[narrator] Let’s take a look at acid hydrations of alkynes.
With alkenes, we remember, we made an alcohol--a markovnikov alcohol with no chance of carbocation
rearrangement.
But, here, because we have two pi bonds, double the reactivity we’re going to end up making
a ketone.
We also know that this reaction is slow for terminal alkynes.
In fact, it’s slow for internal and so what we do is we use a mercury 2+ catalyst, a mercury(II)
catalyst, for terminal.
Now, a lot of places say that it’s optional for internal but I’m going to go ahead and
put it because it’s going to help us.
It’s okay to use the catalyst, although some say it’s not necessary for the internal
alkyne with two R groups.
One on each end, with carbon groups versus a hydrogen here for the terminal.
What do we make?
We make, for example, one, we make a ketone.
If we make this a little interesting for the second we could make this unsymmetrical so
that we would end up with two possible ketone products in the second example for the end
terminal.
But, with a terminal there’s only one possibility.
Let’s look at the mechanism and we’ll look at the terminal.
Our mercury catalyst is going to add simultaneously, concerted, as the pi bond--one of the pi bonds
from the alkyne grabs the mercury, a nucleophilic attack on the Lewis acid.
We get our mercurinium, cyclic mercurinium, catalyst here and at this point we can add
a nucleophilic water molecule to the more substituted position, bigger delta plus than
the less able to carry positive charge terminal carbon here.
We attack here, kick up the mercury, and we have our water addition.
Any time you add a water you have left over, and this is an anti-type addition although
it’s going to end up not mattering here.
We end up having to do an acid-base reaction every time we have water.
There’s plenty of water molecules that can accomplish this next deprotonation.
Here we are with our mercuric alcohol.
We have to eliminate our mercury, at some point, here.
What can we do?
Well, the first thing we do is we tautomerize this to a ketone
and in acidic conditions, like we are, we have to tautomerize, in this fashion, invoking
a hydronium ion.
What do we end up with?
We end up with a protonated ketone with our mercury still sitting there, waiting to leave.
After we tautomerize we’re ready to get rid of our mercury so we dump the mercury
electrons that are bonded to the carbon into a pi bond between carbon and carbon, here,
and then kick up the pi bond onto the oxygen, the existing pi bond, and we’re going to
make our enol.
Now we are ready to tautomerize, again, and form our ketone.
We do this in acidic conditions, again, invoking a hydronium.
The final deprotonation using one of our billions and billions and trillions of solvent molecules
we finally arrive at our ketone.