Azonitriles decomposition

Rate Measurements. Oxidation rates, which were normally carried out under pure oxygen at 760-mm. total pressure, and azonitrile decomposition rates were measured using apparatus previously described. 

Azonitriles are not susceptible to radical-induced decompositions (56) and their decomposition rates are not usually affected by other components of the environment. Cage recombination of the alkyl radicals occurs when azo initiators are used, and results in the formation of toxic tetrasubstituted succinonitrile derivatives (56). This can be a significant drawback to the use of azo initiators. In contrast to some organic peroxides, azonitrile decomposition rates show only minor solvent effects (54—56) and are not affected by transition metals, acids, bases, and many other contaminants. Thus azonitrile decomposition rates are predictable. Azonitriles can be used as thermal initiators for curing resins that contain a variety of extraneous materials since cure rates are not affected. In addition to curing of resins, azonitriles are used for polymerization of commercial vinyl monomers. 

The azonitrile 19 also shows similar decomposition kinetics to AIBN (Table 3.2). The initiators 19 and AIBMe also have greater solubility in organic solvents than AIBN. 

Thermal or photochemical decomposition of azonitriles (e.g. AIBN) affords o-cyanoalkyl radicals (Scheme 3.71 ).29... 

Qualitatively the saipe reactivity pattern was observed for the decomposition of sym. azonitriles 20 (R1 = CN, R2, R3 = alkyl)29 and several symmetrically and un-symmetrically substituted azo compounds30. A selection of these results is found in Table 2. It is apparent from these data that the thermal stability of 20 decreases as the size of the groups R1—R3 increases. Riichardt et al. have observed that a linear relationship exists between the thermolysis rates of Table 2 and the SN 1-solvolysis rates of corresponding f-alkyl-p-nitrobenzoates 21 in 80% acetone-water28d). The... 

During preparation of tetramethylsuccinodinitrile by thermal decomposition of 100 g of the azonitrile by slow warming in unstirred heptane, an explosion occurred. Successive addition of small portions of the nitrile to heptane at 90-92°C is a safer, preferred method. 

This decomposition usually shows little dependence on solvent, so if Ed for decomposition in chloroprene is likewise 30.9 kcal. per mole, then since Eox = 25.1 kcal. per mole Ep = 9.6 kcal. per mole, assuming termination to require no energy of activation. This is 1.2 kcal. per mole larger than kp for styrene oxidation (8). Values of e for azobisisobutyronitrile in oxidation systems usually lie in the range 0.6 to 0.8 if e = 0.7, the above equation for the decomposition of the azonitrile and that given earlier for the initiated oxidation of chloroprene permit calculation of kp/kt1/2 for chloroprene and also the kinetic chain lengths of the oxidations (Table IV). 

In sharp contrast, azonitrile adsorbed on silica produces only the C-C coupling product on photolytical decomposition 324al. The Kolbe electrolysis of optically active carboxylic acids with the asymmetric carbon in the a-position yields inactive coupling products 33S). This result agrees with the postulate of a free radical as an intermediate, while for adsorbed radicals at least some retention of configuration would be expected. 

During the past two decades many fine details of the decomposition mechanism of azonitriles have been delineated and interesting correlations between molecular structure and rate parameters brought to light, in particular by Overberger, Cohen, Wang and co-workers and others. In an elegant series of studies the intervention of free radicals in the reactions was demonstrated by the loss of stereoisomerism in the products diastereomeric azocompounds afforded identical mixtures of... 

The rates of initiation depend on the type of activation ch.osen. In photochemical initiation, Vs = 2 0/, where I is the absorbed light intensity and 0 = the efficiency coefficient. With an average intensity, rates of initiation of approximately 10"7 mole l 1 s 1 are attained. In thermal activation, autoinitiation by interaction between oxygen and aldehyde gives low values of approximately 10 9 mole l-1 s"1 under standard laboratory conditions. When azonitrile is used, since the thermal decomposition rate of this product is approximately first order [62], Vi is given by... 

The azonitrile 19 also shows similar decomposition kinetics to AIBN (Table... 

Overberger et al. (1949) studied the decomposition rates of a series of aliphatic azonitriles in toluene by measuring the volume of Nj evolved as a function of time, at constant temperature. A set of V,/(where V, and are the volumes at time t, and t = °o) data for the decomposition of the azo nitrile compound... 

The decomposition of an unsymmetrical azonitrile (6) results in the formation of 2 ketenimines (7) and (8) in addition to the normal products (Scheme 3). 

Table 2.2. Decomposition Rates of Some Azonitrile Initiators ...

Table 3.2 Decomposition rates of some azonitrile initiators...

The most common process used to generate free radicals is the homolytic thermal decomposition of a molecule containing a fragile symmetrical bond, such as peroxides (R-O-O-R), hydroperoxides (R-O-O-H), azonitriles (R-N=N-R) or peroxodisulphates (see Equations 3.4-3.6). 

The effects of the medium upon the dissociations and efficiencies of typical initiators have been reviewed with some discussion of poly functional initiators. Solvation effects have been considered for azoisobutyronitrile. The rate of decomposition of the azonitrile in dimethylformamide/glycerol mixtures depends upon the composition, but because of inhomogeneities resulting from limited solubility of the initiator and not because of effects such as differences between the viscosities of the various systems. The solubilities of typical peroxides over a range of pressures and. temperatures have been measured to assess problems that might arise from crystallization of initiators at high pressures. ... 

Synthesis and Decomposition of Azo Compounds—Free Radical Initiators. Our laboratory was one of the first groups to be heavily involved in the use of azonitriles as free radical initiators. In particular, we illustrated the fact that virtually no induced decomposition of these azonitriles occurred upon decomposition in solution. We provided detailed studies of the... 

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