|Molar mass||32.05 g/mol|
|Density||1.01 g/mL (liquid)|
1 °C (274 K)
114 °C (387 K)
|Solubility in water||miscible|
|Viscosity||0.9 cP at 25°C|
|Molecular shape||pyramidal at N|
|Dipole moment||1.85 D|
|Flash point||37.78°C (closed cup)|
|Related hydrides||hydrogen peroxide|
|Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
Hydrazine is a chemical compound with the formula N2H4. It has an ammonia-like odor, and its liquid range and density are similar to those of water. It is widely used in chemical synthetic reactions, and it is a component of some rocket fuels. It is, however, very toxic and dangerously unstable, especially when not mixed with water.
Hydrazine has a simple molecular structure. Its formula may be written as H2N-NH2, to indicate that there is a covalent bond between the two nitrogen atoms. Conceptually, this structure would arise by coupling a pair of ammonia (NH3) molecules to form the N-N bond, accompanied by the loss of one hydrogen atom per ammonia molecule.
Within a hydrazine molecule, each H2N-N subunit has a pyramidal structure. The N-N distance is 1.45 angstroms (Å), and the molecule adopts a gauche conformation. 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.
Like ammonia, hydrazine is a chemical base, but it is 15 times weaker than ammonia. It can receive a proton (H+) as follows:
(for ammonia, K = 1.78 x 10-5) Protonated hydrazine can combine with a second proton with difficulty:
Free hydrazine was synthesized for the first time by Theodor Curtius in 1889, by a circuitous route.
Another synthetic process (called the Olin Raschig process) was announced in 1907, involving the use of sodium hypochlorite and ammonia. This method relies on the reaction of chloramine with ammonia.
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. That is 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.
Hydrazine can also be produced via the so-called ketazine and peroxide processes.
In 2001, microbiologist Marc Strous of the University of Nijmegen in the Netherlands discovered that hydrazine is produced from yeast bacteria and the open ocean bacterium anammox (Brocadia anammoxidans). They are the only organisms known to produce hydrazine naturally.
Many substituted hydrazines are known, several of which occur naturally. Some examples include:
Hydrazines are part of many organic syntheses, many of which are of practical significance in pharmaceuticals, such as antituberculants, as well as in textile dyes and in photography.
The acetone azine is an intermediate in the Atofina-PCUK synthesis. Direct alkylation of hydrazines with alkyl halides in the presence of base produces alkyl-substituted hydrazines, but the reaction is typically inefficient due to poor control on the 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.
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 drive the reaction.
Being bifunctional, with two amine groups, 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. In the Einhorn-Brunner reaction, hydrazines react with imides to give triazoles.
Being a good nucleophile, N2H4 is susceptible to attack by sulfonyl halides and acyl halides. The tosylhydrazine also forms hydrazones upon treatment with carbonyls.
Hydrazine is used to cleave N-alkylated phthalimide derivatives. This scission reaction allows the phthalimide anion to be used as an amine precursor in the Gabriel synthesis.
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.
Hydrazine is converted to solid salts by treatment with mineral acids. A common salt is hydrazine hydrogen sulfate, [N2H5]HSO4, which probably should be called hydrazinium bisulfate. Hydrazine bisulfate 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.
Hydrazine is used in many processes. Examples include: production of spandex fibers, as a polymerization catalyst; a blowing agent; 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. A solution of 70 percent hydrazine and 30 percent water 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 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) and in a mixture with methanol (M-Stoff) and hydrogen peroxide 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 were used in both Viking 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 (aluminum oxide) or carbon nanofibers, or more recently molybdenum nitride on alumina, which causes it to decompose into ammonia, nitrogen gas, and hydrogen gas according to the following reactions:
These reactions are extremely exothermic (the catalyst chamber can reach 800 °C in a matter of milliseconds), and they produce large volumes of hot gas from a small volume of liquid hydrazine, making it an efficient thruster propellant.
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 are used as two-component rocket fuel, often together with dinitrogen tetroxide, N2O4.
Hydrazine is highly toxic and dangerously unstable, especially in the anhydrous form. Symptoms of acute exposure to high levels of hydrazine may include irritation of the eyes, nose, and throat, dizziness, headache, nausea, pulmonary edema, seizures, and coma in humans. Acute exposure can also damage the liver, kidneys, and central nervous system in humans. 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.
All links retrieved January 22, 2018.
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