An imine is created when a carbonyl reacts with a primary amine and acid.
You can also use ammonia.
Ammonia would be considered as 0-degree amine.
It’s the same thing.
Either 0 degree or primary would both make an imine.
You need an acid so I’m going to put H+ here.
For the purposes of this mechanism, I personally like it if I combine my proton with my solvent
so that I don’t have multiple types of reagents I’m trying to work with.
Can you guys just go ahead and assume with me that NH3 plus H+ would give me NH4+.
That’s what I’m actually going to react with.
For all of my reactions, for both of these mechanisms, I’m just going to use NH4+,
which is fine.
You can use any acid you want, whatever acid professor or your textbook gives you.
I'm going to go ahead and draw it like this.
What do you guys think is the first step of this acid-catalyzed mechanism?
You got it, protonation.
How did you guess?
Now I’m going to use reversible arrows, not forward arrows.
I’m going to draw that I have a double bond H carbon.
This has a positive charge.
I'm going to go ahead and draw a resonance structure.
I’m going to draw this structure so that I can understand what this positive charge
is doing.
Just so you know, disclaimer, your professor may not draw this resonance structure and
that's okay because it’s still valid.
What I’m trying to say here is that there may be minor differences between my mechanism
and the one that you learned in class, but again this is a valid mechanism and if you
want to throw in a resonance structure, it's totally fine.
Your professor isn’t going to dock you for doing this because all the arrows are going
to the same place eventually.
Now what this is going to do, since I have a positive charge, this is going to yield
a nucleophilic attack.
Let’s go ahead and take our NH¬3.
It has a lone pair, so it's very attracted to that positive charge.
We’re going to do a nucleophilic attack and what we're going to get is a compound
that looks like this.
We’re going to get OH, RR.
Those are just R groups and then NH3+.
Does it look familiar at all?
If you guys have gone through the hemiacetal and acetal mechanism, pretty much everything
has been the same up until this point.
The difference is going to happen here.
We're going to consolidate a few steps.
Instead of deprotonating this thing, we're just going to go ahead and start trying to
get rid of that alcohol.
The way we could do that is through a proton exchange.
A better way to write this NH3+ would be to take the 3, change it to a 2 and then draw
one H out.
That’s going to allow me to do a proton exchange.
This is going to yield a compound that now looks like, this OH2+, NH2, RR.
Let me go ahead and backtrack and write down all the steps.
We said that this first one was protonation.
The next one was nucleophilic attack.
Now we just got proton transfer and now we're about to kick out the leaving group.
But instead of it just leaving, we could actually use the electrons from my nitrogen to kick
it out.
What I could do is I could make a double bond and kick out the water.
What this is going to make is a double bond N.
I just realized that my nitrogen is at the bottom.
I wanted to face it up.
If you don’t mind.
I’m going to flip this molecule.
I’m going to flip it so that now my Nh2 is at the top and it was making a double bond
so I’m going to draw the double bond facing down.
There's two R groups.
I’m going to put the R groups facing down, positive charge.
Do you recognize this intermediate?
This is the iminium cation, plus we would also get water leaving.
Now that is my compound, that’s my imunium cation.
How do you think we can get rid of one of those Hs?
We can use the conjugate base.
Remember I had NH3 hanging around.
I’m going to use NH3 and I’m going to deprotonate.
I’m going to take away an H, give the elections to the N and what I'm going to get is my imine
plus my catalytic acid and plus yeah, we did make a water.
We lost a water like that.
Now my product is an imine.
How do I know this is an imine?
Because it looks like a carbonyl except that has a nitrogen in it.
Does that make sense?
This has a lot of steps but if you've watched the hemiacetal or acetal mechanism, this is
extremely similar.
If you haven’t watched those mechanisms yet, then when you get to them it will seem
very similar to this.
I'm just trying to show you guys how the mechanisms are very, very related.
Also, I think I messed up on an arrow up here.
This is supposed to be reversible.
Sorry about that.
I get too excited with this mechanism.
But you have to make sure to go back if you make mistakes, always change it before you
submit your exam.
We're done with this one.
Let’s move on to enamines.