Hydrazine is aninorganic compound with thechemical formulaN2H4. It is a simplepnictogen hydride, and is a colourless flammable liquid with anammonia-like odour. Hydrazine is highly hazardous unless handled in solution as, for example, hydrazine hydrate (N2H4·xH2O).
Hydrazine is mainly used as afoaming agent in preparingpolymer foams, but applications also include its uses as aprecursor topharmaceuticals andagrochemicals, as well as a long-termstorable propellant for in-space spacecraft propulsion. Additionally, hydrazine is used in variousrocket fuels and to prepare the gas precursors used inairbags. Hydrazine is used within both nuclear and conventional electricalpower plant steam cycles as anoxygen scavenger to control concentrations of dissolved oxygen in an effort to reduce corrosion.[8]As of 2000[update], approximately 120,000 tons of hydrazine hydrate (corresponding to a 64% solution of hydrazine in water by weight) were manufactured worldwide per year.[9]
Hydrazines are a class of organic substances derived by replacing one or more hydrogen atoms in hydrazine by an organic group.[9]
The name "hydrazine" was coined byEmil Fischer in 1875; he was trying to produce organic compounds that consisted of mono-substituted hydrazine.[10] By 1887,Theodor Curtius had produced hydrazine sulfate by treating organic diazides with dilute sulfuric acid; however, he was unable to obtain pure hydrazine, despite repeated efforts.[11][12][13] Pure anhydrous hydrazine was first prepared by the Dutch chemistLobry de Bruyn in 1895.[14][15][16]
The nomenclature is a bi-valent form, with prefixhydr- used to indicate the presence ofhydrogen atoms and suffix beginning with-az-, fromazote, the French word fornitrogen.
Hydrazine is also used as a long-termstorable propellant on boardspace vehicles, such as theDawn mission to Ceres and Vesta, and to both reduce the concentration of dissolved oxygen in and control pH of water used in large industrial boilers. TheF-16 fighter jet,[17]Space Shuttle, andU-2 spy plane use hydrazine to fuel their Emergency Start System in the event of an engine stall.[18] The Space Shuttle'ssolid boosters decomposed hydrazine to provide power for onboard systems.
Hydrazine compounds can be effective as active ingredients in insecticides, miticides,nematicides, fungicides, antiviral agents, attractants, herbicides, or plant growth regulators.[20]
Puch Ms 25 motorcycle with a hydrazine-air fuel cell, arguably the world's first ever fuel cell motorcycle, developed byKarl Kordesch
The Italiancatalyst manufacturer Acta (chemical company) has proposed using hydrazine as an alternative tohydrogen infuel cells. The chief benefit of using hydrazine is that it can produce over 200 mW/cm2 more than a similar hydrogen cell without requiring (expensive)platinum catalysts.[21] Because 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-bondedcarbon-oxygencarbonyl, the fuel reacts and forms a safe solid calledhydrazone. By then flushing the tank with warm water, the liquid hydrazine hydrate is released. Hydrazine has a higherelectromotive force of 1.56V compared to 1.23 V for hydrogen. Hydrazine breaks down in the cell to formnitrogen andhydrogen which bonds with oxygen, releasing water.[21] Hydrazine was used in fuel cells manufactured byAllis-Chalmers Corp., including some that provided electric power in space satellites in the 1960s.
A mixture of 63% hydrazine, 32%hydrazine nitrate and 5% water is a standard propellant for experimentalbulk-loaded liquid propellant artillery. The propellant mixture above is one of the most predictable and stable, with a flat pressure profile during firing. Misfires are usually caused by inadequate ignition. The movement of the shell after a mis-ignition causes a large bubble with a larger ignition surface area, and the greater rate of gas production causes very high pressure, sometimes including catastrophic tube failures (i.e. explosions).[22] From January–June 1991, theU.S. Army Research Laboratory conducted a review of early bulk-loaded liquid propellant gun programs for possible relevance to the electrothermal chemical propulsion program.[22]
TheUnited States Air Force (USAF) regularly uses H-70, a 70% hydrazine 30% water mixture, in operations employing theGeneral Dynamics F-16 Fighting Falcon fighter aircraft and theLockheed U-2 "Dragon Lady" reconnaissance aircraft. The single jet engine F-16 utilizes hydrazine to power itsEmergency Power Unit (EPU), which provides emergency electrical and hydraulic power in the event of an engine flame out. The EPU activates automatically, or manually by pilot control, in the event of loss of hydraulic pressure or electrical power in order to provide emergency flight controls. The single jet engine U-2 utilizes hydrazine to power its Emergency Starting System (ESS), which provides a highly reliable method to restart the engine in flight in the event of a stall.[23]
Anhydrous (pure, not in solution) hydrazine being loaded into theMESSENGER space probe (orbital reconnaissance mission of the planetMercury). The technician is wearing a safety suit in overpressure with an external air supply.
Hydrazine is used as a low-powermonopropellant for the maneuvering (RCS/Reaction control system) thrusters of spacecraft, and was used to power theSpace Shuttle's auxiliary power units (APUs). In addition, mono-propellant hydrazine-fueled rocket engines are often used in terminal descent of spacecraft. Such engines were used on theViking program landers in the 1970s as well as the Mars landersPhoenix (May 2008),Curiosity (August 2012), andPerseverance (February 2021).
In all hydrazine mono-propellant engines, the hydrazine is passed over acatalyst such asiridium metal supported by high-surface-areaalumina (aluminium oxide), which causes it to decompose intoammonia (NH3), nitrogen gas (N2), and hydrogen (H2) gas according to the three following reactions:[27]
Reaction 1:N2H4 → N2 + 2 H2
Reaction 2:3 N2H4 → 4 NH3 + N2
Reaction 3:4 NH3 + N2H4 → 3 N2 + 8 H2
The first two reactions are extremelyexothermic (the catalyst chamber can reach 800 °C in a matter of milliseconds,[28]) and they produce large volumes of hot gas from a small volume of liquid,[29] making hydrazine a fairly efficient thruster propellant with a vacuumspecific impulse of about 220 seconds.[30] Reaction 2 is the most exothermic, but produces a smaller number of molecules than that of reaction 1. Reaction 3 isendothermic and reverts the effect of reaction 2 back to the same effect as reaction 1 alone (lower temperature, greater number of molecules). The catalyst structure affects the proportion of theNH3 that is dissociated in reaction 3; a higher temperature is desirable for rocket thrusters, while more molecules are desirable when the reactions are intended to produce greater quantities of gas.[31]
Since hydrazine is a solid below 2 °C, it is not suitable as a general purpose rocket propellant for military applications. Othervariants of hydrazine that are used as rocket fuel aremonomethylhydrazine,CH3NHNH2, also known as MMH (melting point −52 °C), andunsymmetrical dimethylhydrazine,(CH3)2NNH2, also known as UDMH (melting point −57 °C). These derivatives are used in two-component rocket fuels, often together withdinitrogen tetroxide,N2O4. A 50:50 mixture by weight of hydrazine and UDMH was used in the engine of the service propulsion system of theApollo command and service module, both the ascent and descent engines of theApollo Lunar Module andTitan IIICBMs and is known asAerozine 50.[24] These reactions are extremely exothermic, and the burning is alsohypergolic (it starts burning without any external ignition).[32]
There are ongoing efforts in the aerospace industry to find a replacement for hydrazine, given its potential ban across the European Union.[33][34][35] Promising alternatives includenitrous oxide-based propellant combinations, with development being led by commercial companiesDawn Aerospace,Impulse Space,[36] andLauncher.[37] The first nitrous oxide-based system ever flown in space was byD-Orbit onboard theirION Satellite Carrier in 2021, using six Dawn Aerospace B20 thrusters.[38][39] Another alternative is more safe blends of hydrazine with much lowervapor pressure, hence reduced inhalation hazard.Aerojet Rocketdyne has developed HPB-G28 blend that have 150 times lower vapor pressure, same specific impulse, and 35% higher density specific impulse than neat hydrazine. HPB-G28 can be used with same thrusters and catalysts as hydrazine, but has freezing point of -55°C, making propellant line heating unnecessary. It contains 65% (by mol) hydrazine, 27% hydroxyethylhydrazinium nitrate (HEHN) and 8%hydrazinium nitrate.[40]
Potential routes of hydrazine exposure include dermal, ocular, inhalation and ingestion.[41]
Hydrazine exposure can cause skin irritation/contact dermatitis and burning, irritation to the eyes/nose/throat, nausea/vomiting, shortness of breath, pulmonary edema, headache, dizziness, central nervous system depression, lethargy, temporary blindness, seizures and coma. Exposure can also cause organ damage to the liver, kidneys and central nervous system.[41][42] Hydrazine is documented as a strong skin sensitizer with potential for cross-sensitization to hydrazine derivatives following initial exposure.[43] In addition to occupational uses reviewed above, exposure to hydrazine is also possible in small amounts from tobacco smoke.[42]
The official U.S. guidance on hydrazine as a carcinogen is mixed but generally there is recognition of potential cancer-causing effects. TheNational Institute for Occupational Safety and Health (NIOSH) lists it as a "potential occupational carcinogen". The National Toxicology Program (NTP) finds it is "reasonably anticipated to be a human carcinogen". TheAmerican Conference of Governmental Industrial Hygienists (ACGIH) grades hydrazine as "A3—confirmed animal carcinogen with unknown relevance to humans". The U.S. Environmental Protection Agency (EPA) grades it as "B2—a probable human carcinogen based on animal study evidence".[44]
The International Agency for Research on Cancer (IARC) rates hydrazine as "2A—probably carcinogenic to humans" with a positive association observed between hydrazine exposure and lung cancer.[45] Based on cohort and cross-sectional studies of occupational hydrazine exposure, a committee from theNational Academies of Sciences, Engineering and Medicine concluded that there is suggestive evidence of an association between hydrazine exposure and lung cancer, with insufficient evidence of association with cancer at other sites.[46] TheEuropean Commission'sScientific Committee on Occupational Exposure Limits (SCOEL) places hydrazine in carcinogen "group B—a genotoxic carcinogen". The genotoxic mechanism the committee cited references hydrazine's reaction with endogenous formaldehyde and formation of a DNA-methylating agent.[47]
In the event of a hydrazine exposure-related emergency,NIOSH recommends removing contaminated clothing immediately, washing skin with soap and water, and for eye exposure removing contact lenses and flushing eyes with water for at least 15 minutes.NIOSH also recommends anyone with potential hydrazine exposure to seek medical attention as soon as possible.[41] There are no specific post-exposure laboratory or medical imaging recommendations, and the medical work-up may depend on the type and severity of symptoms. TheWorld Health Organization (WHO) recommends potential exposures be treated symptomatically with special attention given to potential lung and liver damage. Past cases of hydrazine exposure have documented success with pyridoxine (vitamin B6) treatment.[43]
ACGIH Threshold Limit Value (TLV): 0.01 ppm (0.013 mg/m3) 8-hour Time Weighted Average[44]
The odor threshold for hydrazine is 3.7 ppm, thus if a worker is able to smell an ammonia-like odor then they are likely over the exposure limit. However, this odor threshold varies greatly and should not be used to determine potentially hazardous exposures.[48]
For aerospace personnel, theUnited States Air Force uses an emergency exposure guideline, developed by theNational Academy of Sciences Committee on Toxicology, which is utilized for non-routine exposures of the general public and is called the Short-Term Public Emergency Exposure Guideline (SPEGL). The SPEGL, which does not apply to occupational exposures, is defined as the acceptable peak concentration for unpredicted, single, short-term emergency exposures of the general public and represents rare exposures in a worker's lifetime. For hydrazine the 1-hour SPEGL is 2 ppm, with a 24-hour SPEGL of 0.08 ppm.[49]
A complete surveillance programme for hydrazine should include systematic analysis of biologic monitoring, medical screening and morbidity/mortality information. TheCDC recommends surveillance summaries and education be provided for supervisors and workers. Pre-placement and periodic medical screening should be conducted with specific focus on potential effects of hydrazine upon functioning of the eyes, skin, liver, kidneys, hematopoietic, nervous and respiratory systems.[41]
Common controls used for hydrazine include process enclosure, local exhaust ventilation andpersonal protective equipment (PPE).[41] Guidelines for hydrazine PPE include non-permeable gloves and clothing, indirect-vent splash resistant goggles, face shield and in some cases a respirator.[48] The use of respirators for the handling of hydrazine should be the last resort as a method of controlling worker exposure. In cases where respirators are needed, proper respirator selection and a complete respiratory protection program consistent withOSHA guidelines should be implemented.[41]
ForUSAF personnel, Air Force Occupational Safety and Health (AFOSH) Standard 48-8, Attachment 8 reviews the considerations for occupational exposure to hydrazine in missile, aircraft and spacecraft systems. Specific guidance for exposure response includes mandatory emergency shower and eyewash stations and a process for decontaminating protective clothing. The guidance also assigns responsibilities and requirements for proper PPE, employee training, medical surveillance and emergency response.[49] USAF bases requiring the use of hydrazine generally have specific base regulations governing local requirements for safe hydrazine use and emergency response.
Hydrazine,H2N−NH2, contains two amine groupsNH2 connected by a single bond between the two nitrogen atoms. EachN−NH2 subunit is pyramidal. The structure of the free molecules was determined bygas electron diffraction andmicrowave spectroscopy. The N–N single bond length is 1.447(2)Å (144.7(2)pm), the N-H distance is 1.015(2)Å, the N-N-H angles are 106(2)° and 112(2)°, the H-N-H angle is 107°.[50] The molecule adopts agauche conformation with a torsion angle of 91(2)° (dihedral angle between the planes containing the N-N bond and the bisectors of the H-N-H angles). Therotational barrier is twice that ofethane. These structural properties resemble those of gaseoushydrogen peroxide, which adopts a "skewed"anticlinal conformation, and also experiences a strong rotational barrier.
The structure of solid hydrazine was determined by X-ray diffraction. In this phase the N-N bond has a length of 1.46Å and the nearest non-bonded distances are 3.19, 3.25 and 3.30Å.[51]
Diverse synthetic pathways for hydrazine production have been developed.[9] The key step is the creation of theN–N single bond. The many routes can be divided into those that use chlorine oxidants (and generate salt) and those that do not.
Oxidation of ammonia via oxaziridines from peroxide
Hydrazine can be synthesized from ammonia and hydrogen peroxide with a ketone catalyst, in a procedure called thePeroxide process (sometimes called Pechiney-Ugine-Kuhlmann process, the Atofina–PCUK cycle, or ketazine process).[9] The net reaction is:[52]
2 NH3 + H2O2 → N2H4 + 2 H2O
In this route, the ketone and ammonia first condense to give theimine, which is oxidised by hydrogen peroxide to theoxaziridine, a three-membered ring containing carbon, oxygen, and nitrogen. Next, the oxaziridine gives thehydrazone bytreatment with ammonia, which process creates the nitrogen-nitrogen single bond. This hydrazone condenses with one more equivalent of ketone.
The resultingazine is hydrolyzed to give hydrazine and regenerate the ketone,methyl ethyl ketone:
Related to the Raschig process,urea can be oxidized instead of ammonia. Again sodium hypochlorite serves as the oxidant. The net reaction is shown:[54]
The process generates significant by-products and is mainly practised in Asia.[9]
TheBayer Ketazine Process is the predecessor to the peroxide process. It employs sodium hypochlorite as oxidant instead of hydrogen peroxide. Like all hypochlorite-based routes, this method produces an equivalent of salt for each equivalent of hydrazine.[9]
Hydrazine forms amonohydrateN2H4·H2O that is denser (1.032 g/cm3) than theanhydrous formN2H4 (1.021 g/cm3). Hydrazine hasbasic (alkali) chemical properties comparable to those ofammonia:[55]
Exposure to extremely strong bases or alkali metals generates deprotonated hydrazide salts, such assodium hydrazide. Most explode on exposure to air or moisture.[57]
The heat of combustion of hydrazine in oxygen (air) is 19.41 MJ/kg (8345 BTU/lb).[58]
Hydrazine is a convenient reductant because the by-products are typically nitrogen gas and water. This property makes it useful as anantioxidant, an oxygenscavenger, and acorrosion inhibitor in water boilers and heating systems. It also directly reduces salts of less active metals (e.g., bismuth, arsenic, copper, mercury, silver, lead, platinum, and palladium) to the element.[59] That property has commercial application inelectrolessnickel plating andplutonium extraction fromnuclear reactor waste. Some colour photographic processes also use a weak solution of hydrazine as a stabilising wash, as it scavengesdye coupler and unreacted silver halides. Hydrazine is the most common and effective reducing agent used to convertgraphene oxide (GO) to reduced graphene oxide (rGO) via hydrothermal treatment.[60]
Hydrazine can beprotonated to form various solid salts of thehydrazinium cation[N2H5]+, by treatment with mineral acids. A common salt ishydrazinium hydrogensulfate,[N2H5]+[HSO4]−.[61] Hydrazinium hydrogensulfate was investigated as a treatment of cancer-inducedcachexia, but proved ineffective.[62]
Double protonation gives the hydraziniumdication or hydrazinediium,[N2H6]2+, of which various salts are known.[63]
Hydrazines are part of manyorganic syntheses, often those of practical significance inpharmaceuticals (see applications section), as well as in textiledyes and in photography.[9]
Hydrazine is used in theWolff–Kishner reduction, a reaction that transforms thecarbonyl group of aketone into amethylene bridge (or analdehyde into amethyl group) via ahydrazone intermediate. Upon the catalysis with transition-metals, the hydrazones are used as organometallic reagent equivalents (HOME chemistry) for C-C bond formations.[64] The production of the highly stabledinitrogen from the hydrazine derivative helps to drive the reaction.
Being a good nucleophile,N2H4 can attack sulfonyl halides and acyl halides.[66] Thetosylhydrazine also forms hydrazones upon treatment with carbonyls.
Hydrazine is used to cleaveN-alkylated phthalimide derivatives. This scission reaction allows phthalimide anion to be used as amine precursor in theGabriel synthesis.[67]
Illustrative of the condensation of hydrazine with a simple carbonyl is its reaction with acetone to give theacetone azine. The latter reacts further with hydrazine to yieldacetone hydrazone:[68]
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 process. Directalkylation of hydrazines withalkyl halides in the presence of base yields alkyl-substituted hydrazines, but the reaction is typically inefficient due to poor control on level of substitution (same as in ordinaryamines). The reduction ofhydrazones 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 using1,2-diketones intotriazines.
Hydrazine is the intermediate in the anaerobic oxidation of ammonia (anammox) process.[69] It is produced by some yeasts and the open ocean bacterium anammox (Brocadia anammoxidans).[70]
In the novelThe Martian (which was adapted intoa feature film) the main character uses aniridium catalyst to separatehydrogen gas from surplus hydrazine fuel, which he then burns to generate water for survival.
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