so the next type of reaction we want to look at is a hydrohalogenation
from the name we could expect that it's a hydrogen and a halogen that are the two groups being added
to the carbons in the double bond
so this time let's actually look at the mechanism
what we need to understand about addition reactions
is that what we're seeing now
is that a pi bond can behave as a nucleophile
it's electron density that is polarizable, so it can attack an electron deficient something or other
like a proton, so here goes the pi bond going to attack that proton and then
those electrons stay with the bromine
now here's the question, there are two possibilities here
you can almost think of it, it's not terribly accurate but
you can think of it as this pi bond sort of swinging out like a trap door one way or the other
is it the case that the hydrogen will end up bound to this carbon meaning that this carbon
would lose an electron domain and be left with a carbocation
or is it the case that it will swing out this way so that the hydrogen would be attached to this carbon
and we would get a carbocation over here
once again, we know that the more substituted a carbocation is, the more stable it's going to be
so for that reason we are exclusively going to get the secondary carbocation
so this proton which is now implied is attached to this carbon down here, because if that's the way it goes
we are going to get the more stable carbocation intermediate
we're gonna get the carbocation there
then the bromide from before, very easy, negative charge is attracted to a positive charge
and we are going to get our alkyl bromide
we have to learn a new term here, because what we are seeing is that whereas with the hydrogenation
we were highlighting a kind of stereospecificity
meaning a discrepancy between two groups being added from the same side or from opposite sides
now, whereas this is not stereospecific, remember that this is basically like an SN1 reaction
we have a carbocation and then that bromide, the incoming nucleophile
can approach from either side, and if we were generating a chiral product
we would get a racemic mixture
but whereas there is no stereospecificity, there is something called regiospecificity
now that's because these are the two carbons participating in that pi bond
and so it should follow that any nucleophile that is going to attach, like a bromine or something
whichever is the more significant of the two groups
it could either end up on the less substituted carbon
or it could end up on the more substituted carbon
in the case where that nucleophile ends up on the more substituted carbon
you have what's called a markovnikov product
now that is a kind of regiospecificity, so this is a markovnikov hydrohalogenation reaction
because that halogen atom ends up on the more substituted carbon of the two carbons
that were participating in the double bond
primary, secondary, secondary is more substituted.
there's ways to get the anti-markovnikov product, so in this case running this reaction
in the presence of peroxides will promote the anti-markovnikov product
this mechanism is a bit more complicated and it involves radicals so we'll look at that another time
but for the moment we just need to understand that in the presence of peroxides
that will promote anti-markovnikov regiospecificity
so once again, just to reiterate, we need to understand that there can be stereospecificity
which would yield a syn or anti stereospecificity
within a reaction, and there's also regiospecificity
which means the difference between where a functional group could end up on the product
and any addition reaction may or may not be one or both of these things
so with the hydrogenation we saw a stereospecificity but not regiospecificity
whereas with hydrohalogenation we can see that there is not stereospecificity but there is regiospecificity
the next addition reaction we want to take a look at is acid-catalyzed hydration
once again we are going to see that the pi bond behaves as a nucleophile
and the proton as the electrophile
this is the same first step, basically, as hydrohalogenation
and we once again have two options for where the carbocation could end up
and it will exclusively end up on the secondary carbon
because it's more stable. and then, the difference here is that a water molecule could attack
instead of the bromide, now we have water there, this does have a formal positive charge
on the oxygen, and then another molecule of water
can deprotonate this one, neutralize that, and we have our secondary alcohol.
just like the hydrohalogenation, we are seeing that acid-catalyzed hydration is a regiospecific reaction
it is going to yield a markovnikov product for the same reasons as the hydrohalogenation
once again we are seeing that the regiospecificity is directed by the differing stabilities of potential
carbocation intermediates
now what's interesting about acid-catalyzed hydration is that it's exactly the opposite of acid-catalyzed
dehydration we saw during the carbocation rearrangement. we can imagine
protonating this oxygen and getting to here and then water leaving
giving us this carbocation and then an elimination occurring, so you can see that
a lot of these mechanisms are reciprocal and related, and depend on certain aspects of equilibria
to see which way they are being driven
the last addition reaction we want to look at is dihalogenation. as you could guess
this is where two halogen atoms are attached to the two carbons participating in the double bond.
once again we are going to see the pi bond behave as a nucleophile, so that's nothing new
and while it would seem a little bit strange that a non polar bromine molecule could behave as the electrophile
it's simply the case that the bromine atoms are pretty large, so this electron cloud
is pretty polarizable, so the pi bond can generate enough of a momentary dipole
on a bromine molecule that one of them, one of the atoms can become slightly partially positive
and now you have an electrophile
what we end up getting is this bromonium ion intermediate which is a little bit strange
it's as though a bromine atom has inserted itself into that pi bond, we have a formally positively charged bromine
atom, as it's showing six electrons to this lewis dot structure as opposed to its typical valency of seven
each of these carbons is partially positive due to a polar bond to the bromine atom
so now the second step here, the bromide left over from before, this is basically gonna do
a standard SN2 reaction, the only difference is that
the leaving group is going to remain tethered to the molecule, because it has another bond
to another carbon over there. so here's our dihalogenation product
we have our two bromine atoms, and because of the stereospecificity inherent in an SN2 reaction
right here being that it must proceed via the backside attack, that makes this an anti addition reaction.
what we want to be clear about is while there is a kind of stereospecificity in the sense that the second step is
a stereospecific SN2 reaction, that's going to give us an absolute configuration of R and S for these two
but it could have been the other way around, if the bromine initially added on the other side
as it was the bromine atom arrived from this direction but it could have arrived from the other direction
bromine could have been on the dash bonds in the intermediate, and then bromide would have
approached from the front, so there are different gradations of stereospecificity, i guess you could say
so there is a kind of stereospecificity and in general when we are looking at addition reactions
we want to be aware of any possible stereospecificity as well as regiospecificity
though in the case where the two atoms being added are the same, or of the same element
we can not discuss any regiospecificity because it doesn't matter which atom goes where
it's going to be the same structural product
and so there's no regiospecificity here or with the hydration, there was no regiospecificity
or sorry, with the hydrogenation, because those were two hydrogen atoms
whereas with the hydrohalogenation and the hydration, there was no stereospecificity
because we had carbocation intermediates but we did have regiospecificity
in the form of a markonikov or anti-markovnikov product depending on where the hydroxyl
or the bromine ended up
once again be aware of any regiospecificity or stereospecificity if you want to draw
the correct product in an addition reaction
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and as always feel free to email me with questions