Nitrogen Compounds



 

Learning Objective

  • Give examples of applications of nitrogen compounds.

Key Points

    • Nitrogen oxides, called NOxcompounds, are important for their explosive properties. These properties are determined by the extremely strong and stable bond found in molecular, diatomic nitrogen, N2, which has a bond dissociation energy of 945 kJ/mol (226 kcal/mol).
    • The main neutral hydride of nitrogen is ammonia (NH3), which has a pKb of 9.2, and is thus a weak base. The corresponding deprotonated species, NH2, is called an amide and is a strong base (because it’s the conjugate base of ammonia, whose pKa is around 38).
    • Nitrogen is a constituent of molecules in every major drug class in pharmacology and medicine, from antibiotics to neurotransmitters and beyond. One important aspect of nitrogen is that it is the only non-metal that can maintain a positive charge at physiological pH.

Terms

  • propellantFuel, oxidizer, reaction mass or mixture for one or more engines (especially internal combustion engines or jet engines) that is carried within a vehicle prior to use.
  • oxideA binary chemical compound of oxygen with another chemical element.
  • anionA negatively charged ion, as opposed to a cation.
  • bond dissociation energyThe energy required to separate two atoms joined by a particular bond. Expressed in terms of a mole of such bonded atoms. Indicates the strength of the bond.

Survey of Nitrogen Compounds and their Uses

Nitrogen compounds play an important role in many aspects of life and commercial processes, from the industrial production of fertilizers to the building blocks of life.

The nitrogen-nitrogen triple bond in N2 contains 226 kcal/mol of energy, making it one of the strongest bonds known. When nitrogen gas is formed as a product from various reactions, the bond energy associated with the N-N triple bond is released, causing the explosive properties seen in many nitrogen compounds.

Amines

The main neutral hydride of nitrogen is ammonia (NH3), although hydrazine (N2H4) is also commonly used. Ammonia is more basic than water by 6 orders of magnitude. In solution, ammonia forms the ammonium ion (NH4+). The pKa of ammonium chloride is 9.2. Liquid ammonia (boiling point 240 K) is amphiprotic (displaying either Brønsted-Lowry acidic or basic character) and forms ammonium and the less common amide ions (NH2). Ammonia has a pKa of 38, making the corresponding amide ions very strong bases. Singly, doubly, triply and quadruply substituted alkyl compounds of ammonia are called amines (four substitutions, to form commercially and biologically important quaternary amines, results in a positively charged nitrogen, and thus a water-soluble compound).

Azides

Other classes of nitrogen anions (negatively charged ions) are the poisonous azides (N3), which are linear and isoelectronic to carbon dioxide, but which bind to important iron-containing enzymes in the body in a manner resembling cyanide.

Nitrogen Oxides

Another molecule of the same structure is the colorless and relatively inert anesthetic gas nitrous oxide (dinitrogen monoxide, N2O), also known as laughing gas. This is one of a variety of nitrogen oxides that form a family often abbreviated as NOx. Nitric oxide (nitrogen monoxide, NO), is a natural free radical used in signal transduction in both plants and animals. The reddish and poisonous nitrogen dioxide (NO2) contains an unpaired electron and is an important component of smog. Nitrogen molecules containing unpaired electrons show a tendency to dimerize (thus pairing the electrons), and are, in general, highly reactive. The corresponding acids are nitrous (HNO2) and nitric acid (HNO3), with the corresponding salts called nitrites and nitrates.

Nitrogen Compounds used as Explosives and Propellants

One of the earliest uses of a nitrogen compound as an explosive was potassium nitrate, also called saltpeter, used in gunpowder. This is a mixture of potassium nitrate, carbon and sulfur. When the mixture is ignited in an enclosed space, such as a gun-barrel or a firework, the nitrate ions oxidize the carbon and sulfur in a highly exothermic reaction, producing high-temperature gases very rapidly. This can propel a bullet out of a gun or cause a firework to explode.

Potassium NitratePotassium nitrate, or saltpeter, was a constituent of early gunpowder. It serves as an oxidant, along with an oxidizable material such as sugar.

The higher oxides, dinitrogen trioxide (N2O3), dinitrogen tetroxide (N2O4) and dinitrogen pentoxide (N2O5), are unstable and explosive, a consequence of the chemical stability of N2. Nearly every hypergolic (i.e. not requiring ignition) rocket engine uses N2O4 as the oxidizer; their fuels, various forms of hydrazine, are also nitrogen compounds.

These engines were extensively used on spacecraft such as the space shuttle and those of the Apollo Program because their propellants are liquids at room temperature and ignition occurs on contact without an ignition system, allowing many precisely controlled burns. N2O4 is an intermediate in the manufacture of nitric acid HNO3, one of the few acids stronger than the hydronium ion, and a fairly strong oxidizing agent.

Nitrogen is notable for the range of explosively unstable compounds that it can produce. Nitrogen triiodide (NI3) is an extremely sensitive contact explosive. Nitrocellulose, produced by nitration of cellulose with nitric acid, is also known as guncotton. Nitroglycerin, made by nitration of glycerin, is the dangerously unstable explosive ingredient of dynamite. The comparatively stable, but less powerful explosive trinitrotoluene (TNT) is the standard explosive against which the power of nuclear explosions are measured. In all cases, the explosive properties of nitrogen compounds are derived from the extreme stability of the product of these reactions: gaseous molecular nitrogen, N2.

Nitrogen Compounds in Drugs and Medicine

Nitrogen is a constituent of molecules in every major drug class in pharmacology and medicine. Nitric oxide (NO) has recently been discovered to be an important signaling molecule in physiology. Nitrous oxide (N2O) was discovered early in the 19th century to be a partial anesthetic, though it was not used as a surgical anesthetic until later. Called “laughing gas,” it was found to induce a state of social disinhibition resembling drunkenness.

Other notable nitrogen-containing drugs are drugs derived from plant alkaloids, such as morphine. Many alkaloids are known to have pharmacological effects; in some cases, they appear as natural chemical defenses of plants against predation. Drugs that contain nitrogen include all major classes of antibiotics, and organic nitrate drugs like nitroglycerin and nitroprusside that regulate blood pressure and heart action. Amines (alkyl derivatives of nitrogen) are important in pharmacology because they can readily carry a positive charge, as the corresponding protonated ammonium species. This allows for electrostatic interactions between the ammonium cation and various negatively charged or polarizable species in proteins.