Page 214 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

THE HEAVIER PNICTOGENS
194
2−
F
F IV F −
Mn F F F
F F F IV F − F (5B.86)
F F Sb F Mn F + Sb
F F F F
F F F
–
The Mn(IV) state then seems unable to survive in the absence of six terminal F ligands,
which accounts for the observed reduction to Mn(III):

− −
F F
F IV F III
Mn F Mn + F
F F (5B.87)
F F
F F F F

The above synthesis of molecular fluorine is not useful in a practical sense. Much more
important is the application of SbF in the preparation of superacids, which are defined as
5
acids that are stronger (i.e., have greater protonating power) than 100% sulfuric acid. The
acidity of such strong acids is typically quantified by the Hammett acidity function, which
we encourage you to read up on at your convenience.
Anyhydrous FSO H (fluorosulfonic acid) is a superacid, but addition of SbF increases
3 5
its acidity (i.e., protonating power) dramatically. George Olah (Nobel Prize in chemistry,
1994) coined the name “magic acid” for 1:1 FSO H/SbF in view of its ability to protonate
3 5
alkanes. A dramatic demonstration showed a candle, consisting essentially of long-chain
∘
hydrocarbons, dissolving away, when dipped in magic acid. At 140 C, magic acid even
protonates methane, the most unreactive of alkanes, leading ultimately to the relatively
stable t-butyl cation:
+
CH + H → CH 5 +
4
+ +
CH 5 + CH → C H 5 + 2H 2
2
4
+ +
C H 5 + 2CH → (CH ) C + 2H 2 (5B.88)
3 3
4
2
At the cost of a short digression from group 15 chemistry, a brief comment may be
warranted on these exotic reactions. The remarkable species CH 5 + is best thought of as a
complex of CH + and H , with a three-center two-electron bond, reminiscent of B–H–B
3 2
bonding in diborane.
+
H H
C
H
H H