Hydroxylamine

Hydroxylamine is a reactive chemical with formula NH₂OH. It can be considered a hybrid of ammonia and water due to parallels it shares with each. At room temperature pure NH₂OH is ordinarily a white, unstable crystalline, hygroscopic compound;^[1] however it is almost always encountered as an aqueous solution.

Hydroxylamine tends to be explosive, and the nature of the hazard is not entirely understood. At least two factories dealing in hydroxylamine have been destroyed since 1999 with loss of life.^[2] It is known, however, that ferrous and ferric iron accelerate the decomposition of 50% NH₂OH solutions. Hydroxylamine and its derivatives are more safely handled in the form of salts.

NH₂OH is an intermediate in biological nitrification. The oxidation of NH₃ is mediated by HAO (hydroxylamine oxidoreductase).

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Production

NH₂OH can be synthesized via several routes:

Raschig synthesis: Aqueous ammonium nitrite is reduced by HSO₄⁻/SO₂ at 0°C to yield a hydroxylamido-N,N-disulfate anion, which can be hydrolyzed to give (NH₃OH)₂SO₄.

NH₄NO₂ + 2SO₂ + NH₃ + H₂O → [NH₄]₂[N(OH)(OSO₂)₂]

[NH₄]⁺₂[N(OH)(OSO₂)₂]²⁻ + H₂O → [NH₄][NH(OH)(OSO₂)] + [NH₄][HSO₄]

2[NH₄]⁺[NH(OH)(OSO₂)]⁻ + 2H₂O → [NH₃(OH)]₂[SO₄] + [NH₄]₂[SO₄]

Solid NH₂OH can be collected by treatment with liquid ammonia. Ammonium sulfate is insoluble in liquid ammonia and is removed by filtration; the liquid ammonia is evaporated to give the desired product.^[1]

Another method of synthesis is to make hydroxylammonium salts which can then be converted to hydroxylamine.

[NH₃(OH)]Cl + NaOBu → NH₂OH + NaCl + BuOH^[1]

The reduction of nitrous acid or potassium nitrate with bisulfite:

HNO₂ + 2 HSO₃⁻ → [N(OH)(OSO₂)₂]²⁻ + H₂O → [NH(OH)(OSO₂)]⁻ + [HSO₄]⁻

[NH(OH)(OSO₂)]⁻ + H₃O⁺ (100 °C/1 h) → [NH₃(OH)]⁺ + [HSO₄]⁻

Reactions

Hydroxylamine reacts with electrophiles, such as an alkylating agents, which can attack at either the O or N position.

R-X + NH₂OH → R-ONH₂ + HX

R-X + NH₂OH → R-NHOH + HX

The reaction of NH₂OH with an aldyhyde or ketone produces an oxime.

R₂C=O + NH₂OH∙HCl , NaOH → R₂C=NOH + NaCl + H₂O

This reaction is useful in the purification of ketones and aldehydes, Oximes also are employed as ligands, e.g. dimethylglyoxime.

NH₂OH reacts with chlorosulfuric acid to give hydroxylamine-O-sulfonic acid, a useful reagent for the synthesis of caprolactam.

HOSO₂Cl + NH₂OH → NH₂OS₂OH + HCl

The hydroxylamine-O-sulfonic acid, which should be stored at 0 °C, can be checked by iodometric titration.

Hydroxylamine (NH₂OH), or hydroxylamines (R-NHOH) can be reduced to amines.^[3]

NH₂OH (Zn/HCl) → NH₃

R-NHOH (Zn/HCl) → R-NH₂

Uses

Hydroxylamine and its salts are commonly used as reducing agents in a myriad of organic and inorganic reactions. They can also act as antioxidants for fatty acids. Some non-chemical uses include removal of hair from animal hides and photography developing solutions.^[4]

The nitrate salt, hydroxylammonium nitrate, is being researched as a rocket propellant, both in water solution as a monopropellant and in its solid form as a solid propellant.

This has also been used in the past by biologists to introduce random mutations by switching base pairs from A to G, or from C to T. This is to probe functional areas of genes to elucidate what happens if their functions are broken. Nowadays other mutagens are used. Hydroxylamine can also be used to highly selectively cleave asparaginyl-glycine peptide bonds in peptides and proteins. It also bonds to and permanently disables (poisons) heme-containing enzymes. It is used as an irreversible inhibitor of the oxygen-evolving complex of photosynthesis on account of its similar structure to water.

In the semiconductor industry, hydroxylamine is often a component in the "resist stripper" which removes photoresist after lithography.

Safety

Hydroxylamine may explode on heating. It is an irritant to the respiratory tract, skin, eyes, and other mucous membranes. It may be absorbed through the skin, is harmful if swallowed, and is a possible mutagen.^[5]

References