Nitrate ester

Not to be confused with Nitro compound.

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.

Pentaerythritol tetranitrate is a commercially important explosive that contains four nitrate ester groups.

Synthesis and reactions

Nitrate esters are typically prepared by condensation of nitric acid and the alcohol:^[1][2] For example, the simplest nitrate ester, methyl nitrate, is formed by reaction of methanol and nitric acid in the presence of sulfuric acid:^[3]

${\displaystyle {\ce {CH3OH + HNO3 -> CH3ONO2 + H2O}}}$

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.^[4] 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).^[5]

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.^[6][7] 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

Main article: Glyceryl trinitrate (pharmacology)

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.