Page 219 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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28 Main Group Elements

Sulfur dioxide is a bent molecule in the gaseous state, fites on fresh produce, but sulfites are also added to alco-
whereas SO 3 , gas molecules are planar triangular. In the holic beverages, frozen potatoes, fresh seafood, and even
liquid and solid state, SO 3 trimerizes to form S 3 O 9 : prescription medicines. The sulfites commonly employed
are sodium sulfite (Na 2 SO 3 ), sodium bisulfite (NaHSO 3 ),
O O
O
S S potassium bisulfite (KHSO 3 ), and SO 2 .
O O O
3SO 3
O S
O F. Sulfur–Nitrogen Compounds
O
The study of binary compounds of sulfur and nitrogen
In the presence of a trace of H 2 O, SO 3 will polymerize to
has been spurred both by the problems they pose to sim-
form needles of crystalline β-SO 3 [actually HO(SO 3 ) x H,
ple bonding theory and by the observation that polysulfur
where x is very large].
nitride is metallic in its properties.
Although oxygen and sulfur are in the same periodic
E. Oxoacids and Oxoanions family, there is virtually no parallel between nitrogen ox-
ides and nitrogen sulfides. The relatively poor capacity for
Of the approximately 10 oxoacids, only 2 are considered:
π bonding among second long-row elements is reflected
H 2 SO 4 and H 2 SO 3 . Sulfur trioxide is the acid anhydride
by the lack of evidence for the existence of NS and NS 2 ,
of sulfuric acid, H 2 SO 4 , the most important commercially
the sulfur analogues of NO and NO 2 , the two common
prepared compound. Its total synthesis from sulfur is as
oxides of nitrogen.
follows:
Theeasiestofthesulfurnitridestoprepare,andthestart-
S 8 + 8O 2 (g) → 8SO 2 (g) ing point for most of the others, is tetrasulfur tetranitride,
S 4 N 4 . The compound can be prepared from liquid am-
V 2 O 5
2SO 2 (g) + O 2 (g) −−−→ 2SO 3 (g) monia and elemental sulfur or by warming a mixture of
catalyst ammonia and S 2 Cl 2 or SCl 2 . It is kinetically inert in air
but may decompose explosively to the elements on being
SO 3 (g) + H 2 SO 4 (l) → H 2 S 2 O 7 (l)
struck. This instability is due to the very strong bond in N 2
rather than to the inherent weakness of the S N bonds.
H 2 S 2 O 7 (l) + H 2 O → 2H 2 SO 4
The compound assumes a boatlike ring form in which
Although sulfuric acid results from the reaction of there may be some transannular S S bonding, making the
SO 3(g) with H 2 O, a for or mist of tiny H 2 SO 4 droplets structure more like a cluster. No single satisfactory bond-
forms in the reaction chamber. The formation of this mist ing structure can be written for the compound, but rather
can be avoided if the SO 3 is dissolved in pure H 2 SO 4 to an extensive series of resonance structures must be con-
form pyrosulfuric acid, H 2 S 2 O 4 , which is then allowed to sidered. As in other such cases, the bonding is best treated
react with water to yield H 2 SO 4 . by molecular orbital theory.
In terms of mass, H 2 SO 4 ranks first among the chem- Other nitrides include S 2 N 2 ,S 3 N 2 ,S 11 N 2 ,(S 7 N) 2 S x ,
icals produced annually in the United States. In 1989, and S 5 N 6 . Each of these is a colored compound and,
∼40 billion kg was manufactured; this amounts to nearly with the possible exception of (S 7 N) 2 S x ,defies simple
160 kg for each person in the nation. The main uses of two center–two electron bonding descriptions. Notewor-
H 2 SO 4 are the manufacture of phosphate and ammonium thy is that the first three are ring structures whose essen-
fertilizers, the synthesis of other chemicals, petroleum re- tial planarity about the nitrogen atoms suggests some de-
fining, and metallurgical applications. gree of π bonding to the sulfur atoms (S 11 N 2 is bicyclic).
Sulfur dioxide is the acid anhydride of sulfurous acid, S 4 N 2 is formally analogous to the important oxide, N 2 O 4 ,
H 2 SO 3 , a weak acid that does not exist in the free state. but has no comparable acyclic structure. In fact, only the
The salts of the acid, the sulfites, are stable and are com- 1,3-diazahexasulfane ring isomer is known. (S 7 N) 2 S x has
mercially important in food processing, the paper industry, two eight-membered rings joined by an S x (x = 1–5) chain
and photography. In 1985, the Food and Drug Administra- at the two nitrogens. This structure allows a simple Lewis
tion restricted the use of sulfites on fresh produce. Sulfites bonding structure in which a trivalent nitrogen replaces
are used as “freshening agents” to preserve color and re- one sulfur atom in the S 8 ring. S 5 N 6 assumes a cage (or
tard wilting in fruits and vegetables; however, asthmatics quadricyclic) structure. Each of these structures retains
and other sulfite-sensitive persons may experience violent some. Lewis base character.
reactions. Victims often require hospitalization, and some In addition to the neutral binary compounds, a number
deaths have even been reported. The rapidly increasing of binary cations and anions have been isolated and
number of salad bars necessitated the ban on using sul- structurally characterized. These have contributed to the