In organic chemistry, a nitrate ester is an organic functional group with the formula R−ONO2, where R stands for any organyl group. They are the esters of nitric acid and alcohols. A well-known example is nitroglycerin, which is not a nitro compound, despite its name.
Synthesis and reactions Nitrate esters are typically prepared by condensation of nitric acid and the alcohol: For example, the simplest nitrate ester, methyl nitrate, is formed by reaction of methanol and nitric acid in the presence of sulfuric acid:
CH 3 OH + HNO 3 ⟶ CH 3 ONO 2 + H 2 O {\displaystyle {\ce {CH3OH + HNO3 -> CH3ONO2 + H2O}}}
Formation of a nitrate ester is called a nitrooxylation (less commonly, nitroxylation). Most commonly, "mixed acid" (nitric and sulfuric acids) are used, but in the 1980s production of the nitrocellulose with magnesium nitrate as a dehydrating agent was started in the US. In laboratory, phosphoric acid and phosphorus pentoxide or acetic acid and its anhydride may be used for the same purpose, or the nitroxylation can be conducted in anhydrous conditions (such as dichloromethane or chloroform).
Explosive properties The thermal decomposition of nitrate esters mainly yields the gases molecular nitrogen (N2) and carbon dioxide. The considerable chemical energy of the detonation is due to the high strength of the bond in molecular nitrogen. This stoichiometry is illustrated by the equation for the detonation of nitroglycerin.
Illustrative of the highly sensitive nature of some organic nitrates is Si(CH2ONO2)4. A single crystal of this compound detonates even upon contact with a teflon spatula and in fact made full characterization impossible. Another contributor to its exothermic decomposition (inferred from much safer in silico experimentation) is the ability of silicon in its crystal phase to coordinate to two oxygen nitrito groups in addition to regular coordination to the four carbon atoms. This additional coordination would make formation of silicon dioxide (one of the decomposition products) more facile.
Medical use
The nitrate esters isosorbide dinitrate (Isordil) and isosorbide mononitrate (Imdur, Ismo, Monoket, Mononitron) are converted in the body to nitric oxide, a potent natural vasodilator. In medicine, these esters are used as a medicine for angina pectoris (ischemic heart disease).
Related compounds Acetyl nitrate is a nitrate anhydride, being derived from the condensation of nitric and acetic acids.
Atmospheric Occurrence
Atmospheric Abundance Alkyl nitrates (RONO2) compounds are widespread in the boundary layer, accounting for a large portion of total reactive nitrogen oxides in both urban and rural environments and across various hydrocarbon mixtures. In addition, alkyl nitrates are typically complex, multifunctional molecules that are still challenging to detect using chromatographic methods at the concentrations found under ambient conditions. Alkyl nitrates have been found to contribute to the NOy compound budget. In total, 43 (C6–C13) alkyl mononitrates, 24 (C3–C6) alkyl dinitrates, and 19 (C2–C6) hydroxy alkyl nitrates were identified. In the urban air samples, the summed concentrations of 15 (C6–C10) alkyl mononitrates ranged from 2.9 to 11.0 pptv. The total levels of 21 (C3–C6) alkyl dinitrates were between 2.3 and 10.5 pptv, and the sum of 7 (C2–C4) hydroxy alkyl nitrates ranged from 7.3 to 28 pptv. These findings indicate that alkyl dinitrates, hydroxy alkyl nitrates, and alkyl mononitrates make an important contribution to the NOy compound budget. Moreover, no significant differences in either the concentrations or patterns of organic nitrates were observed between the urban air of cities. Organic nitrates, however, occur at comparatively low concentrations. Short-chain alkyl nitrates (C4–C6) exhibit mixing ratios of about 0.2–2.5 pptv, with a pronounced minimum in the tropics and much lower levels in the Southern Hemisphere. For the longer and more functionalized species, the summed mixing ratio of 36 long-chain alkyl mononitrates (C7–C13) lies between 0.02 and 0.43 pptv, that of 23 alkyl dinitrates (C3–C6) ranges from 0.005 to 1.08 pptv, and that of 7 hydroxy alkyl nitrates (C2–C4) ranges from 0.005 to 1.07 pptv.
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