hey everybody, it's professor Dave and I'd like to talk to you about Michael addition
so the Michael addition is very similar to the aldol
condensation in that it is
enolate chemistry however it differs a bit in the substrate
that the enolate attacks so let's take a look
this we've seen already so
the hydroxide can grab this alpha proton
and we will then form
our enolate, we know about the enolate
here's where it's a little bit different, the substrate the enolate is going to
attack is a little bit different
in the Michael addition because it is going to be
a carbonyl containing compound with an alpha-beta
degree of unsaturation, so we have a pi bond here
specifically in the alpha-beta position with respect to the carbonyl carbon so here's
the carbonyl
alpha, beta, this is an alpha-beta
unsaturated system so what this is gonna do is it is gonna delocalize
the site of electrophilicity, it is gonna delocalize where the enolate is going
to attack
because there is some resonance stabilization these two
pi bonds are conjugated so there is a resonance structure where we could draw
this pi bond over here
and this pi electron density from the carbonyl
up there so that means that whereas typically and if this pi bond wasn't
there
then there would be a partially positive charge here
on that carbonyl carbon and that would be the only site of electrophilicity
but now we're saying that there's a resonance structure
where that partial positivity is satisfied to a degree
by the shuffling pi electron density and if we did that there would be a formal
positive charge
on this carbon over here so what we've done is similar to the conjugated
polyunsaturated systems
what we were looking at
a hydrohalogenation we are delocalizing the site of attack
and so what's happening now is no longer is the enolate gonna attack the carbonyl
carbon
in a Michael addition the carbonyl reforms and then this is now going to
attack
the beta carbon and then these pi electrons
are going to go here and then these will go up here
and so this is what we get
now once we form that this can protonate
from water or whatever and
here we have something called an enol
and what an enol is going to tend to do
is tautomerize so what that means
is that we're gonna
shuffle these electrons around and
it's basically it's a game of thermodynamics we are trading
a carbon-carbon pi bond and an oxygen-hydrogen
sigma bond for a carbon
hydrogen bond and a
carbon-oxygen pi bond so because of the
polar nature the carbonyl that is gonna have
it's gonna be a more energetic bond so there's more energy stored
in this than this so that is what is going to
shuffle these bonds around and that process is called tautomerization
so what we end up in the end with our Michael product here
is whereas an aldol
whereas aldol was a 1,2
addition Michael is going to be
a 1,4 addition because if we consider the nucleophilic carbon
to have been carbon 1, the new
functional group of note being this other carbonyl
is at the 4-position so the thing that we want to know as well
this is enolate chemistry similar to the aldol condensation
the products are quite different because whereas with aldol condensation it was
that 1,2
addition where we had the new
the new hydroxyl group or the new pi bond
at the 1,2 position over here. the chemistry that has occurred in the
michael addition is such that
we have a dione. so first of all we have two
carbonyls as opposed to a carbonyl and hydroxide or
a double bond so we have a dione and they're specifically in this
configuration, they will always be precisely this far away from one another
so whether we are looking at products and trying to predict
what kind of reaction will form them if we're doing that then the spatial
distribution of those groups can cue us in to what
reaction occurred but also for just going in the forward direction we can
always check
we have to know that these groups are going to end up this far apart from each
other
so the Michael addition is similar to the aldol condensation in that it is
enolate chemistry, it differs in a couple of regards, number one
the substrate always will have this alpha-beta
unsaturation which will cause the enolate to attack the beta carbon
instead of the carbonyl carbon that's the key difference mechanistically
and then the key difference in the product is the spatial distribution
of the oxygen-containing
functional groups in the product
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and as always feel free to email me with questions