Page 274 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

THE HALOGENS
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F CN

F F NC CN
Polar aprotic solvents
CN + 6 n-Bu F +
6 n-Bu 4 4
− 35 °C RT
F F NC CN
F CN

The following two reactions illustrate well the exceptional reactivity of TBAF anh :

TBAF anh
Br THF, RT, < 5 min F
40–50%
(Remainder: 1-butene)
NO 2 F

TBAF anh
RT, 2 min
F C CF 3 F C CF 3
3
3
> 95%
What might account for the remarkable reactivity of the TBAF anh reagent? What
might be a reason for the researchers’ choice of the highly toxic tetrabutylammonium
cyanide as a starting material for synthesizing TBAF ?
anh
As a postscript to this story, we may add that, as a nucleophilic fluorinating agent,
18
18
[ F]-TBAF has been found to be significantly superior to cryptand-activated K F,
anh
which is the standard 18 F source in positron-emission tomography (PET).

7.1 SOME NOTES ON ELEMENTAL HALOGENS

As reactive electronegative elements, the halogens do not occur in their elemental form
in Nature. They occur most commonly as halide anions, and the molecular elements are
synthesized—and indeed were discovered—via the oxidation of the corresponding halide
ions. As mentioned above, the ease of oxidation of the halide anions increases down the
group, and the oxidizing power of the molecular elements decreases down the group. Thus,
as an oxidant, fluorine is the strongest, followed by chlorine, and so on. The story of the
discovery of the halogens provides for excellent illustrations of these trends. Here we’ll
briefly recount the discovery of bromine by the French chemist Antonie Jérôme Balard
(1802–1876); for the other elements, we’ll refer you to Further reading.
Born of poor parents in Montpellier, France, Balard was adopted and educated by his
godmother. At age 17, he became a laboratory assistant (“préparateur”) at the local Ecole