Nitrate ester

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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

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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]

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

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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

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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).

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Acetyl nitrate is a nitrate anhydride, being derived from the condensation of nitric and acetic acids.

Atmospheric Occurrence

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Atmospheric Abundance

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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.[8] 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.[9] 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.[10]

Alkyl Nitrates Abundance in Variant Geographic Regions
Concentrations (pptv) / Region Urban area Marine air
Alkyl mononitrates 2.9 – 11.0 0.02 – 0.43
Alkyl dinitrates 2.3 – 10.5 0.005 – 1.08
Hydroxy alkyl nitrates 7.3 – 28 0.005 – 1.07

Formation Mechanisms

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There are two primary mechanisms for producing RONO2 in the atmosphere:

(1) oxidation of hydrocarbons initiated by hydroxyl radicals (OH) in the presence of NOx during the daytime.

(2) oxidation of alkenes initiated by nitrate radicals (NO3) at night.

Methyl nitrate (CH3ONO2), ethyl nitrate (C2H5ONO2), and possibly propyl nitrate (C3H7ONO2) are released directly from the oceans, and concentrations of these compounds reaching up to several tens of ppt have been observed in remote marine environments.[11] Apart from these marine emissions, direct emissions are generally not regarded as a major source. RONO2 exhibit a wide range of vapor pressures. It occurs both in gaseous and aerosol particles.[12]

OH-Initiated RONO2 Formation

When OH reacts with a saturated hydrocarbon (1), it first abstracts a hydrogen atom, and almost immediately afterward, oxygen (O2) adds to the resulting radical to form an RO2 radical (2).

RH + OH → R • + H2O (1)

R • + O2 → RO2 • (2)

Two distinct product pathways (3a and 3b) arise from the reaction between RO2 and NO when NOx is present.

RO2 • + NO → RO • + NO2 (3a)

RO2 • + NO → RONO2 (3b)

The dominant pathway 3a, produces NO2 and an alkoxy radical and thereby continues the NOx and HOx catalytic cycles. In contrast, the less frequent pathway 3b, terminates the chain by generating a stable monofunctional organic nitrate. Reaction 3b is a termolecular process and represents only a minor pathway (typically <5%) compared with the major, bimolecular pathway, reaction 3a.

NO3-Initiated RONO2 Formation

During daylight hours, the OH-initiated pathway described previously is the primary source of RONO2. At night, in contrast, RONO2 formation occurs through reactions of NO3 with alkenes and phenols.[13] These nighttime reactions have high nitrate formation amounts. Although the NO3 pathway constitutes only a small portion of overall organic oxidation relative to OH, this pathway can still account for up to about 50% of the regional RONO2 burden.[14]

The process begins with NO3 adding to a carbon–carbon double bond (4). The alkyl radical formed in this step then rapidly reacts with O2, analogous to reaction 2, yielding a peroxy radical (5).

R1=R2 + NO3 → R1(ONO2)- R2 • (4)

R1(ONO2)- R2 • + O2 → R1(ONO2)- R2O2 • (5)

Once formed, this peroxy radical can react with HO2, RO2, or NO3, and these reactions yield stable aldehyde- or alcohol-nitrate products. In contrast, pathways involving NO (3a and 3b) are negligible in the atmosphere because NO3 cannot accumulate where NO is present; NO and NO3 rapidly react together, producing two NO2 molecules. Nitrooxy-peroxynitrates (R1(ONO2)–R2O2NO2) can arise from the reaction of nitrooxy-peroxy radicals with NO2 and have been detected as transient species in laboratory experiments.[15] However, their strong thermal instability and the lack of ambient detections indicate that they mainly influence how laboratory results are interpreted rather than playing a direct role in atmospheric chemistry. NO3 is also theoretically capable of oxidizing alkanes through hydrogen atom abstraction, but this reaction proceeds so slowly that it is not considered important under atmospheric conditions.

Atmospheric Removal

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For gas phase, a RONO2 molecule has several possible fates: it may be transported by atmospheric winds, take part in further chemical reactions, deposit onto the surface, or partition into the aerosol phase. How quickly any of these processes occur depends on the molecule’s specific structure, which controls both its reactivity and its tendency to deposit or move into particles.

Nitrates that contain multiple functional groups, either formed that way in the first oxidation step or made multifunctional after successive reactions, are expected to be particularly reactive. Molecules that still possess a carbon–carbon double bond or have abstractable hydrogen atoms also tend to be processed more rapidly. As one example, first-generation isoprene nitrates react quickly with both O3 and OH, and all eight isomers could survive daytime oxidation but no more than a few hours. With continued oxidation, the rate of resulting higher-generation products would differ from those of the parent nitrate.

The detailed products formed when RONO2 is oxidized by OH, O3, and NO3 are not well constrained. The overall chemistry can be thought as two main pathways:

  1. Oxidation preserves the nitrate group and produces a stable multifunctional nitrate, R1ONO2. Here the original carbon skeleton R has been altered so that the new backbone R1 carries additional functional groups (e.g., carbonyls, alcohols, or extra nitrate groups).
  2. Oxidation removes the nitrate functionality, releasing NO2 and effectively stripping the nitrate group from the molecule.

RONO2 compounds absorb near-UV radiation (λ < 340 nm) and break down through photodissociation, yielding RO• radicals and NO2 with quantum efficiencies close to unity. Experimental studies have determined photolysis rates for numerous C1–C5 alkyl and cycloalkyl nitrates[16], as well as for several difunctional organic nitrates.[17]

For smaller members (C1–C4), photolysis rates rival oxidation by OH. Under typical summer surface conditions, photolytic lifetimes can vary from 3 to 10 days. On the other hand, most RONO2 species photolysis much slower than reactions with the peroxy radical. Because of this, photolysis is not significant to atmospheric loss.[18]

References

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  1. ^ Snyder, H. R.; Handrick, R. G.; Brooks, L. A. (1942). "Imidazole". Organic Syntheses; Collected Volumes, vol. 3, p. 471.
  2. ^ Boschan, Robert; Merrow, Raymond T.; van Dolah, Robert W. (1955-06-01). "The Chemistry of Nitrate Esters". Chemical Reviews. 55 (3): 485–510. Bibcode:1955ChRv...55..485B. doi:10.1021/cr50003a001. ISSN 0009-2665.
  3. ^ Alvin P. Black & Frank H. Babers. "Methyl nitrate". Organic Syntheses; Collected Volumes, vol. 2, p. 412.
  4. ^ Liu, Jiping (2019-03-06). Nitrate Esters Chemistry and Technology. Springer. ISBN 978-981-13-6647-5.
  5. ^ Heinze, T.; Liebert, T. (2012-01-01), Matyjaszewski, Krzysztof; Möller, Martin (eds.), "10.05 - Celluloses and Polyoses/Hemicelluloses", Polymer Science: A Comprehensive Reference, Amsterdam: Elsevier, pp. 83–152, ISBN 978-0-08-087862-1, retrieved 2024-11-11
  6. ^ The Sila-Explosives Si(CH2N3)4 and Si(CH2ONO2)4: Silicon Analogues of the Common Explosives Pentaerythrityl Tetraazide, C(CH2N3)4, and Pentaerythritol Tetranitrate, C(CH2ONO2)4Thomas M. Klapötke, Burkhard Krumm, Rainer Ilg, Dennis Troegel, and Reinhold Tacke J. Am. Chem. Soc.; 2007 doi:10.1021/ja071299p
  7. ^ Sila-Explosives Offer A Better Bang Stephen K. Ritter Chemical & Engineering News May 7 2007Link
  8. ^ Paulot, F.; Henze, D. K.; Wennberg, P. O. (2012-02-02). "Impact of the isoprene photochemical cascade on tropical ozone". Atmospheric Chemistry and Physics. 12 (3): 1307–1325. Bibcode:2012ACP....12.1307P. doi:10.5194/acp-12-1307-2012. ISSN 1680-7316.
  9. ^ Kastler, J.; Jarman, W.; Ballschmiter, K. (2000-09-01). "Multifunctional organic nitrates as constituents in European and US urban photo-smog". Fresenius' Journal of Analytical Chemistry. 368 (2): 244–249. doi:10.1007/s002160000550. ISSN 1432-1130.
  10. ^ Fischer, Ralf G.; Kastler, Jürgen; Ballschmiter, Karlheinz (2000). "Levels and pattern of alkyl nitrates, multifunctional alkyl nitrates, and halocarbons in the air over the Atlantic Ocean". Journal of Geophysical Research: Atmospheres. 105 (D11): 14473–14494. Bibcode:2000JGR...10514473F. doi:10.1029/1999JD900780. ISSN 2156-2202.
  11. ^ Atlas, E.; Pollock, W.; Greenberg, J.; Heidt, L.; Thompson, A. M. (1993). "Alkyl nitrates, nonmethane hydrocarbons, and halocarbon gases over the equatorial Pacific Ocean during SAGA 3". Journal of Geophysical Research: Atmospheres. 98 (D9): 16933–16947. Bibcode:1993JGR....9816933A. doi:10.1029/93JD01005. ISSN 2156-2202.
  12. ^ Blake, Nicola J.; Blake, Donald R.; Wingenter, Oliver W.; Sive, Barkley C.; Kang, Chang Hee; Thornton, Donald C.; Bandy, Alan R.; Atlas, Elliot; Flocke, Frank; Harris, Joyce M.; Rowland, F. Sherwood (1999). "Aircraft measurements of the latitudinal, vertical, and seasonal variations of NMHCs, methyl nitrate, methyl halides, and DMS during the First Aerosol Characterization Experiment (ACE 1)". Journal of Geophysical Research: Atmospheres. 104 (D17): 21803–21817. Bibcode:1999JGR...10421803B. doi:10.1029/1999JD900238. ISSN 2156-2202.
  13. ^ Fry, J. L.; Kiendler-Scharr, A.; Rollins, A. W.; Wooldridge, P. J.; Brown, S. S.; Fuchs, H.; Dubé, W.; Mensah, A.; dal Maso, M.; Tillmann, R.; Dorn, H.-P.; Brauers, T.; Cohen, R. C. (2009-02-23). "Organic nitrate and secondary organic aerosol yield from NO3 oxidation of β-pinene evaluated using a gas-phase kinetics/aerosol partitioning model". Atmospheric Chemistry and Physics. 9 (4): 1431–1449. doi:10.5194/acp-9-1431-2009. ISSN 1680-7316.
  14. ^ Horowitz, Larry W.; Fiore, Arlene M.; Milly, George P.; Cohen, Ronald C.; Perring, Anne; Wooldridge, Paul J.; Hess, Peter G.; Emmons, Louisa K.; Lamarque, Jean-François (2007). "Observational constraints on the chemistry of isoprene nitrates over the eastern United States". Journal of Geophysical Research: Atmospheres. 112 (D12) 2006JD007747. Bibcode:2007JGRD..11212S08H. doi:10.1029/2006JD007747. ISSN 2156-2202.
  15. ^ Wängberg, I.; Barnes, I.; Becker, K. H. (1997-07-01). "Product and Mechanistic Study of the Reaction of NO3 Radicals with α-Pinene". Environmental Science & Technology. 31 (7): 2130–2135. Bibcode:1997EnST...31.2130W. doi:10.1021/es960958n. ISSN 0013-936X.
  16. ^ Carbajo, Paula Gorrotxategi; Orr-Ewing, Andrew J. (2010-06-03). "NO2 quantum yields from ultraviolet photodissociation of methyl and isopropyl nitrate". Physical Chemistry Chemical Physics. 12 (23): 6084–6091. Bibcode:2010PCCP...12.6084C. doi:10.1039/C001425G. ISSN 1463-9084.
  17. ^ Suarez-Bertoa, Ricardo; Saliu, Francesco; Bruschi, Maurizio; Rindone, Bruno (2012-09-30). "Reaction products and mechanism of the regioselective oxidation of N-phenylmorpholine by ozone". Tetrahedron. 68 (39): 8267–8275. doi:10.1016/j.tet.2012.07.055. ISSN 0040-4020.
  18. ^ Perring, A. E.; Pusede, S. E.; Cohen, R. C. (2013-08-14). "An Observational Perspective on the Atmospheric Impacts of Alkyl and Multifunctional Nitrates on Ozone and Secondary Organic Aerosol". Chemical Reviews. 113 (8): 5848–5870. doi:10.1021/cr300520x. ISSN 0009-2665. PMID 23614613.