Radical Chain Mechanisms: Termination & Summary

Uploaded by lamechivanes on 28.12.2010

In the last webcast
we saw how propagation steps rely on reactions
between odd electron radical intermediates
and even electron substrates.
However, these kinds of reactions
are not the only possibilities
when we consider the witches brew of radical intermediates
that may be present in a radical chain reaction
at any given time.
Another possibility is the combination
of two odd electron species to form an even electron product.
In fact, because such a reaction transforms
two highly unstable open shell molecules
to a closed shell even electron product,
it is extremely favorable
when run in a controlled environment
such as the gas phase.
In the context of radical chain mechanisms,
combinations of two radical intermediates
to form undesired even electron products
are called termination steps because they terminate
a radical's progress around the cycle of propagation.
If termination steps are so favorable,
why don't they dominate the reactivity of radicals?
We'll resolve this conundrum shortly,
but first let's look at some examples of termination steps
in the context of hydrobromination.
Any two radical intermediates from the propagation stage
may combine in a conceivable termination step.
In this first example, Br dot,
combines with the product of addition of Br radical
to the alkene, yielding halogenated product.
Because they reacted with each other,
both radicals are removed from the cycle of propagation
by this step.
In the second example, two molecules of alkyl radical
combine to give a C-C coupled product.
Here, substrate is consumed but no new radical is generated,
leaving some HBr unreacted.
The final example is a simple coupling of two Br radicals
to form elemental bromine.
These three steps represent all the possible combinations
of radical intermediates with one another
and represent the full termination phase
of radical hydrobromination.
This final slide is a summary of the radical chain mechanism
we've discussed in the last two webcasts.
The full set of initiation,
propagation and termination steps
constitute a radical mechanism.
But the question remains,
why don't termination-type steps
dominate the chemistry of radicals?
Thankfully, chemists can use the instability of radicals
to their advantage
by taking advantage of concentration effects.
Because radicals are so shortlived,
if they're generated in a sea of even electron species,
they're much more likely
to react with the even electron species than other radicals,
which may be an unreachable distance away.
Using very small amounts of initiator
helps prevent termination steps
from negatively affecting radical chain reactions.
Often, there's a fine balance
between the kinetics of propagation and termination
that organic chemists must consider
when deciding on the optimal concentration
of a radical reaction.