172
REACTION MECHANISMS
For clarity, nonbonding electrons are usually omitted,
though in order to propose meaningful mechanisms
it is important to remember how many electrons are
associated with each atom. The unpaired electron
must always be shown.
In the formation of radicals, a bond is broken and
each atom takes one electron from the pair constituting the bond. Bond-making and bond-breaking processes are indicated by single-headed (fishhook) curly
arrows representing the movement of one electron.
fish-hook curly arrow
representing the movement
of one electron
A B
A
B
radicals
A radical mechanism sequence requires three
distinct types of process: initiation, propagation, and
termination. Initiation is the formation of two radical
species by bond fission, whereas propagation involves
reaction of a radical with a neutral molecule, a
process that leads to generation of a new radical.
Because radicals are so reactive, the propagation
process may continue as long as reagent molecules
are available. Finally, the reaction is brought to a
Cl Cl
H CH 3
Cl H
Cl Cl
H 3 C Cl
H 3 C CH 3
initiation
propagation
termination
(radical
pairing)
fission of single bond (two electrons)
creates two radicals each containing
one unpaired electron)
new bond formed by combination
of one electron from radical, and
one electron from single bond
this creates a
new radical
the new radical reacts further,
generating another radical
eventually two radicals combine
to form a new single bond
Cl
Cl
CH 3
Cl
H 3 C
Cl
H 3 C
C H 3
conclusion by the combination of two radical species,
so that the unpaired electrons, one from each species,
are combined into a new single bond. The radicalpairing termination step is analogous to a reversal of
the initiation step. It occurs readily because of the
reactivity of radicals; it follows, therefore, that the
initiation step will require the input of a considerable
amount of energy in order to dissociate the single
bond.
In the propagation steps shown above, the radical
propagates a further radical by causing fission of a
single bond in the substrate. Many important radical
reactions actually involve compounds with double
bonds as substrates, and the π bond is cleaved during
the radical addition reaction.
C C
C
C C C
radical addition
cleavage of π bond
creation of a new
radical
It makes good sense to draw free-radical mechanisms in the manner shown by these examples.
However, shorter versions may be encountered in
which not all of the arrows are actually drawn. These
versions bear considerable similarity to two-electron
curly arrow mechanisms, in that a fishhook arrow
is shown attacking an atom, and a second fishhook
arrow is then shown leaving this atom. The other electron movement is not shown, but is implicit. This type
of representation is quite clear if the complement of
electrons around a particular atom is counted each
time; but, if in any doubt, use all the necessary fishhook arrows.
C C
C
C C C
alternative representation of radical
addition omitting some curly arrows
electron movement gives
carbon nine electrons;
therefore, one must be lost
by transfer to next atom
REACTION MECHANISMS
For clarity, nonbonding electrons are usually omitted,
though in order to propose meaningful mechanisms
it is important to remember how many electrons are
associated with each atom. The unpaired electron
must always be shown.
In the formation of radicals, a bond is broken and
each atom takes one electron from the pair constituting the bond. Bond-making and bond-breaking processes are indicated by single-headed (fishhook) curly
arrows representing the movement of one electron.
fish-hook curly arrow
representing the movement
of one electron
A B
A
B
radicals
A radical mechanism sequence requires three
distinct types of process: initiation, propagation, and
termination. Initiation is the formation of two radical
species by bond fission, whereas propagation involves
reaction of a radical with a neutral molecule, a
process that leads to generation of a new radical.
Because radicals are so reactive, the propagation
process may continue as long as reagent molecules
are available. Finally, the reaction is brought to a
Cl Cl
H CH 3
Cl H
Cl Cl
H 3 C Cl
H 3 C CH 3
initiation
propagation
termination
(radical
pairing)
fission of single bond (two electrons)
creates two radicals each containing
one unpaired electron)
new bond formed by combination
of one electron from radical, and
one electron from single bond
this creates a
new radical
the new radical reacts further,
generating another radical
eventually two radicals combine
to form a new single bond
Cl
Cl
CH 3
Cl
H 3 C
Cl
H 3 C
C H 3
conclusion by the combination of two radical species,
so that the unpaired electrons, one from each species,
are combined into a new single bond. The radicalpairing termination step is analogous to a reversal of
the initiation step. It occurs readily because of the
reactivity of radicals; it follows, therefore, that the
initiation step will require the input of a considerable
amount of energy in order to dissociate the single
bond.
In the propagation steps shown above, the radical
propagates a further radical by causing fission of a
single bond in the substrate. Many important radical
reactions actually involve compounds with double
bonds as substrates, and the π bond is cleaved during
the radical addition reaction.
C C
C
C C C
radical addition
cleavage of π bond
creation of a new
radical
It makes good sense to draw free-radical mechanisms in the manner shown by these examples.
However, shorter versions may be encountered in
which not all of the arrows are actually drawn. These
versions bear considerable similarity to two-electron
curly arrow mechanisms, in that a fishhook arrow
is shown attacking an atom, and a second fishhook
arrow is then shown leaving this atom. The other electron movement is not shown, but is implicit. This type
of representation is quite clear if the complement of
electrons around a particular atom is counted each
time; but, if in any doubt, use all the necessary fishhook arrows.
C C
C
C C C
alternative representation of radical
addition omitting some curly arrows
electron movement gives
carbon nine electrons;
therefore, one must be lost
by transfer to next atom