284
Chapter 22 Free-Radical and Spin-Paired Diradical Reactions
electron of atom Υ spin-pairs with the unpaired antibonding electron of AB to
generate the “increased-valence” structure Υ — A · B . We indicate again that
because the Y-B bonding (–––––) in this structure is weak when Y and B are nonadjacent atoms, we have for convenience of representation omitted it. Some
examples of this type of free radical reaction, which we have discussed in Chapter
11, are F + NO → FNO and
2
2
F O
FO
.
We have also discussed numerous examples of reactions of type (c), namely
reactions in which both reactants have a Pauling “3-electron bond”. Representative examples are
2 2
2NO N O
,
2
2 3
NO NO
N O
,
2
2 4
2NO
N O
and
2 2
2ClO Cl O
.
A fourth type of free radical reaction, in which one reactant molecule has an
unpaired electron that occupies an atomic orbital, and the other species has an
electron-pair bond, is customarily represented as (d).
(d)
We shall now examine other aspects of the reactions (b)-(d). In particular, we
shall show how the products of these reactions may themselves sometimes involve
electronic rearrangements and decompositions.
22-2 R + O 3 RO + O 2 , with R = H, Cl, and NO
Reactions between 3
O and univalent radicals – in particular, chlorine atoms and
NO – are of ecological concern, because it has been suggested that they might lead
to some destruction of the protective ozone layer in the stratosphere
1,2 . These
reactions generate univalent free radicals, for example ClO and
2
NO
1,2
. To
examine how electronic reorganization might proceed for these types of reactions,
we shall initially examine mechanisms for the reaction of ozone with hydrogen
atoms, using both standard Lewis and “increased-valence” structures.
For 3
O the standard Lewis structures are of type (1), with no unpaired-electrons. If we use it to represent the electronic structure, it is necessary to formulate
the reaction of 3
O with a hydrogen atom according to mechanism (d) of Section
22-1, as follows:
Chapter 22 Free-Radical and Spin-Paired Diradical Reactions
electron of atom Υ spin-pairs with the unpaired antibonding electron of AB to
generate the “increased-valence” structure Υ — A · B . We indicate again that
because the Y-B bonding (–––––) in this structure is weak when Y and B are nonadjacent atoms, we have for convenience of representation omitted it. Some
examples of this type of free radical reaction, which we have discussed in Chapter
11, are F + NO → FNO and
2
2
F O
FO
.
We have also discussed numerous examples of reactions of type (c), namely
reactions in which both reactants have a Pauling “3-electron bond”. Representative examples are
2 2
2NO N O
,
2
2 3
NO NO
N O
,
2
2 4
2NO
N O
and
2 2
2ClO Cl O
.
A fourth type of free radical reaction, in which one reactant molecule has an
unpaired electron that occupies an atomic orbital, and the other species has an
electron-pair bond, is customarily represented as (d).
(d)
We shall now examine other aspects of the reactions (b)-(d). In particular, we
shall show how the products of these reactions may themselves sometimes involve
electronic rearrangements and decompositions.
22-2 R + O 3 RO + O 2 , with R = H, Cl, and NO
Reactions between 3
O and univalent radicals – in particular, chlorine atoms and
NO – are of ecological concern, because it has been suggested that they might lead
to some destruction of the protective ozone layer in the stratosphere
1,2 . These
reactions generate univalent free radicals, for example ClO and
2
NO
1,2
. To
examine how electronic reorganization might proceed for these types of reactions,
we shall initially examine mechanisms for the reaction of ozone with hydrogen
atoms, using both standard Lewis and “increased-valence” structures.
For 3
O the standard Lewis structures are of type (1), with no unpaired-electrons. If we use it to represent the electronic structure, it is necessary to formulate
the reaction of 3
O with a hydrogen atom according to mechanism (d) of Section
22-1, as follows: