According to IUPAC Red book page 140, hyponitrous acid, correctly said as hypodintrous acid, is $\ce{H2N2O2}$ while the compound $\ce{HNO}$, azanone also exists. So why is the dimer considered the hypo of -ous oxyacid of nitrogen over $\ce{HNO}$?
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2$\begingroup$ Useful links for text and formula formatting: Notation basics / Formatting of math/chem expressions / upright vs italic // For more, see Math SE MathJax tutorial. // Keep CH SE Q titles in plain text. $\endgroup$– PoutnikCommented Aug 20, 2022 at 17:40
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1$\begingroup$ @Poutnik It is the actual dimer. $\endgroup$– MithoronCommented Aug 20, 2022 at 18:34
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2$\begingroup$ In fact it HNO dimerises very fast - it's an unstable intermediate, so hyponitrous acid is the product of reaction creating HNO. ncbi.nlm.nih.gov/pmc/articles/PMC124232 $\endgroup$– MithoronCommented Aug 20, 2022 at 18:43
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3$\begingroup$ Well, nitroxyl/azanone is a highly reactive species that spontaneously dimerizes to hyponitrous acid, which dehydrates to give nitrous oxide and water. $$\ce{HNO + HNO -> HON=NOH -> N2O + H2O}$$ with reaction rate constant of $\ce{{\it{k}} = 8 \times 10^6 M^{-1} s^{-1}}$. Hence, HNO cannot be stored or detected for long time. $\endgroup$– Nilay GhoshCommented Aug 21, 2022 at 5:12
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3 Answers
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Let us assume you are discovering hyponitrous acid for the first time. You analyzed the elements and it is found to contain nitrogen, hydrogen and oxygen in a ratio of 1:1:1, and hence the empirical formula should be $\ce{HNO}$, as you wanted. The formula weight of this compound would be 1+14+16=31 (as a rough estimate).
The story of chemical formulae do not end here. People wanted to determine the molecular weight of this new compound. Chemists as early 19th century knew how to determine the molecular weight by relatively simple but elegant experiments. Only high school algebra is required.
Molecular weight measurements would tell the chemists that is 2x31 i.e., 62 not 31. This is just an example to clarify that elemental analysis and its bare formula is not enough to develop the correct formula. Molecular weight measurements are necessary.
Historically, the acid was not analyzed but its silver salts and the early discoverers assigned a formula of the salts as AgNO, implying HNO for hyponitrous acid (Ueber die untersalpetrige Säure, DOI: 10.1002/cber.18770100257).
In short, experimental measurements of the molecular weight decide that it is $\ce{H2N2O2}$ rather than $\ce{HNO}$.
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Technically, per Wikipedia, hyponitrous acid ($\ce{H2N2O2}$) is an isomer of nitramide ($\ce{H2N−NO2}$) and a formal dimer of azanone ($\ce{HNO}$).
If your question is why it it is called hyponitrous acid, my suspected explanation is that the naming convention is following the early tradition established with, for example, the corresponding family of chlorine-based acids, where in moving down from chlorous acid ($\ce{HClO2}$) to hypochlorous acid ($\ce{HClO}$), there is a loss of one atom of oxygen.
So here, think of hyponitrous acid ($\ce{H2N2O2}$) as likely very early posted in the chemical literature as $\ce{HNO}$, which compared to nitrous acid $\ce{HNO2}$, is correspondingly minus one atom of oxygen from the nitrous acid.
My supposition on the $\ce{HNO}$ initial assignment is supported by the following comments from Atomistry, which is a compilation of historical chemical journal commentaries, to quote:
It was shown by Divers in 1871, that the reduction of sodium nitrate with sodium amalgam gave a salt having the empirical composition NaNO, the acid being an isomer of nitramide... A bibliography of hyponitrous acid has been compiled by Divers, to whose researches so much of our knowledge of this interesting compound is due.
I hope this helps, and reading old chemistry texts, at times, may provide some insights.
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I have reused the relevant portion of my answer in the related What is formal dimer?.
Comparing $\ce{HClO}$ as hypochlorous acid and $\ce{HNO}$ as eventual hyponitrous acid is misleading, as they have different structure.
$\ce{H-O-N=N-O-H}$ could be a true dimer of azanone $\ce{H-N=O}$ if the latter had structure $\ce{H-O-N}$, with the equilibrium reaction $\ce{2 H-O-N <=> HON=NOH}$.
As nitrous acid is $\ce{H-O-N=O}$, hypothetical monomeric hyponitrous acid is supposed to be $\ce{H-O-N}$ like $\ce{H-O-Cl}$, and not $\ce{H-N=O}$.
Interesting fact is that dissociation of $\ce{HNO}$ is spin-forbidden quantum event and the rate of the dissociation is very slow.
