Page 233 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

6.5 CYCLOOCTACHALCOGEN RING FORMATION 213

REVIEW PROBLEM 6.3
Elemental sulfur may be oxidized to a variety of polysulfur cations. Thus, oxidation
of S in liquid SO with AsF or SbF leads to S 2+ among other species:
8 2 5 5 8
S + 3AsF → [S ][AsF ] + AsF
8 5 8 6 2 3
The bonding in S 2+ is probably fairly subtle but one contributing resonance form is
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expected to be the following:

S
S S
+ S S +

S S
S

The process might very well involve single-electron transfers from S and various
8
radical intermediates. Here, however, we ask you to write out a polar mechanism for
the reaction.

6.5 CYCLOOCTACHALCOGEN RING FORMATION

We’ll now consider the reverse of what we discussed above, that is, how a cyclooctachalco-
gen ring is assembled, a very common process in chalcogen chemistry. Consider, as an
example, the following reaction:

4Ph PSe + 4 SeCl → Se + 4Ph PCl 2 (6.41)
3
8
3
2
An examination of the products indicates that a large number of Se–Se bonds must
form, strongly suggesting that Se-on-Se nucleophilic attacks must be taking place. With
that insight, the first couple of steps are not difficult to surmise:

− Se Se Se
− Se CI CI Se CI CI
Se + CI Ph
CI P P (6.42)
+ Ph Ph Ph
P Ph
Ph Ph
Ph
Ph
Observe that we have formed a Se–Se linkage. A second nucleophilic attack by Ph PSe
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on the product above now leads to a Se chain, as shown below:
3