Page 273 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

THE HALOGENS 253
Stated differently, the larger the anion, the stronger it is as a reducing agent. Thus,
iodide is quite a strong reducing agent, while fluoride is nonreducing. By the same
token, molecular halogens are oxidants, with oxidizing power decreasing down the
group:
F > Cl > Br > I
2 2 2 2
−
Feel free to have a quick look at Table 1.5 for a sense of the relative X /X reduction
2
potentials. The data will clearly show F to be one of the strongest oxidants. Chlorine,
2
much more moderate by comparison, arguably offers “more bang for the buck” in the
sense that it’s cheap, much easier to handle, and remarkably versatile.
• Halides are nucleophilic. The Swain–Scott nucleophilicity increases down the group
(see, e.g., Table 1.1):
− − − −
I > Br > Cl > F
As emphasized in Section 1.2, however, the relative nucleophilicities of the halides is
a somewhat tricky issue, and in polar, aprotic solvents the order of nucleophilicities
is actually the reverse of the above.
• The higher-valent states of chlorine, bromine, and iodine are powerful electrophiles
and/or oxidants. Again, have a look at Table 1.5 and note the very high reduction
potentials of hypochlorous acid (HOCl), chlorous acid (HClO ), and chlorine dioxide
2
(ClO ). A number of higher-valent halogen compounds interact with organic com-
2
pounds, forming high-valent species that subsequently fall apart via reductive elim-
ination. Thus, higher-valent organoiodine compounds have long been an important
class of reagents in organic chemistry. Recently, even trivalent bromine compounds
are finding unique applications in organic synthesis, a topic we will touch on toward
the end of this chapter.

REVIEW PROBLEM 7.1

Bromine water (Br /H O) added to an aqueous solution of sodium iodide liberates
2
2
molecular iodine. Suggest a mechanistic explanation.

REVIEW PROBLEM 7.2

If you examine Table 1.1, you are likely to arrive at the conclusion that fluoride is not a
particularly good nucleophile. Recently, Stephen DiMagno and coworkers at the Uni-
versity of Nebraska synthesized anhydrous tetrabutylammonium fluoride (TBAF anh )
via the following reaction, and found it to be a highly active form of nucleophilic
fluoride in dry DMSO and other polar aprotic solvents (Sun, H.; Dimagno, S. J. Am.
Chem. Soc. 2005, 127, 2050–2051):