Imperial College London

Henry Rzepa

Chemists love a mystery as much as anyone. And gaps in patterns can be
mysterious. Mendeleev's period table had famous gaps which led to new
discovery. And so from the 1890s onwards, chemists searched for the
perbromate anion, BrO~4~^--^. It represented a gap between perchlorate
and periodate, both of which had long been known. As the failure to turn
up perbromate persisted, the riddle deepened. Finally, in 1968, [the key
was found](http://dx.doi.org/10.1021/ja01009a040){target="_blank"}, but
talk about sledgehammer to crack a nut! It was done by alchemical-like
radioactive transmutation of selenium into bromine:

Se^83^O~4~^2-^ → Br^83^O~4~^--^ + β^--^

Once the psychological barrier had been surmounted, a chemical synthesis
provided enough perbromic acid to show it was a stable, high boiling
liquid. So, the failure to make it was not because it was unstable!

XeF~2~ + NaBrO~3~ → NaBrO~4~

Once quantities were available, the thermodynamic and redox properties
could be
[measured](http://dx.doi.org/10.1021/ic50083a025){target="_blank"}. This
did little to solve the riddle. Although it was found to be a better
oxidant than periodate, this was not considered enough to explain why it
had proved so elusive. The theoreticians got [in on the
act](http://dx.doi.org/10.1080/00268970050025493){target="_blank"}, but
their article too did little to resolve matters; the calculations merely
verified the experimental measurements.

To this day, little perbromate has been made, and so much of its
chemistry remains a mystery. Only in 2011 has a [synthesis
appeared](http://dx.doi.org/10.1021/ic201329q){target="_blank"} which
could potentially result in large and hence cheap quantities, by
formation through the carefully controlled reaction of hypobromite and
bromate ions in an alkaline sodium hypobromite solution.

Periodate has found much utility in organic synthesis as an oxidant, and
perchlorate is a very interesting non-coordinating counter ion in metal
catalysis. Who knows what use might transpire for perbromate!

So, unlike the gaps in the periodic table, plugging the perbromate gap
has not yet resulted in unexpected discoveries. But it is worth
speculating why any given compound may be non-existent. It may be
thermodynamically unstable, and hence have too short a lifetime to be
isolated (not the case for perbromate). Or all of the possible kinetic
pathways to its formation may have unfeasibly large barriers. The key
here is the word *all; *if one searches long enough, a route that works
will probably be found.

The other side of the coin is novel types of compounds that may well
exist, but no-one has anticipated trying to make them precisely because
they are so novel. I am thinking here of the wonderfully entitled
article "[Mindless
Chemistry](http://dx.doi.org/10.1021/jp057107z){target="_blank"}" where
systematic exploration of ALL possible minima for a given molecular
formula revealed a whole zoo of species which the speculative chemist
would never have dreamt of trying to make (in other words, they did not
manifest as obvious gaps in the patterns that constitute our present
chemical knowledge). I do often think about all of these undiscovered
molecules, and if they could indeed be synthesised and their properties
studied. One such [occurred in silicon
chemistry](http://dx.doi.org/10.1126/science.1181771){target="_blank"};
truly the existence of an isomer of hexasilabenzene was not predicted
before it was made, and its properties (aromaticity) did indeed prove
fascinating and new.

Too long the focus of synthetic chemistry has been to try to make
molecules that nature has already synthesized. Perhaps we should focus
as well on molecules that nature has never deigned to make, but which
are nevertheless entirely viable (as was the case for perbromate).

::: {.wp_orcid_field}
[https://orcid.org/0000-0002-8635-8390](https://orcid.org/0000-0002-8635-8390){target="_blank"
rel="author"}
:::


Chemists love a mystery as much as anyone. And gaps in patterns can be mysterious. Mendeleev’s period table had famous gaps which led to new discovery. And so from the 1890s onwards, chemists searched for the perbromate anion, BrO
<sub>
 4
</sub>
<sup>
 –
</sup>
. It represented a gap between perchlorate and periodate, both of which had long been known. As the failure to turn up perbromate persisted, the riddle deepened.

Postagens de Rogue Scholar

Perbromate. A riddle, wrapped in a mystery, inside an enigma; but perhaps there is a key.