Page 243 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

6.8 OZONE 223
until recently, have resulted in significant depletion of the ozone layer. Although otherwise
very stable, these compounds decompose photochemically in the atmosphere producing
chlorine and bromine atoms, the key culprits in ozone depletion. Thus, a chlorine atom
reacts with O to produce hypochlorite radical and O :
3 2
Cl + O → ClO + O 2 (6.72)
3
The hypochlorite radical reacts with a second molecule of O to produce two O molecules
3 2
and a second chlorine atom, which can then continue to destroy ozone:

ClO + O → Cl + 2O 2 (6.73)
3
Since the late 1980s, these organohalogens have been gradually phased out by inter-
national agreement and largely replaced by products such as R-410A, which is a
near-azeotropic (i.e., boiling with little change in composition) mixture of difluoromethane
and pentafluoroethane.
Given that ozone oxidizes many, if not most, common substances, space permits only
a tiny sampler of its chemical reactions. Thus, it detoxifies cyanide, producing the far less
dangerous cyanate anion.
−
−
CN + O → NCO + O (6.74)
3 2
In a simple picture, cyanide may be expected to act as the nucleophile and ozone as the
electrophilic oxygen atom donor:
+ −
− − O N C O + O (6.75)
N C O O O
As in the case of reaction 6.61, don’t be overly concerned that an oxygen with a negative
formal charge acts as the electrophile. DFT calculations in the authors’ laboratory provide
support for such a pathway for a variety of OAT reactions involving p-block elements.
Triphenylphosphite forms an ozone adduct at low temperature, which decomposes on
warming to triphenylphosphate and singlet oxygen:

− 78 °C
(PhO) 3 P(O 3 )
3
(PhO) P + O 3
(6.76)
− 15 °C
(PhO) 3 PO + O O
Ozone reacts quantitatively with iodide, so the following reaction is useful for quantita-
tive determination of ozone:

O 3 + 2 Kl + H 2 O l 2 + 2 KOH + O 2 (6.77)

REVIEW PROBLEM 6.10
Suggest polar mechanisms for the above two reactions, that is, equations 6.76 and
6.77.