temperatures. As the linear dimension d of the charge increases, the rate of heat evolution increases as d, whereas the rate of heat dissipation by conduction increases only as d. Theoretically, therefore, a size must be attainable at which the sample will eventually... 

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