Page 188 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

THE HEAVIER PNICTOGENS
168
A radical mechanism, on the other hand, is also conceivable. In this case, iodide would
begin by transferring a single electron to arsenic acid, producing a tetravalent As interme-
diate, which could fall apart in a number of ways, one of which is shown below:

HO OH −
− + O As −
O
I As OH As I OH + HO OH (5B.24)
OH
HO
O − OH
I
The HOI (hypoiodous acid) thus produced could then be scavenged by a second iodide
ion to produce molecular iodine; this is a well-known reaction for all the halogens, except
fluorine:
OH −
− I I I + OH (5B.25)
I

This chemistry highlights at least two key differences between phosphorus and arsenic.
Compared with arsenic acid, phosphoric acid and phosphates are far more resistant to reduc-
tion. Second, note once again the tricoordinate structure of arsenous acid (H AsO ) and
3
3
recall that phosphorous acid (H PO ) has a different tetracoordinate structure.
3
3
5B.5 ARSENIC TOXICITY AND BIOMETHYLATION

Arsenic is inextricably linked to poisoning and murder. Kings and emperors, famous and
not-so-famous people, have succumbed to arsenic over the ages. In Europe, arsenic as a
murder weapon became popular in the Middle Ages and the Renaissance, especially among
the ruling classes in Italy. By the nineteenth century, it was recognized as an “element of
murder” pretty much everywhere. The so-called arsenic trioxide, As O , colorless, taste-
4 6
less, and readily available, was perfectly suited as a poison administered with food. Along
with a few other substances used for the same purpose, it acquired the nickname inheri-
tance powder in nineteenth-century France and elsewhere. Arsenic poisoning continues to
be widespread today, not so much as a result of human malice but because of groundwater
contamination in many parts of the world.
Arsenic sabotages a great many physiological processes, and even a cursory description
of them is beyond the scope of this chapter. One of arsenate’s most serious biochemical
effects is its inhibition of pyruvate dehydrogenase, the enzyme that decarboxylates pyru-
vate (CH COCOOH), the end product of glycolysis, to acetyl CoA, the substrate for the
3
ATP-forming citric acid cycle. This and other disruptions ultimately bring about death from
multisystem organ failure. Here we will focus on the chemistry of a particular aspect of
arsenic metabolism, namely biomethylation, which is best known as a detoxification pro-
cess in bacteria.

REVIEW PROBLEM 5B.8
Organoarsenic compounds have been used as chemical warfare agents. Chief among
was Lewisite, which was used as a vesicant (blister agent) and lung irritant. The main