Page 224 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

GROUP 16 ELEMENTS: THE CHALCOGENS
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• Catenation is an important property of the heavier chalcogens. A variety of chains,
rings, and cage structures are known for S, Se, and Te.
• One of the most distinctive features of the heavier chalcogens S to Te, relative to oxy-
gen, is their capacity for variable valence. Switching between di, tetra, and hexavalent
states is commonplace for S to Te.
• Metallic character increases down the group. This is reflected in the greater prevalence
of polymeric as opposed to discrete molecular structures and of anionic complexes
2− 2− 2−
such as SeBr 6 ,TeBr 6 , and PoI 6 .
− −
• The lower-valent states in the form of H Ch, HCh , and RCh are fairly strongly
2
nucleophilic.
• The tetravalent state is more variable in behavior—nucleophilic and/or reducing in
some compounds but electrophilic and/or oxidizing in others. Thus, SO acts as both
2
an oxidizing and a reducing agent, though most commonly as a reducing agent; sele-
nium dioxide, on the other hand, is best known as an oxidant.
• The hexavalent state, best known with O and F as terminal ligands, is often remarkably
stable and not particularly prone to reduction.
• Because of its intense radioactivity, polonium has been less studied than the other
chalcogens, and the chemical consequences of the inert-pair effect, primarily a
sixth-period phenomenon, remain largely unexplored for polonium.

Much of our discussion in this chapter will focus on sulfur, which we’ll treat as paradig-
matic of the chalcogen group. As for the remarks by Pauling quoted above, selenium and
tellurium compounds are indeed toxic. With good laboratory practices, however, they can
be safely handled. We will see that selenium reagents allow some unique transformations
in organic chemistry, and the same may be said, to a lesser extent, for tellurium.

6.1 THE DIVALENT STATE: FOCUS ON SULFUR

For group 16 elements, the divalent state is probably the most familiar; it’s the standard,
nonhypervalent state of the elements. Sulfur has a large number of allotropes, and most
of these are made up of rings or chains of divalent sulfur; several allotropes are made up
of puckered S rings in different packing arrangements. Familiar examples include water,
8
alcohols, and ethers; thiols (RSH) and thioethers (RSR’), including the essential amino
acids cysteine (which has a CH SH sidechain) and methionine (which has a CH SCH 3
2
2
sidechain); “catenated” species such as hydrogen peroxide and the sulfanes, H S ; and the
2 n
chlorides SCl and S Cl . The hydrides and the chlorides serve as building blocks of many
2
2
2
other divalent sulfur species. Consider, for example, the following rational synthesis of
cyclohexasulfur.
H S + S Cl → cyclo-S + 2HCl (6.2)
6
2
2 4
2
The SH sulfurs being the negative ends of dipoles are the expected nucleophiles. The
SCl sulfurs are good electrophiles, in large measure because of the weakness of the S–Cl
bonds (bond dissociation energy, BDE, ∼251 kJ/mol) and the resulting efficacy of chloride
as a leaving group: