Thermal Decomposition of Nitrate Esters

Rates of thermal decomposition and solvent rate effects have been measured for a series of nitrate esters. The alkoxy radicals formed by homolysis together with some of their further degradation products have been stabilized by hydrogen donation. Internal and external return of nitrogen dioxide has been demonstrated by solvent cage effects and isotope exchange. Radicalstabilizing substituents favor β-scission. Dinitrates in a 1,5 relationship behave as isolated mononitrates. Dinitrates in a 1,3 or 1,4 relationship exhibit intramolecular reactions. Tertiary nitrate esters in diethyl ether undergo elimination rather than homolysis. Introduction The esters of nitric acid have long been used as explosives and propellants, and their thermal decomposition products and kinetics have been studied as a means to evaluate the thermal hazards of these compounds. The thermal decomposition of ethanolnitrate was examined by a number of researchers, and it was proposed that the first, and rate-determining, step was the reversible loss of NO2: RCH2O-NO2 <--> RCH2O + NO2 Griffiths, Gilligan, and Gray investigated 2-propanol nitrate pyrolysis and found it exhibited more β-cleavage than primary nitrate esters. It yielded nearly equal amounts of 2propanol nitrite and acetaldehyde, as well as small amounts of acetone, nitromethane, and methyl nitrite (Fig. 1). Homolytic cleavage of the RO-NO2 bond would form the 2-propoxy radical, which could subsequently react with nitric oxide to produce 2-propanol nitrite. It could also eliminate methyl radical to form acetaldehyde and methyl-derived products; or it could be converted to acetone by oxidization or loss of a hydrogen atom. Dinitrates of butanediol were investigated by Powling and Smith. They found that 1,4butanediol dinitrate, following cleavage of one NO2 moiety, decomposed to formaldehyde, nitrogen dioxide, and ethylene, whereas 2,3-butanediol dinitrate gave acetaldehyde in place of formaldehyde and ethylene. A similar intramolecular decomposition to gaseous products has been proposed for nitroglycerin. The mechanism of thermolysis of pentaerythritol tetranitrate (PETN) is complex and poorly understood; however, Ng, Field, and Hauser have postulated a mechanism based on time-of-flight mass spectrometric results which involves the formation of the tertiary tris(nitroxymethyl)methyl radical. In this paper the thermal decomposition rates have been measured and mechanisms proposed for selected primary, secondary, and tertiary mononitrates (n-pentanol nitrate, 3-buten-1ol nitrate, ethanol nitrate, neopentanol nitrate, 2-phenylethanol nitrate, 2-propanol nitrate, cyclohexanol nitrate, 2-methyl-2-propanol nitrate, and 2-methyl-2-butanol nitrate). In addition,