Page 223 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

GROUP 16 ELEMENTS: THE CHALCOGENS 203
Interestingly, as a so-called Döbereiner triad (i.e., a triad of elements, whose chemical
similarities were recognized by German chemist Johann Wolfgang Döbereiner in the nine-
teenth century), sulfur, selenium and tellurium played a role in the initial construction of the
periodic table.
Many group 16 compounds are malodorous, hydrogen sulfide (H S) being probably
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the most notorious example. Flatulence, bad breath, and skunk spray all contain hydrogen
sulfide (H S), as well as other volatiles. Hydrogen sulfide is also toxic, so it’s important
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to exercise caution so as to avoid exposure when working with H S in the laboratory.
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Interestingly, against this unattractive backdrop, H S has recently been recognized as a
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gasotransmitter, that is, a gaseous signaling substance in biology, comparable to NO and
CO. Unfortunately, we will not be discussing this important biological role of H Sinthis
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book, partly because the story is still unfolding and key details are still missing.
Dimethyl sulfide (DMS) has a cabbage-like smell, which becomes offensive at higher
concentrations. It is, however, an important ingredient of the flavors of several food
items such as asparagus, beetroot, and truffles. Along with algal pheromones called
dictyoterpenes, DMS is a key part of what people call “the smell of the sea.” We’ll see that
DMS is the end product of various dimethysulfoxide-mediated oxidations (e.g., the Swern
oxidation of alcohols); washing the glassware with bleach destroys much of the DMS
produced.
The group 16 elements or chalcogens (which we will occasionally abbreviate as Ch)
exhibit considerable chemical diversity, with the greatest discontinuity in properties
between oxygen and sulfur. Sulfur and selenium are chemically rather similar. Tellurium
is considered a metalloid and polonium a full-fledged metal. Some general group trends
are as follows.

• Oxygen is perhaps best known for its high electronegativity (3.44 on the Pauling
scale), which is second only to fluorine (3.98). The electronegativities of sulfur and
selenium are much lower—2.58 and 2.55, respectively—essentially the same as that
of carbon (2.55). The difference in electronegativity translates to many differences
between oxygen and the heavier chalcogens. Thus, O–O bonds are high energy enti-
ties, much more fragile than S–S bonds with respect to both homolytic and heterolytic
dissociation.
• Elemental oxygen commonly occurs in a couple of different forms that are unique
for group 16. The stable form of the element is triplet O , which has two unpaired
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electrons and is paramagnetic. Ozone, O , is diamagnetic, a metastable but highly
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endothermic substance:
O → 3∕2O ΔH =−142 kJ∕mol (6.1)
3 2

The closed-shell, double-bonded form singlet O (O=O) is an excited state, about
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94.3 kJ/mol higher in energy than triplet O . In many oxygen-evolving reactions, the
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O is initially produced in the singlet state, and it subsequently decays to the stable
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triplet state. Singlet oxygen is highly reactive, aggressively attacking most organic
matter.
• As a divalent “first-row” (period 2) element, oxygen forms double bonds with many
elements and does so with greater ease compared to the heavier chalcogens.