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Hydrazine: structure, properties, synthesis, uses

It bears a close resemblance to water, since both exist as liquid substances at similar temperature ranges. From hydrazine we can obtain organic derivatives by replacing its hydrogens with carbon groups. Likewise, hydrazones, hydrazides, and salts of hydrazines are synthesized from it.

Structural formula of hydrazine. Source: NEUROtiker via Wikipedia.

Above is the structural formula of hydrazine, which can be written as H 2 NNH 2 or H 2 N-NH 2 . If you look closely, it appears that two NH 3 molecules have bonded to each other. However, not only ammonia molecules intervene as reagents in the formation of the NN bond.

Structure of hydrazine

Molecule and bond

Molecular structure of hydrazine. Source: Ben Mills via Wikipedia.

In the upper image we have the hydrazine molecule represented by a model of spheres and bars. Note that the spatial orientation of the hydrogen atoms, the white spheres, contrasts with the impression given by the structural formula; that is to say, the hydrogens of both -NH 2 are not aligned or eclipsed, but rather have a rotation angle of 95º between them.

In fact, it is so much so that the NN bond is barely able to rotate, having an energy barrier that must be overcome in order to eclipse the four hydrogen atoms.

What is this about? In principle, to the strong electronic repulsions that the two pairs of solitary electrons mutually experience on nitrogen atoms. This angle of 95º is the one that gives the hydrazine molecule the greatest stability.

Geometries

Each nitrogen atom has an sp 3 hybridization , adopting a triangular pyramid geometry. Therefore, with the pairs of free electrons, the hydrazine molecule could be visualized as two pyramids linked by one of its vertices, and in whose centers the nitrogen atoms would be located.

Intermolecular interactions

The formula H 2 NNH 2 could give the false idea that hydrazine is an apolar compound because it has two identical groups linked. However, the molecular structure and its rotated NN bond show that the molecule is not completely symmetric, also mentioning that its pairs of free electrons do not point in opposite directions.

Properties of hydrazine

Hydrazine hydrate. Source: LHcheM / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Physical appearance

Hydrazine is a colorless liquid that is very similar to water in terms of viscosity and density . When it burns, it emits white vapors, as well as reeking of ammonia and fish.

Molar mass

32.0452 g / mol

Melting point

2 ºC.

Boiling point

114 ° C. Note that hydrazine exists in a liquid state within the temperature range 2-114 ºC, very similar to that of water, 0-100 ºC.

Density

1,021 g / cm 3

Vapor density

1.1 in relation to air.

Vapor pressure

Around 0.010 atm at 30.7 ºC

Solubility

Hydrazine is miscible with water, as well as with various primary alcohols. Hydrazine forms a hydrate with water, N 2 H 4 · H 2 O, which is characterized by being more dense than pure or anhydrous hydrazine. This hydrate is the most commercialized product, since it is more stable and cheaper.

Dipole moment

1.85 D. It is a fairly polar substance, almost the same as water.

Viscosity

0.876 cP

Flash point

52 ºC

Autoignition temperature

It varies depending on the substances or surfaces with which it comes in contact. For example, on iron oxides it burns at 23ºC, while on glass or other containers it does not start to ignite until up to 270ºC.

Decomposition

When it burns in air, it decomposes into nitrogen oxides, NO x . But with regard to its catalytic decomposition, that is, that accelerated by metallic oxides or metals, products such as ammonia, nitrogen and hydrogen are obtained, and an immense amount of heat, which end up forming a burning but invisible flame.

Reactivity

Hydrazine is a good reducing agent, since when it oxidizes it produces water and nitrogen, both compounds that do not undergo secondary reactions.

On the other hand, hydrazine is a nucleophile, as it uses its pairs of free electrons to bind to other organic molecules. Likewise, it can undergo substitution of any of its four hydrogens by alkyl groups, such as CH 3 , to produce methylated derivatives: CH 3 NHNH 2 or (CH 3 ) 2 NNH 2 .

Reacts with strong acids to form hydrazinium salts, N 2 H 5 X, and hydrazonium, N 2 H 6 X 2 . The latter are the least common. Hydrazinium sulfate, [N 2 H 5 ] HSO 4 , is prepared by reacting hydrazine hydrate with sulfuric acid. 

Basicity

Hydrazine is a weaker base than ammonia:

2 H 4 (aq) + H 2 O (l) ⇌ N 2 H + (aq) + OH  (aq)

b = 1.3 10 -6

This is because the hydrazinium conjugated acid, N 2 H + or H 3 N + -NH 2 , has its charge destabilized by the attractor effect of the NH 2 group ; that is, instead of adding electronic density to the positive nitrogen, it removes it.

Hydrazine is capable of undergoing a second protonation, much less likely, which generates an even more unstable conjugated acid:

2 H + (aq) + H 2 O (l) ⇌ N 2 H + (aq) + OH  (aq)

b = 8.4 10 -16

Hydrazonium, N 2 H + or H 3 N + -N + H 3 , is very unstable because of the repulsions between the positive charges of both nitrogen atoms. However, in the company of certain anions it can exist as a salt.

Toxicity

Anhydrous hydrazine is an extremely toxic, caustic, and possibly carcinogenic substance, not to mention that it is highly explosive, so it should only be handled under strict safety protocols.

Synthesis

The most widely used method to synthesize hydrazine is the Raschig Process. This consists, in summary, of the following two reactions:

NH 3 (aq) + NaOCl (aq) → NH 2 Cl (aq) + NaOH (aq)

2NH 3 (aq) + NH 2 Cl (aq) → N 2 H 4 (aq) + NH 4 Cl (aq)

At the end, the hydrate N 2 H 4 · H 2 O is formed, which is distilled in the presence of KOH or NaOH as drying solids:

2 H 4 H 2 O → N 2 H 4

In this way the anhydrous hydrazine is obtained and the NN bond is formed.

Uses of hydrazine

Hydrazine is one of the most highly valued substances in the aerospace boom. Source: Pxhere.

– Hydrazine, in the presence of metallic catalysts, decomposes exothermically to produce nitrogen and hydrogen, thus supplying enough energy to power airplanes, rockets and satellites.

– This tendency to decompose and originate gases is used precisely to inflate polymeric materials, such as rubber or polyethylene, turning them into their respective foams.

– Also, when releasing gases, these can drag others that are dissolved in the liquids in the pipes, so it is very useful to remove oxygen from the boilers, thus preventing them from corroding.

– Hydrazine is a precursor material for many organic compounds with pharmacological, antifungal, herbicide, insecticide, fertilizer, etc. functions. This is due to the fact that a great variety of cyclic nitrogen derivatives are synthesized from it.

– Finally, being a good reducing agent, hydrazine and its hydrazinium salts are used to reduce precious metal ores, being of great value in the metallurgical industry.

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