The objective of our research was to measure accurate rate constants for the thermal, unimolecular decomposition of organic nitrates. Our research confirms that the rate-determining step is homolytic cleavage of the weak O-N bond to form alkoxy radical and NO 2 , but the rate constants reported in the past are incorrect. The alkoxy radical and NO 2 engage in secondary reactions that ultimately generate stable products such as carbonyl and nitro compounds. Infrared spectroscopy (IR) and gas chromatography/mass spectrometry (GC/MS) were used to monitor the time dependent loss of organic nitrate and to characterize the products of the thermal reaction. Past research indicates that oxygen slows the rate of homolytic O-N bond cleavage to form radicals. Our research shows that the rates are the same in nitrogen as they are in air. The reaction in air produces R-β unsaturated ketones/aldehydes, which are not generated in a nitrogen atmosphere. The unsaturated ketone/aldehydes complicate the IR analysis, giving the appearance that the loss of organic nitrate slows down. Unhindered, linear organic nitrates have lower reaction rate constants than hindered organic nitrates. Activation energies were found to be lower for hindered organic nitrates. The steric strain present in the hindered organic nitrates may account for the weaker O-N bonds and faster thermal reaction rates. Reaction rates for thermal decomposition under nitrogen were found to decrease as the viscosity of the solvent increased.
Our previous research showed that the rates for unimolecular thermal fragmentation of the O-N bond of organic nitrates are the same in air as they are under a nitrogen atmosphere. This is contrary to what literature studies have reported. When the rate of reaction was followed by infrared (IR) spectroscopy, as in previous literature studies, the IR absorption of the unsaturated carbonyls generated during the reaction complicated the IR analysis and the rates deceptively appeared to slow in air. The current study shows that unsaturated carbonyls can also be formed under a nitrogen atmosphere when hydroperoxide is added. Hydroperoxides created naturally most likely give rise to the unsaturated carbonyl compounds when the reaction is carried out in air. Evidence suggests that the products of organic nitrate thermal chemistry accelerate the decomposition of the model hydroperoxide. The details of this chemistry are critical for controlling the oxidation of hydrocarbons. The reaction of organic nitrates with various catalytic compounds has also been studied. The catalysts fall into three categories: (1) no effect on the rate of loss of organic nitrate or the type of products generated over the thermal base case, (2) accelerate the loss of organic nitrate, but the products remain the same as the thermal base case, and (3) accelerate the loss of organic nitrate and generate different products than the thermal base case. Catalytic amounts (3 mol %) of copper(II) oleate and iron(III) acetyl acetonate each increased the rate of loss of organic nitrates at 170 °C under N 2 by a factor of 1.5 and 2, respectively. The reactions remained first order (or pseudo-first order). Changes in product distribution, in some cases, indicate that the mechanism may be non-radical. Copper and iron compounds convert organic nitrates to stable products before they can cleave thermally and form radicals or react directly with other molecules. Hydroperoxides react with some of the same catalysts, but they also react with compounds that have no effect on organic nitrates.
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