Pyro acids are prepared by removing one water molecule from two molecules of oxyacid. $\ce{H3BO3}$ has only one acidic hydrogen so two of that molecule will have two acidic hydrogens. If we remove that, then there won't be any acidic hydrogen and hence it shouldn't be able to form pyroboric acid or at least it should not be called an acid as it doesn't have $\ce{H+}$ ion to donate. This is the case with hypophosphorus acid, but why it isn't with boric acid?
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$\begingroup$ $\ce{B(OH)3 + H2O <=> B(OH)4- + H+}$ // $\ce{HBO2}$ is a cyclical trimer with alternating O and BOH. $\endgroup$– PoutnikCommented Jan 13, 2022 at 6:06
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$\begingroup$ It is mono basic Lewis acid. But not an Arrhenius acid. So it doesn't have any acidic hydrogen. $\endgroup$– InfiniteCommented Jan 13, 2022 at 9:46
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$\begingroup$ @Infinite That isn't true. Each one of them can be abstracted, it's just like with any other protons. $\endgroup$– MithoronCommented Jan 13, 2022 at 14:10
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$\begingroup$ @Mithoron I thought that boric acid is acidic because it abstracts HO- from water rather than because is adds protons to water which makes that possibility irrelevant in aqueous solution. $\endgroup$– matt_blackCommented Jan 14, 2022 at 15:46
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$\begingroup$ @matt_black It does both, to a similar extent. $\endgroup$– MithoronCommented Jan 14, 2022 at 16:08
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2 Answers
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From my answer:
The IUPAC goldbook states that:
"pyro" is used as a prefix designating compounds formed by heating a compound, usually with the elimination of water, carbon dioxide, or other simple molecule, e.g. pyroglutamic acid from glutamic acid.
Since Pyroboric acid is made by heating boric acid, it is prefixed as pyro. You can also call it tetraboric acid.
$$\ce{\underset{orthoboric acid}{4H3BO3} ->[373 K] \underset{metaboric acid}{4HBO2} ->[435 K] \underset{pyroboric acid}{H2B4O7} ->[red hot] B2O3 }$$
answered Jan 13, 2022 at 6:01
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Hydrogen atoms need not bevunalterqbly acidic or non-acid for all time. Chemical reactions elsewhere is a molecule can change a hydrogen atoms acid-base reactivity
Suppose you were to react acetoacetic acid, $\ce{CH3-CO-\color{blue}{CH2}-CO-OH}$, with a strong base such as a Grignard reagent. The base extracts the catboxyl proton, and that's it. In the resulting anion no more hydrogen atoms, including those in the blue methylene group, have significant acidic properties.
Now suppose you esterofy the acid with ethanol, making what is commonly called acetoacetic ester, $\ce{CH3-CO-\color{blue}{CH2}-CO-OC2H5}$, and then apply the strong base. The base now deprotonates one of those hydrogen atoms in that methylene group. Removing the original acidic hydrogen from a different functional group has converted the methylene hydrogen from non-acidic to acidic.
Similarly, when boric acid polymerizes, it is essentially esterifying itself; but it's proton or Lewis acidity is not necessarily destroyed. It could be transferred to alternate sites.
answered Jan 13, 2022 at 16:00