Page 214 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN009F-398 July 6, 2001 20:34

Main Group Elements 23

derivatized, however, the chain polymerization reaction This series of reactions, one of the most useful and
no longer occurs, in general. It was determined, however, versatile for alkene synthesis, is usually called the Wittig
that the polymerization reaction occurs in two stages. The reaction to honor its discoverer and developer, the Ger-
first stage leaves a soluble, low molecular weight polymer man chemist Georg Wittig. In 1979, Wittig shared the
on which the same substitution reactions can be accom- Nobel Prize in chemistry with H. C. Brown, whose boron
plished. Further heat treatment gives fully substituted high reagents also made possible the facile syntheses of numer-
polymers, many of which have highly desirable properties. ous organic compounds. Wittig reagents are practical and
Several are in current small-scale commercial production. economical, and they have even been adopted by chemical
Synthesis problems remain, however, in providing the and drug firms for the large-scale commercial syntheses
complete range of potentially useful polymer substituents of special pharmaceuticals and bioorganic molecules.
and functional groups. One approach to this lies in the de-
velopment of alternative routes to the polymers. A recent
report describes a condensation–polymerization route in IX. ARSENIC, ANTIMONY, AND BISMUTH
which alkyl substituents and, in some circumstances,
Arsenic, antimony, and bismuth are the last three mem-
mixed alkyl substituents can be introduced.
bers of Group 15 (VA). The first two members of the fam-
The high polymers are characterized by a highly flexible
ily, nitrogen and phosphorus, are decidedly nonmetallic;
backbone, which permits facile conformational changes
however, arsenic is a metalloid, antimony is partially a
and leads in most cases to materials that retain their elas-
metalloid, and bismuth is definitely a metal. Arsenic and
tomeric character to very low temperatures. The freedom
antimony, like phosphorus, exist in allotropic modifica-
to introduce a wide variety of substituent groups onto the
tions. Arsenic, antimony, and bismuth occur in nature in
polymer chain makes possible a wide array of properties.
both the free state and combined states, usually as sul-
For example, when the side chain is an amino function
fides. The principal ores of arsenic are realgar (As 2 S 2 ),
(e.g., NHMe), the high polymer is water soluble and re-
orpiment (As 2 S 3 ), and arsenopyrite (FeAsS); the princi-
tains significant activity as a ligand for transition metals. In
pal ore of antimony is stibnite (Sb 2 S 3 ); and the major ore
contrast, when the substituent groups are OCH 2 CF 3 , the
of bismuth is bismuthinite (Bi 2 S 3 ). None of the elements
polyphosphazene is so water repellant that it does not in-
is abundant in crustal rock; the parts-per-million value of
teract with living tissue, making it a good candidate for use
arsenic is 1.8, that of antimony is 0.20, and that of bis-
in artificial organ devices. This ease of functionalization
muth is only 0.008. In terms of abundance, the rankings
also shows promise in providing polymer-attached thera-
among all elements are fifty-first for arsenic, sixty-second
peutic agents, catalysts, and polymeric solid electrolytes.
for antimony, and sixty-ninth for bismuth.
The pyrometallurgy of all three elements is similar; the
I. Phosphorus Ylides
sulfide is first roasted and then reduced with carbon (E =
Phosphorus ylides are dipolar species in which positive As, Sb, Bi):
and negative charges are located on adjacent atoms. A
2E 2 S 3 + 9O 2 → 2E 2 O 3 + 6SO 2
typical example is the following:
E 2 O 3 + 3C → 2E + 3CO
( ) ( )
Ar 3 P C R Ar Phenyl group A considerable amount of bismuth is recovered as a by-
R H or Alkyl group
R product of the processes involved in refining the metals
lead, zinc, and copper.
These strange-looking species are not merely interest-
Arsenic is a gray crystalline solid that does not melt
ing laboratory curiosities; on the contrary, they are ex-
at atmospheric pressure but simply volatilizes to give a
tremely valuable laboratory workhorses that facilitate the
dense, malodorous yellow vapor. Its main use as an ele-
conversion of an aldehyde or ketone to an alkene via in-
mentistohardenlead–antimonyalloys,forexample,those
termediates known as betaines and oxaphosphatenes:
in storage batteries or lead shot. Arsenic compounds are
( ) ( ) highly toxic; hence, many have been used as potent her-
O O PAr 3 O PAr 3
( ) bicides and insecticides.
R′ C C R R′ C CR 2 R C CR 2 Antimony, more metallic than arsenic, is an opaque,
H Ar 3 P R
H H lustrous, tin–white, brittle solid. Both the element and its
A betaine An oxaphosphatene
compounds are poisonous. Like arsenic, antimony is used
O R′ R
to improve the properties of lead alloys.
Ar 3 P C C
Triphenyl H R As expected, bismuth is more metallic and more lus-
phosphine oxide
Alkene trous than either arsenic or antimony. A brittle, white metal