Radical Stability Trends, BDEs, & Initiation

Uploaded by lamechivanes on 28.12.2010

Radicals in general are highly unstable intermediates.
However, there are two reliable trends
in carbon radical stability
that are analogous to trends in cation stability
that we can take note of.
First of all,
increased substitution around the radical center
increases the stability and lowers the energy
of the radical.
Because more substituted radicals are more stable
than less substituted ones,
they are more likely to be formed as intermediates
in radical reactions.
In a later webcast,
we'll see how this stability trend can be exploited
to selectively add HBr across an alkene
in an anti-Markovnikov fashion
opposite the trend for HBr addition involving cations.
The second trend involves electron delocalization.
More delocalized or resonance-stabilized radicals
are more stable than radicals not involved in resonance.
Delocalization lowers the energy of filled molecular orbitals,
lowering the energy of the radical overall
and increasing its stability.
In addition, an important point to note here
is that resonance imparts radical character
to multiple atoms in the structure
which may lead to product mixtures.
Until this point,
we've talked only about carbon-centered radicals,
and you may be wondering about trends in radical stability
as a function of atom type.
When considering radicals
centered on different atoms, however,
it becomes critical to examine the origin of each radical.
In other words,
we have to look at the stability of the bonds that broke
to form the radicals.
Bond dissociation energies, or BDEs,
provide a direct measure of the likelihood
that a two-electron bond will break to form two radicals,
a process known as homolytic cleavage.
A lower bond dissociation energy
indicates more favorable homolytic cleavage,
and, in general, larger, more diffuse atoms
exhibit lower BDEs than the smaller, second row atoms.
Thus, larger atoms such as the halogens and silicon
generate radicals more easily than the second row atoms.
If you take a look at the table shown here,
you'll notice that many of the bonds
that have been used to generate radicals historically,
such as Br-Br and I-I,
have notably low bond dissociation energies.
The first step of any radical reaction
is the generation of a small amount of radical
from an even-electron species, called a radical initiator,
by a homolytic cleavage of some kind.
Let's look now at a few examples of initiators
that are commonly used to start off radical reactions.
Peroxides contain the labile O-O bond
which can break homolytically
to give two alkoxy radicals.
The alkoxy radical is able to abstract hydrogen
from organic substrates, generating organic radicals
that then participate in radical reactions.
The lengthily named azobisisobutyronitrile, or AIBN
is an interesting substrate
in which two bonds break homolytically.
The C-N bonds on the left and on the right of the azo group,
which is the N-N double bond,
both break to yield a molecule of nitrogen gas,
which is lost,
and two radicals in which the radical is adjacent
to a cyano group that stabilizes it by resonance.
The kinetic loss of nitrogen gas
drives this reaction nearly to completion.
You should also note that in this drawing,
I've omitted two arrows
-- those arrows going back to the carbons
on which the radicals reside.
You'll sometimes see these arrows omitted
because they're implied
by the red arrows that I've drawn here.
Finally, the elemental halogens, such as Br2 and I2,
are commonly used to initiate radical reactions.
Under the influence of light or heat,
the Br-Br bond can break homolytically
to reveal two molecules of bromine radical
which, like the alkoxy radicals generated by peroxides,
are able to abstract hydrogen from organic substrates
to initiate radical reactions.
One important point to keep in mind for all of these
is that generating the radical
is an endothermic process in all cases.
And so an energetic input of light or heat
is necessary in order to initiate the reaction
of the initiator.
In the next webcast,
we'll take a detailed look at radical chain reactions
from initiation all the way to propagation
and product formation.
It all begins with small amounts of a radical initiator
which goes on to react with one of the substrates
to kick off the radical chain reaction.