CHEMICALLY INDUCED DECOMPOSITION

As mentioned above one of the fundamental attributes ascribed to homogeneous catalysts is superior activity at low temperature. However, even within classes of such catalysts, improvements in catalyst activity can be made allowing operation at lower temperatures, thus reducing or avoiding completely this mode of catalyst decay. One such example can found in recent advances in palladium catalysed ethene carbonylation (Equation 1.1). 

Equation 1.1. Ethene carbonylation leading to alkyl (R) propionates (n = 1) or to ethene carbon monoxide 

2 [11]. On occasion these over ligated complexes are materials that can be identified in solution or perhaps more tellingly isolated from catalytic reactions. Such reactions can often be reversed by removal of the excess reagent. Such processes are not considered in the context of this book as resulting in loss of overall turnover number. 

The loss of expensive catalyst from the reactor system can be fatal for any process. Physical loss involves the removal of active catalyst from the closed loop of the process. This can include the plating out of metal or oxides on the internal surfaces of the manufacturing plant, failure to recover potentially active catalyst from purge streams and the decomposition of active catalyst by the process of product recovery. The first two can be alleviated to some extent by improvements in catalyst or process design, the last is an intrinsic problem for all manufacturing operations and is the subject of this book. 

Catalysts are traditionally designed and optimised based on their performance in the reactor and not for their ability to withstand traditional separation processes. However, on taking any system from the laboratory to the pilot plant and beyond, this need to isolate product whilst efficiently recovering the catalyst often becomes the most important single issue. The best option is selection of a product isolation method that maintains the integrity of the catalyst and requires no further treatment of the catalyst prior to reintroduction into the reactor, or leaves the catalyst in the reactor at all times. 

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Chemically induced decomposition, of which two further categories can be considered namely substrate induced decomposition and poisoning by impurities or products. 

Although ionic liquids do not boil it is useful to know the temperature range under which they can operate. For many ionic liquids, the onset of decomposition takes place at temperatures exceeding 250°C and often much higher/181 However, 1,3-dialkylimidazolium phosphates were found to slowly decompose below 200°C 19 and when the liquid is composed of less common cations or anions, the thermal stability may be considerably lower. Nevertheless, since the overwhelming majority of catalysed reactions take place at much lower temperatures, thermally induced solvent decomposition tends not to be a problem, although chemically induced decomposition can be a real concern For example, thermal decomposition of [C4Ciim][BF4] takes place a much lower temperature if nucleophiles are present/201... 

Bones, A.M. and Rossiter, J.T. (2006) The enzymic and chemically induced decomposition of glucosinolates. Phytochemistry, 67,1053-67. 

By comparison to peroxides, the azo compounds are generally not susceptible to chemically induced decompositions. It was shown,however, that it is possible to accelerate the decomposition of a,a -azobisisobutyronitrile by reacting it with bis(-)-ephedrine-copper(II) chelate. The mechanism was postulated to involve reductive decyanation of azobisisobutyronitrile through coordination to the chelate. Initiations of polymerizations of vinyl chloride and styrene with a,a -azobisisobutyronitrile coupled to aluminum alkyls were investigated. Gas evolution measurements indicated some accelerated decomposition. Also, additions of large amounts of tin tetrachloride to either a,a -azobisisobutyronitrile or to dimethyl-a,a -azobisis-obutyrate increase the decomposition rates. Molar ratios of [SnCl4]/[AIBN]= 21.65 and [SnCl4]/[MAIB] = 19.53 increase the rates by factors of 4.5 and 17, respectively. Decomposition rates are also enhanced by donor solvents, like ethyl acetate or propionitrile in the presence of tin tetrachloride. ... 

Chemical off—on switching of the chemiluminescence of a 1,2-dioxetane (9-benzyhdene-10-methylacridan-l,2-dioxetane [66762-83-2] (9)) was first described in 1980 (33). No chemiluminescence was observed when excess acetic acid was added to (9) but chemiluminescence was recovered when triethylamine was added. The off—on switching was attributed to reversible protonation of the nitrogen lone pair and modulation of chemically induced electron-exchange luminescence (CIEEL). Base-induced decomposition of a 1,2-dioxetane of 2-phen5l-3-(4 -hydroxyphenyl)-l,4-dioxetane (10) by deprotonation of the phenoHc hydroxy group has also been described (34). 

The reaction of iV-acyloxy-iV-alkoxyamides and aromatic amines, the thermal decomposition of iV,iV -diacyl-iV,iV -dialkoxyhydrazines as well as the base-induced decomposition of azodicarboxyiates are examples of the HERON reaction, one of the newest-named reactions in the chemical literature , but one that seems to be common for these as well as a range of other bisheteroatom-substituted amides . No doubt new examples will be discovered and it is entirely predicated upon the configurational and conformational properties of anomeric amides, attributes arising directly as a consequence of the dual electron demands of the heteroatoms at nitrogen. 

B. Electron-transfer-induced Decomposition Chemically Initiated... 

Chemical Properties. Acyclic di-ferf-alkyl peroxides efficiently generate alkoxy free radicals by thermal or photolytic homolysis. Primary and secondary dialkyl peroxides undergo thermal decompositions more rapidly than expected owing to radical-induced decompositions. Such radical-induced peroxide decompositions result in inefficient generation of free radicals. 

In the literature (e.g. refer to [16]), one can find analogous schemes of induced decomposition of hydrogen peroxide, the simplest representative of peroxy compounds. Nevertheless, mechanical translation of the concept of induced initiator decomposition to H202 dissociation induces conclusions contradicting the notion of chemical induction and conjugated processes. 

In addition to the chemical activation non-equilibrium systems, the thermally induced decomposition of hydrocarbons and hydrocarbon radicals has also been widely encountered. The earliest hydrocarbon reactions to be studied were the thermal unimolecular decompositions of alkanes10 and alkyl radicals11 in which mirror removal techniques were used to demonstrate the actual presence of the radicals. These thermal reaction systems tend to be complex and, despite continued investigation, 12-13 many are not fully understood. 

The overwhelming majority of foams are TPs, but TSs are also foamed with CBAs, although some of them do create problems. Popular TS foams are made from polyurethane, polyester, phenolic, epoxy, and rubber. Thermal decomposition of the blowing agent with certain plastics such as TS polyesters cannot be applied in this system because the heat of polymerization is not high enough to induce decomposition. But chemical reactions simultaneously produce gas and free radicals they typically involve oxidation and reduction of a hydrazine derivative and peroxide. The reactions are catalyzed by metals, which can be used repeatedly. 

L13. Lehnig, M., Radical mechanisms of the decomposition of peroxynitrite and the peroxynitrite-CO(2) adduct and of reactions with 1-tyrosine and related compounds as studied by (15)N chemically induced dynamic nuclear polarization. Arch. Biochem. Biophys. 368, 303-318... 

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