Hydrazine is a chemical compound with the formula N₂H₄. It has an ammonia-like odor and is derived from the same industrial chemistry processes that manufacture ammonia. However, hydrazine has physical properties that are more similar to those of water.

Hydrazine is usually handled as 60% (saturated) aqueous solution for product-safety reasons.

Hydrazine is mainly used as a blowing agent in preparing polymer foams, but significant applications also include its uses as a precursor to polymerization catalysts and pharmaceuticals.

Hydrazine is used as rocket fuel and to prepare the gas precursors used in air bags. Approximately 260,000 tons are manufactured annually.^[1]

Molecular structure and properties

Hydrazine can arise via coupling a pair of ammonia molecules by removal of one hydrogen per molecule. Each H₂N-N subunit is pyramidal in shape. The N-N distance is 1.45 Å (145 pm), and the molecule adopts a gauche conformation.^[2] The rotational barrier is twice that of ethane. These structural properties resemble those of gaseous hydrogen peroxide, which adopts a "skewed" anticlinal conformation, and also experiences a strong rotational barrier.

Hydrazine has basic (alkali) chemical properties comparable to those of ammonia but about 1/15 as strong.

N₂H₄ + H₂O ? [N₂H₅]⁺ + OH^- K_b = 3.0 x 10^-6

(for ammonia K_b = 1.78 x 10^-5) Hydrazine can be diprotonated only with difficulty:^[3]

[N₂H₅]⁺ + H₂O ? [N₂H₆]²⁺ + OH^- K_b = 8.4 x 10^-16

Synthesis and manufacture

Theodor Curtius synthesized free hydrazine for the first time in 1889 via a circuitous route.^[4]

Hydrazine is produced in the Olin Raschig process from sodium hypochlorite (the active ingredient in many bleaches) and ammonia, a process announced in 1907. This method relies on the reaction of chloramine with ammonia.^[5] Ammonia is readily available from the Haber process.

The Olin Raschig route to hydrazine involves oxidation of urea with sodium hypochlorite:^[6]

(H₂N)₂C=O + NaOCl + 2 NaOH ? N₂H₄ + H₂O + NaCl + Na₂CO₃

In the Atofina-PCUK cycle, hydrazine is produced in several steps from acetone, ammonia, and hydrogen peroxide. Acetone and ammonia first react to give the imine followed by oxidation with hydrogen peroxide to the oxaziridine, a three-membered ring containing carbon, oxygen, and nitrogen, followed by ammonolysis to the hydrazone, a process that couples two nitrogen atoms. This hydrazone reacts with one more equivalent of acetone, and the resulting azine is hydrolyzed to give hydrazine, regenerating acetone. Unlike the Raschig process, this process does not produce salt. The PCUK stands for Produits Chimiques Ugine Kuhlmann, a French chemical manufacturer.^[7]

Hydrazine can also be produced via the so-called ketazine and peroxide processes.

It was recently discovered that hydrazine is produced by some yeasts and the open ocean bacterium anammox (Brocadia anammoxidans). They are the only discovered organisms to naturally produce hydrazine.^[8]

Hydrazine derivatives

Many substituted hydrazines are known, and several occur naturally. Some examples:

• gyromitrin and agaritine are phenylhydrazines found in the commercially produced mushroom species Agaricus bisporus. Gyromitrin is metabolized into monomethyl hydrazine.
• Isoniazid, iproniazid, hydralazine and phenelzine are hydrazine-containing medications.
• 1,1-dimethylhydrazine and 1,2-dimethylhydrazine are hydrazines where two hydrogen atoms are replaced by methyl groups.
• 2,4-dinitrophenylhydrazine (2,4-DNP) is commonly used to test for ketones and aldehydes in organic chemistry.
• phenylhydrazine, C₆H₅NHNH₂, the first hydrazine to be discovered.

Applications

The majority use of hydrazine is as a precursor to blowing agents. Specific compounds include azodicarbonamide and azobis(isobutyronitrile), which yield 100-200 mL of gas per gram of precursor. In a somewhat related application, sodium azide, the gas-forming agent in air bags, is produced from hydrazine by reaction with sodium nitrite.^[1] Hydrazine is also used in satellites to make adjustments while in orbit. It is also used as a propellant on board space vehicles. And to both reduce the concentration of oxygen dissolved in primary water and pH control in nuclear pressurized water reactor.

Organic chemistry

Hydrazines are part of many organic syntheses, often those of practical significance in pharmaceuticals, such as the antituberculant Isoniazid and the antifungal Fluconazole, as well as in textile dyes and in photography.^[1]

Hydrazone formation

Illustrative of the condensation of hydrazine with a simple carbonyl is its reaction with propanone to give the diisopropylidene hydrazine. The latter reacts further with hydrazine to afford the hydrazone:^[9]

2 (CH₃)₂CO + N₂H₄ ? 2 H₂O + [(CH₃)₂C=N]₂

[(CH₃)₂C=N]₂ + N₂H₄ ? 2 (CH₃)₂C=NNH₂

The propanone azine is an intermediate in the Atofina-PCUK synthesis. Direct alkylation of hydrazines with alkyl halides in the presence of base affords alkyl-substituted hydrazines, but the reaction is typically inefficient due to poor control on level of substitution (same as in ordinary amines). The reduction of hydrazones to hydrazines present a clean way to produce 1,1-dialkylated hydrazines.

In a related reaction, 2-cyanopyridines react with hydrazine to form amide hydrazides, which can be converted using 1,2-diketones into triazines.

Wolff-Kishner reduction

Hydrazine is used in the Wolff-Kishner reduction, a reaction that transforms the carbonyl group of a ketone or aldehyde into a methylene (or methyl) group via a hydrazone intermediate. The production of the highly-stable dinitrogen from the hydrazine derivative helps to drive the reaction.

Heterocyclic chemistry

Being bifunctional, with two amines, hydrazine is a key building block for the preparation of many heterocyclic compounds via condensation with a range of difunctional electrophiles. With 2,4-pentanedione, it condenses to give the 3,5-dimethylpyrazole.^[10] In the Einhorn-Brunner reaction hydrazines react with imides to give triazoles.

Sulfonation

Being a good nucleophile, N₂H₄ can attack sulfonyl halides and acyl halides.^[11] The tosylhydrazine also forms hydrazones upon treatment with carbonyls.

Deprotection of phthalimides

Hydrazine is used to cleave N-alkylated phthalimide derivatives. This scission reaction allows phthalimide anion to be used as amine precursor in the Gabriel synthesis.^[12]

Reducing agent

Hydrazine is a convenient reductant because the by-products are typically nitrogen gas and water. Thus, it is used as an antioxidant, an oxygen scavenger, and a corrosion inhibitor in water boilers and heating systems. It is also used to reduce metal salts and oxides to the pure metals in electroless nickel plating and plutonium extraction from nuclear reactor waste.

Hydrazinium salts

Hydrazine is converted to solid salts by treatment with mineral acids. A common salt is hydrazine sulfate, [N₂H₅]HSO₄, called hydrazinium sulfate^[13]. Hydrazine sulfate is used as an alternative treatment of cancer-induced cachexia. The salt of hydrazine and hydrazoic acid N₅H₅ was of scientific interest, because of the high nitrogen content and the explosive properties.

Other industrial uses

Hydrazine is used in many processes including: production of spandex fibers, as a polymerization catalyst; in fuel cells, solder fluxes; and photographic developers, as a chain extender in urethane polymerizations, and heat stabilizers. In addition, a semiconductor deposition technique using hydrazine has recently been demonstrated, with possible application to the manufacture of thin-film transistors used in liquid crystal displays. Hydrazine in a 70% hydrazine, 30% water solution is used to power the EPU (emergency power unit) on the F-16 fighter plane. The explosive Astrolite is made by combining hydrazine with ammonium nitrate. Hydrazine has also historically been used as an oxygen scavenger in boiler water treatment. However due to the toxicity and certain undesired effects, namely increased rates of flow accelerated corrosion (FAC), this practice is discouraged.

Rocket fuel

Hydrazine was first used as a rocket fuel during World War II for the Messerschmitt Me 163B (the first rocket-powered fighter plane), under the name B-Stoff (hydrazine hydrate). If mixed with methanol (M-Stoff) and water it was called C-Stoff.

Hydrazine is also used as a low-power monopropellant for the maneuvering thrusters of spacecraft, and the Space Shuttle's Auxiliary Power Units. In addition, monopropellant hydrazine-fueled rocket engines are often used in terminal descent of spacecraft. A collection of such engines was used in both Viking program landers as well as the Phoenix lander launched in August 2007.

In all hydrazine monopropellant engines, the hydrazine is passed by a catalyst such as iridium metal supported by high-surface-area alumina (aluminium oxide) or carbon nanofibers,^[14] or more recently molybdenum nitride on alumina,^[15] which causes it to decompose into ammonia, nitrogen gas, and hydrogen gas according to the following reactions:

1. 3 N₂H₄ ? 4 NH₃ + N₂
2. N₂H₄ ? N₂ + 2 H₂
3. 4 NH₃ + N₂H₄ ? 3 N₂ + 8 H₂

These reactions are extremely exothermic (the catalyst chamber can reach 800 °C in a matter of milliseconds,^[14]) and they produce large volumes of hot gas from a small volume of liquid hydrazine,^[15] making it a fairly efficient thruster propellant with a vacuum specific impulse of about 220 seconds.^[16]

Other variants of hydrazine that are used as rocket fuel are monomethylhydrazine, CH₃NHNH₂ (also known as MMH) and unsymmetrical dimethylhydrazine, (CH₃)₂NNH₂ (also known as UDMH). These derivatives are used in two-component rocket fuels, often together with dinitrogen tetroxide, N₂O₄, also known as nitrogen tetroxide. This reaction is extremely exothermic, as a rocket fuel should be, and it is also hypergolic, which means that the burning starts without any external ignition source.

Fuel cells

The Italian catalyst manufacturer Acta has proposed using hydrazine as an alternative to hydrogen in fuel cells. The chief benefit of using hydrazine is that it can produce over 200 mW/cm² more than a similar hydrogen cell without the need to use expensive platinum catalysts. As the fuel is liquid at room temperature, it can be handled and stored more easily than hydrogen. By storing the hydrazine in a tank full of a double-bonded carbon-oxygen carbonyl, the fuel reacts and forms a safe solid called hydrazone. By then flushing the tank with warm water, the liquid hydrazine hydrate is released. Hydrazine has a higher electromotive force of 1.56 V compared to 1.23 V for hydrogen. Hydrazine breaks down in the cell to form nitrogen and hydrogen which bonds with oxygen, releasing water.^[17] Hydrazine was used in fuel cells manufactured by Allis-Chalmers Corp., including some that provided electric power in space satellites in the 1960s.

Emergency Inflight Power

Can be used to generate emergency power in jet aircraft experiencing engine failure.^[18]

Safety

Hydrazine is highly toxic and dangerously unstable, especially in the anhydrous form. According to the U.S. Environmental Protection Agency:

Symptoms of acute (short-term) exposure to high levels of hydrazine may include irritation of the eyes, nose, and throat, dizziness, headache, nausea, pulmonary edema, seizures, coma in humans. Acute exposure can also damage the liver, kidneys, and central nervous system. The liquid is corrosive and may produce dermatitis from skin contact in humans and animals. Effects to the lungs, liver, spleen, and thyroid have been reported in animals chronically exposed to hydrazine via inhalation. Increased incidences of lung, nasal cavity, and liver tumors have been observed in rodents exposed to hydrazine.^[19]

Limit tests for hydrazine in pharmaceuticals suggest that it should be in the low ppm range ^[20]

At least one human is known to have died from exposure to hydrazine hydrate.^[21]

On February 21, 2008, the United States government destroyed the disabled spy satellite USA 193 with a sea-launched missile, purportedly due to the potential danger of a hydrazine release if it re-entered the Earth's atmosphere intact^[22].

References

See also

• List of Stoffs
• USA 193

External links

• The Late Show with Rob! Tonight?s Special Guest: Hydrazine (PDF) — Robert Matunas
• Hydrazine - chemical product info: properties, production, applications.
• Hydrazine toxicity

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