February 08, 2021
I produce greenery and soon may be even “greener”.
What molecule am I?

Ammonia is a colorless, poisonous gas with a familiar noxious odor. It occurs in nature, primarily produced by anaerobic decay of plant and animal matter; and it also has been detected in outer space. Some plants, mainly legumes, in combination with rhizobia bacteria, “fix” atmospheric nitrogen to produce ammonia.

Ammonia has been known by its odor since ancient times. It was isolated in the 18th century by notable chemists Joseph Black (Scotland), Peter Woulfe (Ireland), Carl Wilhelm Scheele (Sweden/Germany), and Joseph Priestley (England). In 1785, French chemist Claude Louis Berthollet determined its elemental composition.

Ammonia is produced commercially via the catalytic reaction of nitrogen and hydrogen at high temperature and pressure. The process was developed in 1909 by German chemists Fritz Haber and Carl Bosch. Both received the Nobel Prize in Chemistry for their work, but in widely separated years: Haber in 1918 and Bosch in 1931. The fundamental Haber–Bosch process is still in use today.

In 2020, the worldwide ammonia production capacity was 224 million tonnes (Mt). Actual production was 187 Mt. It ranks ninth among chemicals produced globally.

Most ammonia production—≈85%—is used directly or indirectly in agriculture. Chemical fertilizers made from ammonia include urea, ammonium phosphate, ammonium nitrate, and other nitrates. Other important chemicals produced from ammonia include nitric acid, hydrazine, cyanides, and amino acids.

Ammonia was once used widely as a refrigerant. It has largely been displaced by chlorofluorocarbons and hydrochlorofluorocarbons, which are also under environmental scrutiny. Probably the most familiar household use of ammonia is in glass cleaners.

Ammonia is highly soluble in water; its exact solubility depends on temperature (see fast facts). Aqueous ammonia is also called ammonium hydroxide, but that molecule cannot be isolated. When ammonia is used as a ligand in coordination complexes, it is called “ammine”.

Currently ammonia is made from fossil fuel–derived hydrogen and is therefore not a “green” product, despite its widespread use in agriculture. But environmentally green ammonia may be on the horizon if the hydrogen is made by other means, such as wind- or solar-powered electrolysis of water.

Ammonia can be burned as a fuel in standard engines. A study by the catalyst company Haldor Topsoe (Kongens Lyngby, Denmark) concluded that replacing conventional ship fuels with green ammonia would be cost-efficient and would eliminate a significant source of greenhouse gases. It potentially can be used in aircraft fuels as well. During a transition period, ammonia could be mixed with conventional fuels. 

Ammonia hazard information

Hazard class* Hazard statement
Gases under pressure, liquefied gas H280—Contains gas under pressure; may explode if heated  Chemical Safety Warning
Skin corrosion/irritation, category 1B H314—Causes severe skin burns Chemical Safety Warning
Serious eye damage/eye irritation, category 1 H318—Causes serious eye damage Chemical Safety Warning
Acute toxicity, inhalation, category 3 H331—Toxic if inhaled Chemical Safety Warning
Hazardous to the aquatic environment, acute hazard, category 1 H400—Very toxic to aquatic life Chemical Safety Warning
Hazardous to the aquatic environment, long-term hazard, category 2 H411—Toxic to aquatic life with long-lasting effects Chemical Safety Warning

*Globally Harmonized System of Classification and Labeling of Chemicals. Explanation of pictograms. 

MOTW update

L-Tryptophan was the Molecule of the Week for November 20, 2017. It is an essential amino acid that must be part of the human diet. Among other functions, it enables the body to synthesize serotonin and melatonin. Recently, tryptophan appeared in two biochemical studies.

In one, researchers at Pennsylvania State University (University Park) and the Massachusetts Institute of Technology (Cambridge) discovered that the enzyme tryptophan 2C methyltransferase, which catalyzes the addition of a methyl group to tryptophan’s indole ring in the synthesis of the antibiotic thiostrepton, does not use a radical mechanism as do most enzymes in its class. Instead, a carboxylate group in the enzyme acts as a general base to deprotonate the tryptophan substrate.

In the second study, A. Keith Dunker at Indiana University (Bloomington) and Klára Hlouchová at Charles University (Prague) collaborated in a quest to discover how life on Earth began. They hypothesized that inherently disordered proteins (IDPs), ones that don’t hold to a specific conformation, but move around, were involved early in the process. They believe that the unstructured proteins formed by IDPs were eventually augmented by the later-forming aromatic amino acids tryptophan and tyrosine, which are larger and give proteins a greater degree of structure. They tested their ideas by replacing the aromatic amino acids in a key enzyme with leucine; the modified, less structured enzyme retained some activity, but ultimately failed to perform as efficiently as the original.

Ammonia fast facts

CAS Reg. No. 7664-41-7
Empirical formula H3N
Molar mass 17.03 g/mol
Appearance Colorless gas
Boiling point –33.3 ºC
Water solubility ≈530 g/L (20 ºC)
≈320 g/L (25 ºC)

MOTW update:
March 15, 2021

Ammonia's main use is in agriculture, but now it may have a “greener” use—as a transportation fuel. This past week, an article in Chemical & Engineering News presented an industrial update of the options and obstacles that will determine whether ammonia is indeed the “fuel of the future”.

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