Hyponitrites

The hyponitrites (16), esters of hyponitrous acid (HO N=N OH), are low temperature sources of alkoxy or acyloxy radicals. A detailed study of the effect of substituents on for the hyponitrite esters has been reported by Quinga and Mendenhall. 

While di-/-butyl (34) and dicumyl hyponitrites (35) have proved convenient sources of t-butoxy and cumyloxy radicals respectively in the laboratory the utilization of hyponitrites as initiators of polymerization has been limited by difficulties in synthesis and commercial availability. Dialkyl hyponitrites (16) show only weak absorption at X 290 nm and their photochemistry is largely a neglected area. The triplet sensitized decomposition of these materials has been investigated by Mendenhall et 

Approximately 5% of radicals undergo cage recombination when dicumyl hyponitrite (35) is decomposed in bulk MM A or S at 60 Dicumyl peroxide, the product of cage recombination is likely to be stable under the conditions where hyponitrites are usually employed. Nonetheless, its formation is a concern since 

---

At room temperature, Htde reaction occurs between carbon dioxide and sodium, but burning sodium reacts vigorously. Under controUed conditions, sodium formate or oxalate may be obtained (8,16). On impact, sodium is reported to react explosively with soHd carbon dioxide. In addition to the carbide-forrning reaction, carbon monoxide reacts with sodium at 250—340°C to yield sodium carbonyl, (NaCO) (39,40). Above 1100°C, the temperature of the DeviHe process, carbon monoxide and sodium do not react. Sodium reacts with nitrous oxide to form sodium oxide and bums in nitric oxide to form a mixture of nitrite and hyponitrite. At low temperature, Hquid nitrogen pentoxide reacts with sodium to produce nitrogen dioxide and sodium nitrate. 

Nitrous oxide may also be obtained by the controlled reduction of nitrates or nitrites, decomposition of hyponitrites, or thermal decomposition of hydroxylamine. 

Hyponitrites can be prepared in variable (low) yields by several routes of which the commonest are reduction of aqueous nitrite solutions using sodium (or magnesium) amalgam, and condensation of organic nitrites with hydroxylamine in NaOEt/EtOH ... 

Vibrational spectroscopy indicates that the hyponitrite ion has the trans- (C2/1) configuration (1) in the above salts. 

As implied by the preparative methods employed, hyponitrites are usually stable towards... 

There is also considerable current environmental interest in hyponitrite oxidation because it is implicated in the oxidation of ammonia to nitrite, an important step in the nitrogen cycle (p. 410). Specifically, it seems likely that the oxidation proceeds from ammonia through hydroxylamine and hyponitrous acid to nitrite (or N2O). 

With liquid N2O4 stepwise oxidation of hyponitrites occurs to give Na2N20 c (x = 3-6) ... 

In contrast to the stepwise oxidation of sodium hyponitrite in liquid N2O4, the oxidation goes rapidly to the nitrate ion in an inert solvent of high dielectric constant such as nitromethane ... 

Tertiary hyponitrites are not particularly susceptible to induced decomposition. However, the same is not true of primary and secondary hyponitrites. Isopropyl hyponitrite is reported123 to undergo induced decomposition by a mechanism involving initial abstraction of a a-hydrogen (Scheme 3.24). 

Oxygen-centered radicals are arguably the most common of initiator-derived species generated during initiation of polymerization and many studies have dealt with these species. The class includes alkoxy, hydroxy and aeyloxy radicals and tire sulfate radical anion (formed as primary radicals by homolysis of peroxides or hyponitrites) and alkylperoxy radicals (produced by the interaction of carbon-centered radicals with molecular oxygen or by the induced decomposition of hydroperoxides). 

The reactions of /-butoxy radicals are amongst the most studied of all radical processes. These radicals are generated by thermal or photochemical decomposition of peroxides or hyponitrites (Scheme 3.75). 

Relatively few studies have dealt with the reactions of primary and secondary aikoxy radicals (isopropoxy, methoxy, etc.) with monomers. These radicals are conveniently generated from the corresponding hyponitrites (Scheme 3.77).1"3 402... 

Silanes (R3SiH) also add to alkenes to form a new alkyl silane (RaSi—R ) in the presence of a hyponitrite. Silanes add to dienes with a palladium catalyst, and asymmetric induction is achieved by using a binapthyl additive. ° ... 

In the presence of metal catalysts such as rhodium compounds, aldehydes can add directly to alkenes to form ketones. The reaction of co-alkenyl aldehydes with rhodium catalyst leads to cyclic ketones, with high enantioselectivity if chiral ligands are employed. Aldehydes also add to vinyl esters in the presence of hyponitrites and thioglycolates. ° ... 

The action of sodium on ammonium nitrate results in a violent explosion. It is assumed to be caused by the decomposition of a hyponitrite formed as follows ... 

With sodium nitrate, pure sodium (or in liquid ammonia) gives rise to an explosive yellow substance, which may be the same hyponitrite as the one above. 

However, reduced ceria is able, alone, to dissociate NO. Martinez-Arias et al. [85] have first investigated by electron paramagnetic resonance (EPR) and FTIR spectroscopies NO reaction on ceria pre-outgassed at different temperatures and showed the role of superoxides differentially coordinated in the formation of hyponitrites species further decomposed into NzO. Later Haneda et al. [86] have demonstrated that reduced ceria and reduced praseodymium oxide dissociate NO even though the presence of a noble metal (Pt) significantly increases the formation of N2 or N20. The main results of this study are summarized in Table 8.9. 

---