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Nitric Oxide (NO) – Introduction, Preparation, Properties, Uses, Significance, Practice Problems and FAQ

Nitric Oxide (NO) – Introduction, Preparation, Properties, Uses, Significance, Practice Problems and FAQ

Nitrogen and oxygen are like those two happy parents, whose children are exemplary in society. Often we hear things like, “Look at Mr and Mrs Tata. One is a doctor, one is a pilot, one is a criminal lawyer and one is a politician.”

They seem to be the happiest parents, aren't they?

From nitrous oxide is a non-toxic anaesthetic to nitrogen dioxide, an important air pollutant, the oxides of nitrogen have a vivid variety of importance on Earth.

Nitric oxide, the oxide of discussion for today, is infamous for being a major air pollutant (precursor to nitrogen dioxide) impacting global climate change. But this is just one side of the coin when we are talking about ‘Nitric oxide’.

It is true that when nitric oxide is present in the atmosphere as an emission from industries and vehicles, it acts as a pollutant and a greenhouse gas. But, did you know that this very compound is also produced naturally inside our body and that it has immense positive impacts in maintaining various physiological functioning of our body, significantly improving blood circulation to our heart?

Let us find out more about this particular oxide of nitrogen- “Nitric Oxide”


  • Nitric Oxide – Introduction
  • Nitric Oxide – Structure
  • Nitric Oxide – Preparation
  • Nitric Oxide – Physical Properties
  • Nitric Oxide – Chemical Properties
  • Nitric Oxide – Significance
  • Nitric Oxide – Uses
  • Practice Problems
  • Frequently Asked Questions – FAQ

Nitric Oxide – Introduction

Nitrogen monoxide or nitrogen oxide are other names for the colourless gas known as nitric oxide (•N=O). It is also regarded as a free radical since it has an unpaired free electron. Because a free electron is present, the oxide is unstable, but it is extremely important for biological processes. Human bodies naturally produce the molecule nitric oxide, which is essential for many aspects of our health. Its main purpose is vasodilation, which entails relaxing the blood artery's inner muscles so that they can open up and improve circulation. Because it allows for correct and efficient blood, nutrient, and oxygen flow to all parts, nitric oxide synthesis is essential for overall health.

Nitric Oxide – Structure

Nitric oxide is linear in shape. It is an odd electron molecule and is hence paramagnetic in nature. The oxidation state of nitrogen in NO is +2. Nitric oxide dimerizes to dinitrogen dioxide when it condenses to a liquid, however, this connection is weak and reversible. Nearly twice as far apart as N-O, the N-N distance in crystalline NO is 218 pm; in the gaseous state, it the N-N distance 115 pm. The structure of NO is denoted as:

Nitric Oxide – Preparation

  • Lightning causes an uncatalyzed endothermic reaction between oxygen (O2) and nitrogen (N2) that results in the formation of NO at high temperatures (>2000 °C).

N2 + O2 → 2 NO

  • In the Ostwald process, nitric oxide is generated in industrial settings by oxidising ammonia with platinum as a catalyst at 750–900 °C (typically 850 °C). This is an intermediate step in the production of nitric acid (Ostwald’s process)

4NH3+ 5O2→ 4NO + 6 H2O

  • Another method is reacting sodium nitrite with ferrous sulphate and sulphuric acid.

2NaNO2(aq) + 2FeSO4(aq) + 3H2SO4(aq)

→ Fe2(SO4)3 (aq)+ 2 NaHSO4(aq) + 2H2O(l) + 2NO(g)

  • Nitric oxide is easily produced in the laboratory by reducing dilute nitric acid with copper.

3Cu + 8HNO3(dil.) 3CuNO32+2NO+ 4H2O

  • Nitric oxide is produced through the reduction of nitrous acid in the form of sodium nitrite or potassium nitrite.

2NaNO2 + 2FeSO4 + 3H2SO4  Fe2(SO4)3 + 2NaHSO4 + 2H2O + 2NO

2NaNO2 + 2NaI + 2H2SO4  I2 + 2 Na2SO4+ 2H2O + 2NO

3 KNO2 + KNO3+ Cr2O3  2K2CrO4 + 4NO

Nitric Oxide – Physical Properties

  • Nitric oxide is a neutral oxide.
  • The molar mass of nitric oxide is 30.006 g mol-1.
  • Its density is 1.3402 g L-1.
  • The melting point of nitric oxide is -164 °C (−263 °F; 109 K) and the boiling point is −152 °C (−242 °F; 121 K)
  • The solubility of NO in water is 0.0098 g/100 mL at 0 °C and 0.0056 g / 100 mL at 20 °C.

Nitric Oxide – Chemical Properties

  • Nitric oxide combines with oxygen in water to generate nitrous acid (HNO2).

4NO + O2+ 2H2O  4HNO2

  • Nitrosyl halides such as nitrosyl chloride, are formed when nitric oxide interacts with fluorine, chlorine, and bromine.

2NO + Cl2 → 2NOCl

  • Nitric oxide combines with nitrogen dioxide (also a radical) to form dinitrogen trioxide.

NO +NO2 ⇌ N2O3

It can be represented as:

NO +NO2 ⇌ ON-NO2

  • Metal nitrosyls are formed when nitric oxide combines with transition metals.

E.g. [FeCN5NO]2-. It forms sodium nitroprusside Na2[FeCN5NO], an important analytical reagent.

  • A one-electron pseudohalide is nitric oxide. The M-N-O group in these compounds is distinguished by an angle between 120° and 140°. In a number of configurations, the nitrogen atom serves as a bridge for the NO group to connect two metal centres.
  • Nitric oxide is used in organic reactions for nitrosylation. The addition of NO moiety to an organic molecule is termed nitrosylation.
  • For example, Traube’s Reaction. For instance, nitric oxide forms a diazonium dioxide on each position when it combines with acetone and an alkoxide, with the naturally accompanied loss of methyl acetate as a byproduct.

  • On exposure to oxygen, nitric oxide readily converts to nitrogen dioxide.

2NO+O2 2NO2

Nitric Oxide – Significance

  • It plays a crucial role in many chemical and inorganic processes, including nitrosylation.
  • Our body's ability to inflate and constrict blood arteries appears to be aided by nitric oxide. Your blood pressure may go down as a result, which is good for your heart.
  • Because it enables blood, nutrients, and oxygen to reach every region of your body effectively and efficiently, nitric oxide production is crucial for general health. In actuality, erectile dysfunction, diabetes, and heart disease are all linked to a reduced ability to create nitric oxide.
  • Despite being toxic in large doses, nitric oxide acts as a neurotransmitter and is an important signalling chemical in mammals.
  • As a messenger molecule, it sends information to cells in the cardiovascular, nervous, and immune systems.
  • Due to its small size, it has the ability to penetrate through cell membranes and walls to carry out a range of signalling functions in many organ systems. The nitric oxide molecule is substantially more reactive than other signalling molecules due to the presence of a free radical.
  • The body's natural amino acid, L-arginine, is converted into NO by the enzyme known as "NO-synthase."
  • Nitric oxide (NO) inhaled has been demonstrated to impede the respiratory coronavirus's ability to reproduce, which sets it apart from other vasodilators. These results point to the need for further study into the most effective ways to restore pulmonary physiology and suggest that NO may play a significant role in the treatment of COVID-19.

Nitric Oxide – Uses

  • It is used as inhaled nitric oxide (iNO) to treat newborns (term and near-term) with respiratory failure brought on by pulmonary hypertension. Nitric oxide is used with a breathing machine (ventilator) and other medications.
  • The treatment and prevention of diabetes (Type -2) have been found to be benefitted from nitric oxide-boosting medicines.
  • It is used as a powerful vasodilator and helps relax blood vessels and lower blood pressure in adults too.
  • In 1992, nitric oxide was named the "Molecule of the Year." Nitric oxide's position as a cardiovascular signalling molecule was clarified through research, earning it the 1998 Nobel Prize.
  • In ancient times, it used to be used in Britain, as a trigger causing elated happiness or euphoria in high-class parties.
  • It is a non-toxic laughing gas that is used to treat depression at times, in controlled quantities.

Practice Problems

Q. 1. What is the oxidation state of N in NO?

  1. +1
  2. +2
  3. -1
  4. -2

Answer: Let the oxidation state of N in NO be x.

So, x+(-2)= 0

∴ x= +2

Thus, the oxidation state of N in NO is +2.

So, option B) is the correct answer.

Q. 2. How can nitric oxide be assayed quantitatively?

Answer: A chemiluminescent process involving ozone can be used to measure the quantity of nitric oxide. Nitric oxide-containing samples are blended with a lot of ozone. Nitric oxide and ozone react to form oxygen and nitrogen dioxide, along with the emission of light known as chemiluminescence.

NO + O3→ NO2 + O2 + hν

This can be measured using a photodetector. The amount of light obtained is directly proportional to the amount of nitric acid in the sample.

Q. 3. Nitric oxide is responsible for the breakdown of the ozone layer. Elaborate using a reaction.

Answer: NO contributes to the thinning of the ozone layer as it reacts with ozone in the higher stratospheric zone in the presence of sunlight and breaks down ozone. O2 and nitrogen dioxide are produced when nitric oxide and stratospheric ozone interact:

NO + O3 → NO2 + O2

Q. 4. Is NO molecule paramagnetic or diamagnetic?

Answer: The electronic configuration of the molecular orbital of NO which contains 15 electrons, can be represented as(σ1s2) (σ*1s2) (σ2s2) (σ*2s2) (π2px2) (π2py2)  (σ2pz2) ( π*2px1).

So, as it is evident, there is one unpaired electron in NO. Hence, it is paramagnetic in nature.

Frequently Asked Questions – FAQ

1. How is nitric oxide formed inside our body?
The enzyme nitric oxide synthase helps the body produce nitric oxide from the amino acid L-arginine. Although all other types of cells can also manufacture the chemical, the endothelium, the inner layer of blood vessels, is the primary source of synthesis.

2. What is NO signalling?
Nitric oxide is a signalling chemical that can pass across cellular membranes in both an autocrine and paracrine manner. The same cells that create the chemicals signal to one another through an autocrine process. They influence neighbouring cells during paracrine signalling.

It acts for only a brief period of time (a few seconds), and its primary physiological role is to help maintain the vasculature's homeostasis. Nitric oxide prevents leukocyte-endothelial adhesion, platelet aggregation, smooth muscle contraction, and growth. Nitric oxide, which is produced by phagocytes and works as a free radical to kill bacteria, is also involved in immune protection.

It has been demonstrated that several illness conditions result in changes in nitric oxide levels and signalling. Nitric oxide levels are often lower overall in diabetics, and nitric oxide signalling pathways are compromised in atherosclerotic blood arteries.

3. Which foods are most rich in nitric oxide?
One of the best nutritional sources of nitrates, which your body transforms into nitric oxide, is beetroot. Apart from this, there are red spinach, celery, green leafy vegetables, garlic, lettuce, cress, cabbage, and pomegranate. As you age, including each of these in your meals will help you feel more balanced and energised since they can all support maintaining appropriate levels of nitric oxide. Exercising on a daily basis is a fantastic strategy to increase nitric oxide in your body. Increasing blood flow maintains the health of the endothelial cells, which create nitric oxide

4. What are the environmental impacts of NO emissions?
When oxygen and nitrogen interact at high temperatures in the atmosphere, NO and NO2 are both produced. Both the emissions from electrical power generation plants and the exhaust fumes from vehicles and trucks are significant sources of nitrogen oxides. The major environmental impacts of nitric oxide are visualised in the forms of acid rain and ozone layer depletion.

Nitrogen dioxide (NO2) is created when nitric oxide combines with the hydroperoxy radical (HO2•), and it can then react with a hydroxyl radical (HO•) to create nitric acid (HNO3). This leads to acid rain.

NO + HO2• → NO2 + HO

NO2• + HO → HNO3

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