chemistry

Nitroxyl

Nitroxyl is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Nitroxyl rather than just read about it. In short: Nitroxyl (common name) or azanone (IUPAC name) is the chemical compound HNO. It is well known in the gas phase.

Key takeaways

Nitroxyl belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Nitroxyl to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Nitroxyl from memory before moving on to harder problems.

Reference excerpt

Nitroxyl (common name) or azanone (IUPAC name) is the chemical compound HNO. It is well known in the gas phase. Nitroxyl can be formed as a short-lived intermediate in solution. Its conjugate base, NO−, the nitroxide anion, is the reduced form of nitric oxide (NO) and is isoelectronic with dioxygen. The bond dissociation energy of H−NO is 49.5 kcal/mol (207 kJ/mol), which is unusually weak for a bond to the hydrogen atom.

Generation Nitroxyl is produced from the reagents Angeli's salt (Na2N2O3) and Piloty's acid (PhSO2NHOH). Other notable studies on the production of HNO exploit cycloadducts of acyl nitroso species, which are known to decompose via hydrolysis to HNO and acyl acid. Upon photolysis these compounds release the acyl nitroso species which then further decompose. HNO is generated via organic oxidation of cyclohexanone oxime with lead tetraacetate to form 1-nitrosocyclohexyl acetate:

This compound can be hydrolyzed under basic conditions in a phosphate buffer to HNO, acetic acid, and cyclohexanone. Dichloramine reacts with the hydroxide ion, which is always present in water, to yield nitroxyl and the chloride ion. Alkali metals react with nitric oxide to give salts of the form MNO (M = metal). However, generation of the (unstable) free acid from these salts is not entirely straightforward (see below).

Reactions Nitroxyl is a weak acid, with pKa of about 11, the conjugate base being the triplet state of NO−, sometimes called nitroxide. Nitroxyl itself, however, is a singlet ground state. Thus, deprotonation of nitroxyl uniquely involves the forbidden spin crossing from the singlet state starting material to triplet state product:

1HNO + B− → 3NO− + BH Due to the spin-forbidden nature of deprotonation, proton abstraction is many orders of magnitude slower (k = 4.9×104 M−1 s−1 for deprotonation by OH−) than what one would expect for a heteroatom proton-transfer process (processes that are so fast that they are sometimes diffusion-controlled). The Ka of starting from or ending with the electronic excited states has also been determined. When process of deprotonating singlet state HNO to obtain singlet state NO− has a pKa is about 23. On the other hand, when deprotonating triplet HNO to obtain triplet NO−, the pKa is about −1.8. Nitroxyl rapidly decomposes by a bimolecular pathway to nitrous oxide (k at 298 K = 8×106 M s):

2 HNO → N2O + H2O The reaction proceeds via dimerization to hyponitrous acid, H2N2O2, which subsequently undergoes dehydration. Therefore, HNO is generally prepared in situ as described above. Nitroxyl is very reactive towards nucleophiles, including thiols. The initial adduct rearranges to a sulfinamide:

HNO + RSH → RS(O)NH2

Detection In biological samples, nitroxyl can be detected using fluorescent sensors, many of which are based on the reduction of copper(II) to copper(I) with concomitant increase in fluorescence.

Medicinal chemistry Nitroxyl donors, known as nitroso compounds, show potential in the treatment of heart failure and ongoing research is focused on finding new molecules for this task.

See also Nitroxyl radicals (also called aminoxyl radicals) — chemical species containing the R2N−O• functional group

References

Illustrations

Worked examples

Example 1 — a first encounter with Nitroxyl

Start with the simplest possible case. Write down what Nitroxyl claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Nitroxyl before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Nitroxyl ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Nitroxyl

In research: Nitroxyl appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Nitroxyl in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Nitroxyl is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aldehyde dehydrogenase inhibitors, Hydrogen compounds, Nitrogen(I) compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Nitroxyl outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Nitroxyl in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Nitroxyl means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Nitroxyl out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Nitroxyl in simple terms?

Nitroxyl (common name) or azanone (IUPAC name) is the chemical compound HNO. It is well known in the gas phase.

Why does Nitroxyl matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Nitroxyl?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Nitroxyl.