Hydrazine (diamide) H2N-NH2 is a colorless, volatile, corrosive liquid with an ammonia-like odor. It fumes on air. It is fully miscible in water, liquid ammonia, methanol, ethanol. It has a poor solubility in nonpolar organic solvents, such as benzene, toluene, aliphatic hydrocarbons, carbon tetrachloride. Each H2N-N subunit in hydrazine molecule is close to pyramidal. The N-N distance is 0.149 nm, N-H - 0.1021 nm. Valent angles H-N-H and N-N-H are 106 and 112 degrees respectively. The molecule adopts a gauche conformation, the rotational barrier is twice that of ethane. Free hydrazine was synthesized for the first time by German chemist Theodor Curtius in 1887.
Hydrazine forms hydrates. Monohydrate is a stable compound (CAS 7803-57-8, m.p. 51.6°C, b.p. 118.5°C). Tetrahydrate is stable at low temperatures (m.p. -80°C). Hydrazine forms an azeotrope with water (b.p. 120.5°C; mole fraction of hydrazine is 0.585).
In anhydrous hydrazine molecules are associated. Partial ionization take place:
2N2H4 ↔ N2H5+ + N2H3-
Hydrazine dissolves many inorganic salts such as lithium chloride LiCl, calcium chloride CaCl2, sodium nitrate NaNO3, sodium perchlorate NaClO4 and so on. Almost of these salts crystallize from anhydrous hydrazine to form stable solvates.
Hydrazine is a very labile compound. It thermally decomposes with liberation of great amount of heat and a large volume of gazeous products in a matter of milliseconds. There are two mechanisms of hydrazine thermal decomposition:
1. 3N2H4 → 4NH3 + N2 + 111.8KJ
2. N2H4 → 4N2 + 2H2 + 50.5KJ
The way of decomposition can be governed by catalyst. Rhodium, palladium or platinum accelerate second process. Non-catalytic decomposition occurs mainly through first way. Metals (excluding Rh, Pd and Pt) also accelerate first process.
Hydrazine shows a basic properties in water solution. It basisity is 15 times weaker than ammonia one. It forms a stable salts with a number of mineral acids: hydrazine sulfate [N2H5](HSO4), hydrazine hydrochloride N2H4*HCl and others. It can be protonated in water solution. Second protonation occurs with difficalty.
N2H4 + H+ ↔ [N2H5]+
Hydrazine is a strong reducing agent. It can be oxidized by air to produce gaseous nitrogen, ammonia and water. Its vapours burn with a blue flame. It can be oxidized in water solution with a number of metal salts, e.g. Ce4+, Fe3+, Mn3+, Co3+. Hydrazine is able to recover metals from their salts and oxides.
It can also reduce carbonyl group in organic compounds (aldehydes and ketones) resulting in methylene group formation (Wolff-Kishner reduction). This reaction proceeds through intermediate hydrazone and azine formation (see below):
R1R2C=O + H2N-NH2 → R1R2CH2 + H2O + N2↑
Hydrazine condenses with organic carbonyl compounds to give hydrazones and azines:
R1R2C=O + H2N-NH2 → H2O + R1R2C=N-NH2 (hydrazone)
R1R2C=N-NH2 + O=CR2R1 → H2O + R1R2C=N-N=CR2R1 (azine)
Hydrazine can be alkylated or arylated to give different substituted derivatives. Most known of these compounds are 1,1-dimethylhydrazine (rocket fuel), (2,4-dinitrophenyl)hydrazine (test for ketones and aldehydes in organic chemistry). Being a lone pair donor, it forms a stable complexes with a Lewis acids. It can attach one or two electrophiles, for example N2H4*BH3 and hydrazine bis(borane) N2H4*2BH3. Hydrogen in hydrazine molecule can be replaced with metal by action of alkaly metal itself or their amides and hydrides to give highly explosive hydrazides:
H2N-NH2 + Na → H2N-NHNa + ½H2↑
Hydrazine and its water solutions are highly toxic for human and animals. Symptoms may include irritation of the eyes, nose, and throat, dizziness, headache, nausea, pulmonary edema, seizures, and coma. It affects liver and central nervous system. It is dangerously unstable compound and can explore on casual handling.
Hydrazine is produced industrially by several methods:
- Olin Raschig process. It occurs through oxidation of ammonia with sodium hypochlorite. Chloramine formed at the first stage treated with ammonia resulting in hydrazine hydrochloride. Reaction is conducted at 160°C and pressure 2.5-3.0MPa:
NH3 + NaOCl → NH2Cl + NaOH
NH2Cl + NH3 → N2H5Cl
- by oxidation of carbamide (urea) with sodium hypochlorite:
(H2N)2C=O + NaOCl + 2NaOH → N2H4 + H2O + NaCl + Na2CO3
- From acetone, ammonia, and hydrogen peroxide in several steps (Atofina-PCUK cycle). Acetone is treated with ammonia to give imine followed by oxidation with hydrogen peroxide resulting in 3,3-dimethyloxaziridine and then ammonolysis to the hydrazone, which reacts with one more equivalent of acetone. The resulting azine is hydrolysed to produce hydrazine and regenerating two molecules of acetone:
- as an oxygen scavenger for water boiler feed and heating systems to prevent corrosion damage;
- as a monopropellant fuel in rocket engines;
- as a reducing component of rocket fuel (as itself or in the mixture with 1,1-dimethylhydrazine);
- as an energy source in fuel elements;
- as a reducing agent for the recovery of metals (copper, nickel and others);
- as an intermediate in making of blowing agents (e.g. benzene sulfonylhydrazide);
- as an intermediate in insecticides, herbicides, explosives, plant growth regulators, pharmaceuticals, dyes, flame-retardants, polymerization catalysts and other chemical products making;
- it is added into glass mass to remove dimness of glass;
- as a reagent for scrubbing carbon dioxide and mercaptanes from industrial gases to prevent their release into the atmosphere;
- as a chain extender in urethane polymerization;
- as a polymerization catalyst;
- as a component of developers in photography;