Azobis , decomposition

Azo compounds having functional groups that stabilize the radical products are especially reactive. The stabilizing effect of the cyano substituent is responsible for the easy decomposition of azobis(isobutyronitrile) (AIBN), which is frequently used as a radical initiator. 

The preparation of tetramethylsuccinonitrile by decomposition at 90-92°C in azobis-isobutyronitrile (AIBN) heptane gives rise to a detonation. The medium was not stirred and the accident is related to the instability of this diazoic compound, which had probabiy accumulated (the nitrile groups are not responsible for it). If AIBN Is introduced into heptane slowly, at 90°C and under vigorous stirring, the reaction seems safer. 

Meso- and (+ )-azobis[6-(6-cyanododecanoic acid)] were synthesized by Porter et al. (1983) as an amphipathic free radical initiator that could deliver the radical center to a bilayer structure controllably for the study of free radical processes in membranes. The decomposition pathways of the diazenes are illustrated in Fig. 36. When the initiator was decomposed in a DPPC multilamellar vesicle matrix, the diazenes showed stereo-retention yielding unprecedented diastereomeric excesses, as high as 70%, in the recombination of the radicals to form meso- and (+ )-succinodinitriles (Brittain et al., 1984). When the methyl esters of the diazene surfactants were decomposed in a chlorobenzene solution, poor diastereoselectivity was observed, diastereomeric excesses of 2.6% and 7.4% for meso- and ( )-isomers respectively, which is typical of free radical processes in isotropic media (Greene et al, 1970). 

Fig. 36 Structure of meso- and (+ )-azobis[6-(6-cyanododecanoic acid)] and their decomposition scheme.

The thermal decomposition of azoinitiators, e.g., azobis isobutyronitrile can be represented as under... 

Combining thianthrene radical ion(l+) with free radicals to produce thianthrenium salts has also been achieved. Decomposition of various cumene hydroperoxides (83MI6) and of azobis(2-phenoxy-2-propane) (85MI1) gave 5-arylthianthrenium ions together with 5-(propen-2-yl)thianthrenium perchlorate in the latter case. 

The two most important classes of radical initiators are azo-compounds and peroxides (Fig. 10.1). The most commonly used azo-initiators are 2,2/-azobis(isobutyronitrile) (AIBN) and l,l/-azobis(cyclohexane-l-carbonitrile) (ACN). The shorter half-life of AIBN ( ti/2 = 1.24 h at 80°C in benzene) has led to international shipping restrictions and ACN (ti/2 = 29.61 h under the same conditions [l])isbecominganincreasinglyvaluablereplace-ment. As explained in Section 10.4, however, this appreciable difference in decomposition rate means that ACN cannot replace AIBN as an initiator for all radical reactions. Furthermore, AIBN is soluble in a wider range of solvents, polar as well as non-polar (including alcohols, acetonitrile and benzene), compared with ACN which is restricted to use in nonpolar solvents, such as benzene, toluene and cyclohexane. 

Several years later, DeSimone and co-workers (Guan et al., 1993) examined the decomposition of the free-radical initiator 2,2 azobis(isobutyro-nitrile) (AIBN, Scheme 4.6) in sc C02 as a function of pressure. The rate constant was found to increase with increasing pressure (reaching a maximum at approximately 250 bar). At higher pressures, the rate constant... 

Guan, Z. Combes, J. R. Menceloglu, Y. Z. DeSimone, J. M. Homogeneous Free Radical Polymerizations in Supercritical Carbon Dioxide 2. Thermal Decomposition of 2,2 -Azobis(isobutyronitrile), Macromolecules 1993, 26, 2663-2669. 

The definition of an antioxidant suggests a functional assay of antioxidants by measuring inhibition of appropriate (easy to study) oxidation reactions. Such assays can be called inhibition assays for antioxidants (Fig. 2). Various oxidants are used in TAC assays. In many cases, thermal decomposition of 2,2 -azobis (2-amidopropane) (ABAP) is the source of oxidizing radicals. ABAP undergoes temperature-dependent homolysis. The primary radicals produced by thermal decomposition of the initiator react with oxygen to produce peroxyl and alkoxyl radicals, which are oxidizing species in the system (Fig. 3). The amount of free radicals formed in an aqueous medium by decomposition of ABAP at pH 7.4 and at 37°C equals 1.36 x 10-6 [ABAP] x t, where t is time in seconds and [ABAP] is in mol L-1 (N8). 

Although radical chain processes occur spontaneously at moderate temperatures, it is usually desirable to faciliate the chain propagation by addition of an initiator. 2,2 - Azobi-sisobutyronitrile (AIBN = 2,2 -dimethyl-2,2 -azobis[propanenitrile]) is an ideal initiator, its decomposition rate is solvent independent. Such a reaction is described for a prototype 5-hexenylbromide with tributyltin hydride initiated by In [23] as an illustration in Scheme VII/10. 

Other advantages of C02-based polymerizations are that there is no chain transfer to the solvent, and that the production of unstable end groups can be dramatically reduced. Guan et al [4] studied the decomposition of 2,2 -azobis-(isobutyronitrile) (AIBN) in SCCO2. It was found that initiator efficiencies greater than 80% were possible due to the low viscosity of CO2 and negligible solvent cage effects. Additionally, analysis of the decomposition products showed that there was no chain transfer to CO2. 

Using other diazo compounds [di(3-hydroxybutyl)-2,2 -azobisisobutyrate, di(2-hydroxypropyl)-2,2 -azobisisobutyrate and di(2-hydroxyethyl)-2,2 -azobisisobuty-rate] hydroxytelechelic polybutadienes containing primary or secondary hydroxyl terminal groups were prepared 32). The decomposition kinetics of di(4-hydroxybutyl)-2,2 -azobisisobutyrate and di(3-hydroxybutyl)-2,2 -azobisisobutyrate is faster than that of 4,4 -azobis(4-cyano-n-pentanol). Usually, at the same temperature and in the same solvent, the decomposition of di(x-hydroxyalkyl)-2,2 -azobisisobutyrate is faster than that of AIBN 35). The solvent polarity has no effect on the decomposition kinetics. 

Studies of photoisomerization in azoalkanes and azobenzene derivatives have also been described. The E-a,a-dimethylallylazoalkane (4) is converted on irradiation with a nitrogen laser at 25 °C into the thermally labile Z-isomer (5). The triplet-sensitized decomposition of azobis(isobutyronitrile), however, has clearly been shown not to involve an analogous photoisomerization to a thermally labile Z-isomer. Photoisomerization of an azo-group is also implicated in the observed photochromism of benzazoylformazans. ... 

Measure oxygen consumption during controlled lipid oxidation induced by thermal decomposition of 2,2 Azobis(2-aminopropane) hydrochloride AAPH Based on inhibition of production of 2,2 Azinobis(3-ethylbenzthiazoline)-6-sulfonic acid ABTS radical cation and mM concentration of a Trolox solution having antioxidant capacity equivalent to 1.0-mM solution of test substance... 

It is obvious that the absence of the melting point is characteristic of the DTA curve of a powdery chemical of the TD type. That is, a powdery chemical of this type decomposes prior to melting. The self-heating behavior of a powdery chemical, such as 98 % O, a -azobis(isobutyronitrile) (AIBN), which decomposes explosively prior to any remarkable exothennic decomposition reaction, is also of the TD type, so that the F-K equation is applied to calculate its 1. When confined in the closed cell and subjected to the adiabatic selfheating test started from a in the range of 65 to 74 C, 2 cm of AIBN shows such a self-healing behavior, which is typical of liquid, or powdery, chemicals of the TD type, as exemplified in Fig. 15. 

The ability of flavonoids to enhance the resistance to oxidation and to terminate free-radical chain reactions in lipophilic systems can be monitored using low-density lipoproteins (LDL) as a model (Rice-Evans et al., 1996). The LDL oxidation is initiated either by copper or by a peroxyl radical [2,2 azobis(2-amidinopropane hydrochloride) (AAPH)] (Abuja et al., 1998). Hexanal liberated from the decomposition of oxidized n-6 polysaturated fatty acids in LDL may be determined by static headspace gas chromatography (Frankel and Meyer, 1998). Also, bleaching of P-carotene (Velioglu et al., 1998 Fukumoto and Mazza, 2000) and the tracing by HPLC (Fukumoto and Mazza, 2000) of malonaldehyde formed in lipid emulsion systems in the presence of iron (Tsuda et al., 1994) have been used to measure antioxidants in lipophilic systems. 

The effect of pressure and composition on the kinetics of the decomposition of 2, 2-azobis (isobutyronitrile) (AIBN) in CO- ethanol were studied at 333.15 K near the critical region of the mixed solvent. 

Only free radical polymerisation, which requires the formation of reactive free radical species to initiate polymerisation, appears to have been used to form MIPs. Free radicals are produced by the decomposition of an initiator species by the action of heat or light. Commonly used initiators are benzoyl peroxide and azobis compounds such as azobisisobutyronitrile (AIBN) or 2,2 -azobis(2,4-dimethylvaleronitrile) (ABDV) Figure 6.20). 

---