Nitric oxide


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 * align="center" colspan="2" bgcolor="#ffffff" | [[Image:Nitric-oxide-2D.png|150px|Nitric oxide]] [[Image:Nitric-oxide-3D-vdW.png|150px|Nitric oxide]]


 * Density and phase
 * 1.3 × 103 kg m−3 (liquid) 1.34 g dm−3 (vapour)


 * NFPA 704
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The chemical compound nitric oxide  is a gas with chemical formula NO. It is an important signaling molecule in the body of mammals including humans, one of the few gaseous signaling molecules known.

Nitric oxide (NO) should not be confused with nitrous oxide (N2O), a general anaesthetic, or with nitrogen dioxide (NO2) which is another poisonous air pollutant.

The nitric oxide molecule is a free radical, which makes it very reactive and unstable. In air, it quickly reacts with oxygen 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 reaction 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 industrial 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. As a raw material it is used 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
In the body, nitric oxide (known as the 'endothelium-derived relaxing factor', or 'EDRF', before its chemical structure was elucidated) is synthesized from arginine and oxygen by various nitric oxide synthase (NOS) enzymes and by sequential 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 noted to be increased in high-altitude populations for this effect, which helps to avoid hypoxia in thin air. Nitric oxide is a key biological messenger, playing a role in a variety of biological process. These include blood vessel dilatation, neurotransmission, modulation of the hair cycle, and penile erections. "Nitro" powerful vasodilators such as nitroglycerin 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.

Nitric oxide can contribute to reperfusion injury when excessive nitric oxide 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 nitric oxide synthase (as in animals), or by plasma membrane-bound nitrate reductase or by mitochondrial electron transport chain or 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.

A biologically important reaction of nitric oxide is S-nitrosation (or S-nitrosylation), the covalent attachment of a nitrogen monoxide group to the thiol side chain 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.

Chemistry
The chemistry of nitric oxide is extensive. Specifically, NO doesn't generate salts and demonstrates both reducing and oxidizing properties as indicated by the following overview.

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 from nitric acid:
 * 8HNO3 + 3Cu → 3Cu(NO3)2 + 4H2O + 2NO

or from the following aqueous reactions,
 * 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

The iron(II) sulfate reaction is a simple method that has been used in undergraduate laboratory experiments. NO can be produced from the following non-aqueous reagents,
 * 3 KNO2(l) +  KNO3 (l)  +  Cr2O3(s) →   2 K2CrO4(s)  +  4 NO (g)

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 can also serve as a ligand in transition metal complexes, such species are 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. The μ2-symmetric or unsymmetric, μ3 and μ4 bonding modes are possible.

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 using 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.