Nitric oxide
| Nitric oxide | ||
|---|---|---|
 
| ||
| General | ||
| Molecular formula | NO | |
| Molar mass | 30.0061 g/mol | |
| Appearance | colourless gas | |
| CAS number | 10102-43-9 | |
| Properties | ||
| Density and phase | 1.3 × 103 kg m−3 (liquid) 1.34 g dm−3 (vapour) | |
| Solubility in water | ||
| Melting point | −163.6 °C (109.6 K) | |
| Boiling point | −151.7 °C (121.4 K) | |
| Structure | ||
| Molecular shape | linear | |
| Dipole moment | 0.15 D | |
| Thermodynamic data | ||
| Std enthalpy of formation ΔfH |
+90.2 kJ/mol | |
| Hazards | ||
| MSDS | External MSDS | |
| EU classification | Toxic (T), corrosive (C) | |
| NFPA 704 |
| |
| R-phrases | R23, R24, R25, R34, R44 | |
| S-phrases | S23, S36, S37, S39 | |
| Supplementary data page | ||
| Structure and properties |
n, εr, etc. | |
| Thermodynamic data |
Phase behaviour Solid, liquid, gas | |
| Spectral data | UV, IR, NMR, MS | |
| Related compounds | ||
| Related nitrogen oxides | Nitrous oxide Nitrogen dioxide Dinitrogen trioxide Dinitrogen tetroxide Dinitrogen pentoxide | |
| Related compounds | Nitric acid Nitrous acid | |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) | ||
The chemical compound nitric oxide is a gas with formula NO. It is an important signaling molecule in the body of mammals, including humans—one of the few gaseous signaling molecules known. It is also a toxic air pollutant produced by automobile engines and power plants.
Nitric oxide (NO) should not be confused with nitrous oxide (N2O), a general anesthetic, or with nitrogen dioxide (NO2), which is another poisonous air pollutant.
The nitric oxide molecule is a free radical, which is relevant to understanding its high reactivity. It reacts with the oxygen in air to form nitrogen dioxide, signalled by the appearance of the reddish-brown colour.
Production and environmental effects
From a thermodynamic perspective, NO is unstable with respect to O2 and N2, although this conversion is very slow at ambient temperatures in the absence of a catalyst. Because the heat of formation of NO is endothermic, its synthesis from molecular nitrogen and oxygen requires elevated temperatures, >1000 °C. A major natural source is lightning. The use of internal combustion engines has drastically increased the presence of nitric oxide in the environment. One purpose of catalytic converters in cars is to minimize NO formation by catalytic reversion to O2 and N2.
Nitric oxide in the air may convert to nitric acid, which has been implicated in acid rain. Furthermore, both NO and NO2 participate in ozone layer depletion. Nitric oxide (NO) is a small highly diffusible gas and a ubiquitous bioactive molecule.
Technical applications
Although NO has relatively few direct uses, it is produced on a massive scale as an intermediate in the Ostwald process for the synthesis of nitric acid from ammonia. In 2005, the US alone produced 6M metric tonnes of nitric acid.[1] It finds use in the semiconductor industry for various processes. In one of its applications it is used along with nitrous oxide to form oxynitride gates in CMOS devices.
Miscellaneous applications
Nitric oxide can be used for detecting surface radicals on polymers. Quenching of surface radicals with nitric oxide results in incorporation of nitrogen, which can be quantified by means of X-ray photoelectron spectroscopy.
Biological functions
Nitric oxide is a key biological messenger, playing a role in a variety of biological process. Nitric oxide, known as the 'endothelium-derived relaxing factor', or 'EDRF', is biosynthesised from arginine and oxygen by various nitric oxide synthase (NOS) enzymes and by reduction of inorganic nitrate. The endothelium (inner lining) of blood vessels use nitric oxide to signal the surrounding smooth muscle to relax, thus dilating the artery and increasing blood flow. The production of nitric oxide is elevated in populations living at high-altitudes, which helps these people avoid hypoxia. Effects include blood vessel dilatation, neurotransmission, modulation of the hair cycle, and penile erections. Nitroglycerin and amyl nitrite serve as vasodilators because they are converted to nitric oxide in the body.
Nitric oxide is also generated by macrophages and neutrophils as part of the human immune response. Nitric oxide is toxic to bacteria and other human pathogens. Many bacterial pathogens have evolved mechanisms for nitric oxide resistance.[2]
Nitric oxide can contribute to reperfusion injury when excessive amount produced during reperfusion (following a period of ischemia) reacts with superoxide to produce the damaging free radical peroxynitrite. Inhaled nitric oxide has been shown to help survival and recovery from paraquat poisoning, which produces lung tissue damaging superoxide and hinders NOS metabolism.
In plants, nitric oxide can be produced by any of four routes: (i) nitric oxide synthase (as in animals), (ii) by plasma membrane-bound nitrate reductase, (iii) by mitochondrial electron transport chain, or (iv) by non enzymatic reactions. It is a signaling molecule, acts mainly against oxidative stress and also plays a role in plant pathogen interactions. Treating cut flowers and other plants with nitric oxide has been shown to lengthen the time before wilting.[3]
A biologically important reaction of nitric oxide is S-nitrosation (or S-nitrosylation), the covalent attachment of a nitric oxide to the thiol group of cysteine within proteins. S-Nitrosylation has been described by some of its proponents as a mechanism for dynamic, post-translational regulation of most or all main classes of protein. Firm evidence to support this claim is limited.
Reactions
When exposed to oxygen, NO is converted into NO2.
- 2NO + O2 → 2NO2
This conversion has been speculated as occurring via the ONOONO intermediate. In water, NO react with oxygen and water to form HNO2 or nitrous acid. The reaction is thought to proceed via the following stoichiometry:
- 4 NO + O2 + 2 H2O → 4 HNO2
NO will react with fluorine, chlorine, and bromine to from the XNO species, known as the nitrosyl halides, such as nitrosyl chloride. Nitrosyl iodide can form but is an extremely short lived species and tends to reform I2.
- 2NO + Cl2 → 2NOCl
Nitroxyl (HNO) is the reduced form of nitric oxide.
Preparation
As stated above, nitric oxide is produced industrially by the direct reaction of O2 and N2 at high temperatures. In the laboratory, it is conveniently generated by reduction of nitric acid:
or by the reduction of nitrous acid:
- 2 NaNO2 + 2 NaI + 2 H2SO4 → I2 + 4 NaHSO4 + 2 NO
- 2 NaNO2 + 2 FeSO4 + 3 H2SO4 → Fe2(SO4)3 + 2 NaHSO4 + 2 H2O + 2 NO
- 3 KNO2(l) + KNO3 (l) + Cr2O3(s) → 2 K2CrO4(s) + 4 NO (g)
The iron(II) sulfate route is simple and has been used in undergraduate laboratory experiments.
Commercially, NO is produced by the oxidation of ammonia at 750 to 900 °C (normally at 850 °C) in the presence of platinum as catalyst:
- 4NH3 + 5O2 → 4NO + 6H2O
The uncatalyzed endothermic reaction of O2 and N2 which is performed at high temperature (>2000°C) with lightning has not been developed into a practical commercial synthesis:
- N2 + O2 → 2NO
Coordination Chemistry
NO forms complexes with all transition metals to give complexes called metal nitrosyls. The most common bonding mode of NO is the terminal linear type (M-NO). The angle of the M-N-O group can vary from 160-180° but are still termed as "linear". In this case the NO group is formally considered a 3-electron donor. Alternatively, one can view such complexes as derived from NO+, which is isoelectronic with CO.
Nitric oxide can serve as a one-electron pseudohalide. In such complexes, the M-N-O group is characterized by an angle between 120-140°.
The NO group can also bridge between metal centers through the nitrogen atom in a variety of geometries.
Measurement of nitric oxide concentration
The concentration of nitric oxide can be determined using a simple chemiluminescent reaction involving ozone: A sample containing nitric oxide is mixed with a large quantity of ozone. The nitric oxide reacts with the ozone to produce oxygen and nitrogen dioxide. This reaction also produces light (chemiluminescence), which can be measured with a photodetector. The amount of light produced is proportional to the amount of nitric oxide in the sample.
- NO + O3 → NO2 + O2 + light
Other methods of testing include electroanalysis, where NO reacts with an electrode to induce a current or voltage change.
Notes
- ↑ “Production: Growth is the Norm” Chemical and Engineering News, July 1 0, 2006, p. 59.
- ↑ C. A. Janeway, et al. (2005). Immunobiology: the immune system in health and disease, 6th ed., New York: Garland Science. ISBN 0-8153-4101-6.
- ↑ Judy Siegel-Itzkovich. Viagra makes flowers stand up straight. Student BMJ, September 1999.
ReferencesISBN links support NWE through referral fees
- Cotton, F.A., G. Wilkinson, C.A. Murillo, and M. Bochmann. 1999. Advanced Inorganic Chemistry. 6th ed. New York: Wiley-Interscience.
- K.J. Gupta , M. Stoimenova, and W. M. Kaiser "In higher plants, only root mitochondria, but not leaf mitochondria reduce nitrite to NO, in vitro and in situ" Journal of Experimental Botany 2005 56(420):2601-2609.
- E.Planchet, K.J. Gupta, M .Sonada & W.M.Kaiser (2005) "Nitric oxide emission from tobacco leaves and cell suspensions: rate limiting factors and evidence for the involvement of mitochondrial electron transport"The Plant Journal 41 (5), 732-743.
- Stöhr, C.; Stremlau, S. "Formation and possible roles of nitric oxide in plant roots" Journal of Experimental Botany 2006 57(3):463-470.
- Pacher, P.; Beckman, J. S.; Liaudet, L.; “Nitric Oxide and Peroxynitrite: in Health and disease” Physiological Reviews 2007, volume 87(1), page 315-424. PMID 17237348.
External links
- National Pollutant Inventory - Oxides of nitrogen Fact Sheet
- Nitric Oxide: Biology and Chemistry, peer reviewed scientific journal
- 1998 Nobel Prize in Physiology/Medicine for discovery of NO's role in cardiovascular regulation
- Nitric Oxide and its Role in Diabetes, Wound Healing and Peripheral Neuropathy
- Microscale Gas Chemistry: Experiments with Nitrogen Oxides
- Nitric Oxide as a Modulator of Hair Growth and the Hair Cycle
- "Nitric oxide may help some premature babies — Study: Inhaling small amounts could prevent potentially fatal lung disease"
- Your Brain Boots Up Like a Computer - new insights about the biological role of nitric dioxide.
- Assessing The Potential of Nitric Oxide in the Diabetic Foot
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