Hydrazine
Hydrazine is an inorganic compound with the chemical formula N₂H₄, consisting of two amino groups linked by a nitrogen–nitrogen single bond.[1] It exists as a colorless, fuming liquid with an ammonia-like odor, a density of 1.004 g/mL at 25 °C, a boiling point of 113.5 °C, and a melting point of 2 °C.[2] First synthesized in 1887 by Theodor Curtius through the reduction of dimethyl sulfate-derived intermediates, hydrazine is produced industrially via the Raschig process, involving the reaction of chloramine with ammonia.[3] Hydrazine serves as a high-energy monopropellant in aerospace applications, decomposing exothermically over catalysts to provide thrust for spacecraft attitude control and orbital maneuvers, as seen in missions like those of the Hubble Space Telescope.[4] Its hypergolic reactivity with oxidizers such as nitrogen tetroxide enables reliable ignition without igniters, contributing to its use in bipropellant rocket engines.[5] Beyond propulsion, it functions as a reducing agent in boiler water treatment to scavenge dissolved oxygen, preventing corrosion, and as a precursor in synthesizing pharmaceuticals like isoniazid for tuberculosis treatment.[3] Despite its utility, hydrazine poses severe health risks, acting as a potent irritant to skin, eyes, and mucous membranes upon contact or inhalation, with acute exposure potentially causing convulsions, liver damage, and death.[5] The U.S. Environmental Protection Agency classifies it as a probable human carcinogen (Group B2) based on animal studies showing tumors in multiple organs, while the International Agency for Research on Cancer lists it as possibly carcinogenic to humans (Group 2B).[6][7] Its inherent instability also necessitates stringent handling protocols to mitigate explosion hazards when contaminated or mixed with strong oxidants.[1]Chemical and Physical Properties
Molecular Structure and Bonding
Hydrazine possesses the molecular formula N₂H₄ and features two nitrogen atoms linked by a single covalent bond, denoted as H₂N–NH₂. Each nitrogen atom exhibits sp³ hybridization, with a tetrahedral arrangement of four electron pairs: three bonding pairs to hydrogen or the adjacent nitrogen and one lone pair. This electron configuration yields a pyramidal geometry around each nitrogen center, akin to ammonia.[1][8] The N–N bond length measures 1.45 Å, longer than a typical N–N single bond in acyclic amines due to repulsion between adjacent lone pairs, which weakens the bond. Bond angles deviate from the ideal tetrahedral 109.5°; the H–N–H angle approximates 107°, while the H–N–N angle is roughly 112°, reflecting lone pair-bond pair repulsions. These structural features endow the lone pairs with high nucleophilicity, enabling hydrazine to act as a strong nucleophile in reactions with carbonyl compounds and alkyl halides.[9][10] Quantum chemical calculations, including density functional theory, confirm the preference for skew conformations in hydrazine, minimizing lone pair repulsions across the N–N bond, similar to the torsional barrier in hydrogen peroxide. The barrier arises primarily from electrostatic interactions between lone pairs rather than hyperconjugation, with the anti conformer being a transition state. Stability is further influenced by hyperconjugative interactions between N–H σ bonds and the N–N σ* orbital, which delocalize electron density and modulate reactivity.[11][12]Thermodynamic and Spectroscopic Properties
The standard enthalpy of formation of liquid hydrazine (N₂H₄, l) is +50.63 kJ/mol at 298 K, while for the gas phase it is +95.4 kJ/mol, reflecting the endothermic nature of the N-N bond relative to separated N₂ and H₂.[13] [14] These values, derived from combustion calorimetry and equilibrium measurements, enable Gibbs free energy calculations for reactions involving hydrazine, such as its decomposition to ammonia or nitrogen, where the positive Δ_f H° favors exothermic processes under standard conditions.[15] The standard molar entropy S° for liquid hydrazine is 121.5 J mol⁻¹ K⁻¹ at 298 K and 1 bar, increasing to approximately 238 J mol⁻¹ K⁻¹ in the gas phase due to translational and rotational contributions.[16] Heat capacity data, obtained from adiabatic calorimetry, show C_p for the liquid at ~98 J mol⁻¹ K⁻¹ near 298 K, with gaseous C_p following the Shomate equation (e.g., coefficients A=48.18, B=170.5 for 298–800 K range), allowing prediction of temperature-dependent enthalpy changes.[15] [16]| Property | Liquid (298 K) | Gas (298 K) |
|---|---|---|
| Δ_f H° (kJ/mol) | +50.63 | +95.4 |
| S° (J mol⁻¹ K⁻¹) | 121.5 | ~238 |
| C_p (J mol⁻¹ K⁻¹) | ~98 | ~57 (ideal gas limit) |