Free Radicals

hey everyone, professor Dave again. I want to tell you some things about free radical


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


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


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


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


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