Page 293 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

7.7 HEPTAVALENT CHLORINE 273

REVIEW PROBLEM 7.14
Cl O is hydrolyzed by aqueous alkali to chlorate and perchlorate. Draw a
2 6
mechanism.

We won’t discuss standard preparative routes for perchloric acid or perchlorates (pri-
marily for reasons of space), except for the following. Thermal decomposition of potassium
chlorate (KClO ) results in potassium perchlorate (KClO ):
4
3
4KClO → 3KClO + KCl (7.57)
3 4
Or, in ionic form:
− −
4ClO 3 → 3ClO 4 + KCl (7.58)

As in another example (disproportionation of hypochlorite) earlier in the chapter (Section
7.2), the mechanism of this reaction is somewhat hard to come to terms with because multi-
−
ple negatively charged ClO 3 ions must come together to generate the products. Clearly, the
+
K counterions play a major role in facilitating the interaction, as does the high temperature
under which the reaction occurs.
By analogy with the earlier example, the mechanism may then be envisioned as simply
a series of oxygen atom transfers:
−
− − Cl − O
O O 2+ O −
O − Cl
3+ Cl O + − −
2+ Cl O + O
O
O − O −
− O
−
−
− − Cl − O −
O O + O 3+ Cl O − + Cl O
2+ Cl (7.59)
O O
O − −
− O
−
− − −
O O Cl O
− −
2+ Cl 3+ Cl O + Cl
O
O − O
− O −
−
An awkward aspect of each of the above steps is that the nucleophile is a chlorine with a
positive formal charge. Once again, when dealing with higher-valent compounds, this need
not be particularly upsetting. Formal charges are simply an electron-bookkeeping device
and do not provide a realistic indication of the electrostatic character of atoms.