Page 296 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

THE HALOGENS
276
In addition, mention should also be made of the following ligand exchange reaction of IF :
7

IF + POF → IOF + PF (7.70)
7 3 5 5
We already discussed possible mechanisms of this reaction in Section 1.19. We pointed
out that such reactions typically involve bridged intermediates. Thus, for this reaction, an
oxo-bridged intermediate was considered plausible.
The bromine fluorides BrF and BrF are important compounds. Bromine trifluoride
3
5
and trivalent organobromine compounds are rapidly gaining acceptance as exciting new
reagents in organic chemistry; we will have much more to say about them in Sections 7.14
and 7.15. BrF is of interest as an oxidizer in liquid rocket propellants and as a fluorinating
5
agent for the processing of uranium. Both compounds can be prepared by direct reaction of
the elements under different conditions.
The uncharged heptavalent fluorides ClF and BrF do not exist, presumably for steric
7 7
+
+
reasons. The unique heptavalent cations [ClF ] and [BrF ] , however, may be prepared
6 6
from the corresponding pentafluorides, XF (X = Cl, Br), with such strong oxidants as PtF 6
5
+
or KrF :
+ − + −
2ClF + PtF → ClF PtF 6 + ClF PtF 6 (7.71)
6
5
4
6
+ − + −
XF + KrF SbF → XF SbF + Kr (X = Cl, Br) (7.72)
5 6 6 6
REVIEW PROBLEM 7.17
Use arrow pushing to rationalize reactions 7.64 through 7.69.

REVIEW PROBLEM 7.18

BrF reacts violently with water to yield bromic acid and HF:
5
BrF + 3H O → HBrO + 5HF
5 2 3
Write out a detailed mechanism.

7.9* HALOGENS IN ORGANIC SYNTHESIS: SOME CLASSICAL
REACTIONS

We are now in a good position to appreciate a variety of halogen-based reagents that are
part of the modern organic chemist’s repertoire. Before we do so, however, it may be useful
to briefly recapitulate some of the classical applications of halogens in organic chemistry.
As briefly discussed in Chapter 1 (Section 1.11), molecular bromine adds to
carbon–carbon double bonds by way of an ionic mechanism. The first step of the addition
results in a cyclic bromonium ion, which is then opened up by a bromide ion. Such a