Page 55 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

1.20 RADICAL REACTIONS 35
At present, we do not know whether fluoride 1,3-shifts provide a low energy pathway or
not. Therefore, we cannot state categorically which of the above two pathways, or for that
matter a different one, is the one that operates in reality. Note that both pathways require
the formation of an oxo-bridged intermediate. That appears to be a general feature of ligand
exchange reactions of this type. When dealing with such reactions, simply join up the two
reactants via a lone pair on one of the migrating groups; subsequent D and A reactions, or
ligand 1,3-shifts, would then lead to the final products.
A word is in order on the thermodynamic driving forces underlying the above reaction.
Relief of steric strain at the 7-coordinate iodine is a possible factor, but the main driving
force is undoubtedly the formation of two highly stable P–F bonds, whose combined BDEs
+ −
(∼490 kJ/mol each) more than outweigh that of one P –O unit (∼544 kJ/mol).

1.20 RADICAL REACTIONS

Although our focus is clearly on polar or ionic mechanisms, we should not and will not
ignore radical pathways altogether. A very brief introduction is therefore provided here.
Observe that, in the discussion below, single-headed fishhook arrows indicate “movement”
of unpaired electrons.
Radicals are typically produced by thermal or photochemical homolytic cleavage of a
weak single bond:

Homolysis: A B A + B (1.72)

Homolysis refers to the separation of a bonding electron pair into two unpaired electrons,
that is, radicals. Heterolytic mechanisms, by contrast, are characterized by a bond-breaking
step where the electron pair constituting the bond leaves with one of the fragments, as shown
below:
+ −
A B A + B
Heterolysis: (1.73)
− +
A B A + B

The term “heterolytic mechanism” is thus more or less synonymous with a polar or ionic
mechanism. Some classic radical-generating reactions are as follows:

(a) Br Br Br + Br

R
(b) O O 2 RO
(1.74)
R
R
(c) N N 2 R + N N
R