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Hydrazine is the chemical compound with formula N2H4. This species is widely used in chemical synthesis. It is perhaps best known as a component in rocket fuel. Hydrazine is also found in tobacco products.
Molecular structure and properties Conceptually, hydrazine arises via coupling a pair of ammonia molecules by removal of one H per molecule. Each H2N-N subunit is pyramidal. The N-N distance is 1.45 Å. The molecule adopts a gauche conformation, as seen also in hydrogen peroxide. The rotational barrier is twice that of ethane. Hydrazine has a liquid range and density like water's. It has basic properties comparable to ammonia but 15-times weaker. It can be di-protonated only with difficulty: N2H4 + H+ → N2H5+ K = 8.5 x 10-7 (for ammonia K = 1.78 x 10-5) N2H5+ + H+ → N2H62+ K = 8.4 x 10-16 Synthesis Theodor Curtius synthesized free hydrazine for the first time in 1889 via a circuitous route. Hydrazine is produced in the Olin Raschig process from sodium hypochlorite and ammonia, a process developed in 1907. 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, 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. Hydrazine can also be produced via the ketazine and the peroxide processes. Hydrazine derivatives Many substituted hydrazines are known, and several occur naturally. Some examples: Uses in chemistry Hydrazines are part of many organic syntheses, often those of practical significance in pharmaceuticals, such as antituberculants, as well as in textile dyes and photography. Hydrazone formation Illustrative of the condensation of hydrazine with a simple carbonyl is its reaction with acetone to give the azine. This azine reacts further with hydrazine to afford the hydrazone: 2 (CH3)2CO + N2H4 → 2 H2O + (CH3)2C=N2 (CH3)2C=N2 + N2H4 → 2 (CH3)2C=NNH2 The acetone 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. 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 dimethylpyrazole. In the Einhorn-Brunner reaction hydrazines react with amides to give triazoles. Sulfonation Being a good nucleophile, N2H4 is susceptible to attack by sulfonyl halides and acyl halides. 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. 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 electrodeless 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, N2H5HSO4, which probably should be called hydrazinium sulfate. Other industrial uses Hydrazine is used in many processes including: 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. Hydrazine in a 70% solution is used to power the EPU (emergency power unit) on the F-16 fighter plane. Rocket fuel Hydrazine is also used as rocket fuel starting in World War II for the Messerschmitt Me 163, under name B-Stoff (hydrazine hydrate) and in a mixture with methanol (M-Stoff) as 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 scheduled to launch 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 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 rections: 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. The explosive Astrolite is made by combining hydrazine with ammonium nitrate. Safety Hydrazine is highly toxic and dangerously unstable, especially in the anhydrous form. See also | |||||||||||||||||||
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