Radical decomposition reactions

Radical decompositions are unimolecular reactions and show complex temperature and pressure dependence. Section 2.4.l(i) introduces the framework (the Lindemann mechanism) with which unimolecular reactions can be understood. Models of unimolecular reactions are vital to provide rate data under conditions where no experimental data exist and also to interpret and compare experimental results. We briefly examine one empirical method of modelling unimolecular reactions which is based on the Lindemann mechanism. We shall return to more detailed models which provide more physically realistic parameters (but may be unrealistically large for incorporation into combustion models) in Section 2.4.3. 

Direct experimental measurements of radical decompositions are relatively rare but recently a number of techniques have been applied and are producing new and interesting information. In Section 2.4.2. we shall mainly focus on the technique of laser flash photolysis coupled with photoionization mass spectrometry as a method of monitoring radical decompositions although other techniques will be briefly mentioned. 

In the final subsection we shall look in more detail at one of the crucial aspects of decomposition reactions, the true unimolecular and energy dependent dissociation of an activated molecule. Exciting new developments, both theoretical and experimental, are taking place in this area confirming and interpreting some of the proposed theories of unimolecular reactions. 

The rate of collisional deactivation is much greater than that of reaction 

At low pressures the rate of deactivating collisions will be very small, almost all of the activated molecules formed will dissociate and hence the rate determining step will be the bimolecular activation process. Now equation (2.18) reduces to 

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Radiative cooling, 23 13-14 Radiative heating/cooling, 23 25-26 Radical catalysts, 14 274 Radical cations, 12 249 Radical chain reactions, 14 274 Radical cyclization approach, 21 147 Radical decomposition reaction, 10 600 Radical generating systems, alternative, 14 299... 

For addition of Cl atoms, the dissociation energy of the radicals AX has been estimated at about 20-22 kcal./mole. At room temperature k ifki(A) should be well below 10-3 and mechanism (C) should be obeyed and has indeed been frequently observed. At higher temperatures (about 225°C.) k 2/k2(A) 10-3 and a change to mechanism (B) should occur. This has been confirmed experimentally by Adam et al. (1) in a study of the photochlorination of tetrachlorethylene. They observed a maximum in the rate and a change in mechanism at about 180°C., as a result of the increased importance of the radical decomposition reaction (—2). From their data they were able to deduce the Arrhenius parameters for this reaction. In extensions of this work Goldfinger and his collaborators have carried out competitive experiments with a number of hydrocarbons and chlorinated hydrocarbons. 

Both of these suggestions are defective because of the absence of methane (route A) and the much greater quantities of TMMD produced compared with DMA (route B with Reaction 19 as the precursor of methylmethylene imine). A further route to TMMD could be provided by methylene insertion into the NN bond of TMH. This, though theoretically feasible, seems unlikely and requires the production of methylene from dimethylamino radicals by a surface reaction. The radical decomposition reactions (29 and 30) proposed by Gesser, MuUhaupt, and Griffiths (15) are not confirmed by our results. 

Section 2.5 examines addition reactions which are the reverse of the radical decomposition reactions considered in Section 2.4. These reactions in themselves are comparatively unimportant in hydrocarbon oxidation, but they have provided a good source of thermodynamic data on radicals. Thermodynamic parameters are central to the modelling of autoignition because of the importance of heat release, but also because of their use in determining the rate parameters for the reverse of well characterized reactions. Section 2.5 includes a brief review of the currently accepted alkyl radical heats of formation. This field has been in turmoil in recent years because of disagreements on the values, which largely derive from kinetic measurements. Consensus is emerging but controversy still remains. 

Radical decomposition reactions radical concentration following a reaction such as... 

Radical decomposition reactions or indirectly via the generation of chlorine atoms... 

Butyl radicals decompose quickly to form ethylene and propylene. At high temperatures, alkyl radical decomposition reactions constitute an important reaction class and the prevailing fate of alkyl radicals. Take 1-propyl and 1-butyl radicals, for example. These primary alkyl radicals give rise to the following / -decomposition reactions ... 

Alkyl radical decomposition reactions (to form primary radicals) ... 

As previously discussed, alkyl radicals decomposition reactions constitute an important fate and reaction path of alkyl radicals. Due to the very short lifetimes of alkyl radicals, Rice and Herzfeld (1933, 1934) suggested a complete decomposition mechanism where all the radicals larger than methyl were considered instantaneously decomposed into alkenes and H and CH3 radicals. In this mechanism, all the intermediate alkyl radicals decompose to directly form alkenes and smaller alkyl radicals. This would mean that the final ethylene production from a steam cracking process would be significantly overestimated when compared with the experimental measurements. For instance, the net and final result of the successive decomposition mechanism of 1-decyl radical would be 5 moles of ethylene and one H radical. 

The rate of abstraction of H atoms from n-butane by lerl.-butoxy radicals was studied relative to the radical decomposition reaction... 

Levy (1956b) extended his studies to f-CaH70N0, -CaH70NO, and t-C4H9ONO. He found that the rates of thermal decomposition of diese nitrites were sharply decelerated by the addition of NO. The NO reduced the importance of alkoxyl radical decomposition reactions but increased the N2O produced. Thus... 

Figure 9. Calculated reaction coordinate of the radical decomposition reaction butyl transition state ethyl + ethylene. Structures are fully optimized at the MP2/6-31G level. The units of energies are in kcal/mol and bond lengths in A.

The reduction in the butadiene formation with increasing H2S concentration reflects the change in the radical decomposition reaction (Figure 8). Hydrogen atom adducts abstract hydrogen from the H2S or other hydrocarbon molecules at rates which are competitive with, or faster than, those for radical decomposition 6) Therefore, products such as linear pentenes are formed in preference to butadiene. 

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