How is a carbon free radical generated
Free Radicals
hey everyone, professor Dave again. I want to tell you some things about free radical
chemistry
so we're used to electrons as existing in pairs
either within a covalent bond or as a lone pair but
electrons can also be unpaired and these will be called free radicals
so let's learn a little bit about free radicals
if we're looking at a carbon radical this system is sp2 hybridized
and therefore trigonal planar so it's a lot like a carbocation here
there this carbon is missing a fourth electron domain
which is why it is sp2 hybridized and trigonal planar but here this carbon is
actually neutral because it is contributing four electrons
to this lewis dot structure, it is just that one of them is unpaired
and so the carbon radical
is sitting in this unhybridized p orbital so it is
sp2 hybridized overall and planar so that's very important to understand the
geometry of these things
and then a little bit about the stability very similar again to
carbocations
the more substituted a carbon radical is the more stable it is
because a carbon radical even though it is neutral is a form of electron
deficiency
so the carbon radical trend is gonna very closely follow
the carbocation stability trend so the least stable would be the methyl
just as the methyl cation would be very unstable and then
as this becomes more substituted it becomes more stable
because of the hyperconjugation that is occurring from neighboring
alkyl groups just the same way neighboring alkyl groups
would stabilize a carbocation. and then the most stable is this allylic
just as with the carbocation we can have resonance here so that is a
stabilizing factor
so these are some things that we need to know about all radicals
now let's take a look at how radicals formed
so up until now when we've seen electrons rearranging we're used to seeing
heterolytic bond cleavage
so there's something like let's say a leaving group leaving a molecule
so the pair of electrons would leave with one of the atoms
and we have a cation and an anion, that's heterolysis. so that
is a pair of electrons moving
we can also have homolytic bond cleavage, that would be a case where
the two electrons in the covalent bond
one would stay with one atom and then the other would stay
with the other atom, sort of the reversal of the formation of a covalent
bond
and so that would leave us with radicals
and there are certain things that can promote homolysis so we would wonder
why are we seeing this, what is it that would promote this. well
as it happens there are certain kinds of covalent bonds, for example
oxygen-oxygen covalent bonds and halogen-halogen covalent bonds
that if that bond is
heated or struck with a photon of
a very specific energy usually UV light that will actually
promote homolysis because it will excite one of the electrons from the
highest occupied molecular orbital
to the lowest unoccupied molecular orbital which in this case would be
the sigma antibonding orbital so if we have a covalent bond where two electrons
are sitting in the bonding orbital
those atoms will remain bound but because one of the electrons in this bond
is struck by let's say
a photon of UV light and is promoted to the
antibonding orbital them we'll have one in bonding one in antibonding
and the bond order will be zero, that is what promotes homolysis
and this kind of situation. so those are the
that's basically what's promoting free radical reactions when we have homolysis
of these
oxygen-oxygen bonds, halogen-halogen bonds, there are some other
possibilities there but now we understand that we want to look
at the the three steps that are involved in
any radical reaction and so they have names and there are characteristics about
them that will be true
no matter which radical reaction we're looking at so
every radical reaction has an initiation step
so an initiation step is the homeless step
this is where a covalent compound becomes
two radicals so we have one covalent compound
and then this will become two radicals. now one thing that we need to understand
is that up until now we've been using double-headed
electron pushing arrows. now we must use single-handed
electron pushing arrows because a double-headed electron pushing arrow
denotes the movement of a pair of electrons
now that we are talking about the movement of singular electrons
we must use single headed arrows to describe exactly that so here we have a
single electron
from this covalent bond staying with A and a single-electron
staying with B generating two radical species
now every initiation reaction
is going to be endothermic, it is going to be enthalpically
unfavorable because some energy must be absorbed by this system
be it heat energy or the energy from the photon of UV light
or whatever is happening this system must
absorb energy for homolytic bond cleavage to occur.
so initiation will always be endothermic
once we have a radical species we will have one or more
propagation steps. now there could be many propagation steps, there could be one
propagation step, it depends on
exactly which reaction we are looking at but
there will always be propagation steps and that is a situation where
one radical species and one covalent species
generates a different radical species and a different covalent species
so we're looking at
the radical, so this electron
and one electron from this covalent bond are going to now
form
the covalent bond between A and B. the
other electron in this covalent bond is staying with C
to generate the other radical species so propagation once again
one radical species one covalent species generating a different radical
and a different covalent species
now in terms of enthalpy you cannot
predict whether a propagation step will be endothermic
or exothermic because it depends on the bond energies
involved so you could go to your table of
thermodynamic data and calculate yourself
what the energies are stored in this bond
versus the bond that you're generating and then you can go ahead and calculate
the delta H of that reaction. but a propagation step is not inherently
endothermic or exothermic. then lastly
there's going to be a termination step typically, or at least that's what
radicals can do
and so this is a situation where two
radicals go to form a single covalent species here we can see
the electron that is a radical from each species going
together to form this covalent this is essentially the reversal
of the initiation step although it need not be the two species
that generated the radicals in the initiation step. basically
any two radicals be they halogen, alkyl or what have you
if they generate a covalent bond between them that would be considered
a termination step. now just as initiation will always be
endothermic termination will always be exothermic
because we have these very high-energy unstable
radical species that would be very high on an energy diagram then going to form
a covalent bond between them
which is a storage of energy, any bond is a storage of energy
so that will bring this whole system to a much lower energy so
initiation always endothermic termination always exothermic
propagation depends on which bonds are breaking and forming.
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and as always feel free to email me with questions
