hey guys, this is professor Dave and I want to tell you some things about
addition on conjugated polyunsaturated systems
so if we consider a fully saturated hydrocarbon to be
one with maximum hydrogen content then any pi bond is going to be a degree of
unsaturation because if the carbons have additional bonds between them then they
cannot be bound to as many
hydrogens as they could. so any pi bond is a degree of unsaturation so a
polyunsaturated system
is a system with multiple pi bonds. now furthermore if it is a conjugated polyunsaturated system
that means that those are arranged in an alternating single double single
double
format so that is what allows for resonance
so a conjugated polyunsaturated system is sort of a special system
now let's go back to the addition reactions and see what is gonna change
here due to the conjugated
polyunsaturated quality of this molecule. so we're gonna be
use a little bit of a simplified situation by
looking at a symmetrical molecule so this would be a little more complicated
if there was, say another carbon atom or something like that because this is
symmetrical that means that it's not going to matter which pi
bond we use in the reaction so we're just gonna see what happens using one
of them
so let's go ahead and do this hydrohalogenation, let's say this pi bond
is gonna act nucleophilically, grab this proton
so we're gonna get a carbocation intermediate
because these are both secondary carbons
we would expect no preference or no regiospecificity
in where that carbocation ends up on the intermediate but
if we take a look at the intermediate we have to realize
something here, it's exclusively gonna go to this carbon because this
is the position that will be resonance stabilized
by the other pi bond so resonance stabilization
is something that's going to bring the energy of the system down so that is going to
be preferred
so whereas this position is not resonance stabilized because this pi
electron density
is just too far away to interact, this will be and so it is exclusively going
to go
there. now what we have to realize is that
there is another resonance structure here if we can move that pi bond
and and put the carbocation over there and so we look at the
composite resonance structure we see that what's going on is we've
delocalized the formal positive charge and so there is
a delocalized positive charge and partial pi electron density
all about this portion of the molecule. so here's where things get different
because
if we had delocalized the site of electrophilicity
then we must necessarily have delocalized the
the point of nucleophilic attack so remember that
neither of these resonance structures actually exists
these are ways that we describe molecules so that we can draw mechanisms
and have them make sense but the composite resonance structure is what truly exists
so as a bromide approaches the system
it is not necessarily going to have a specific preference
for this carbon over this carbon, you can either approach this carbon
pushing the electron density back over to this side or can approach here
and push the electron density back over to that side so what has to happen is
that we need to
draw another step in the mechanism starting from
each of these resonance structures to get all of the possible products
so let's say that the bromide attacks this resonance structure
and the bromide goes there we're gonna get a racemic mixture
of compounds with the bromine at that position on the molecule
but just because this is an equally valid resonance structure
we can have the bromide attack here and
likewise get a racemic mixture of compounds with the bromine
at that position on the molecule
so what we need to understand is that
because of the resonance stabilization here
we're creating the possibility for a much larger number of products in
the product mixture
and this would only be
much moreso the case if this were not a symmetrical molecule
because then we would have to go through this again
utilizing the other pi bond as the nucleophile, you have
quite a greater number of products in the product mixture.
thanks for watching guys. subscribe to my channel for more tutorials
and as always feel free to email me with questions