Leah here from leah4sci.com and in this video, we'll continue with the Diels Alder reaction
by looking at cyclo reactants and bicyclo products.
The tricks refer to in this video were taught in part 1 so it you haven't seen that, go
back to my website leah4sci.com/dielsalder where you'll find the entire video series
alongwith the Diels Alder reaction guide and practice problem set.
In part 1 I showed you how to setup this little key as a trick for the diels alder reaction.
And we also used a molecule that was oriented differently and made it fit that trick.
But what if I tell you that you're starting with two molecules of one three cyclopentadiene
and reacting with heat to form a diels alder product.
How do we make the ring sit that little trick?
that little key here?
Well, organic chemistry is now an art class.
You may feel that way but as long as you're drawing the molecule correctly, it doesn't
have to be pretty.
So to start my reaction, I always wanna start with a pattern of 1,2,3,4 for the diene.
Remember, it's not an art class so if I close the ring over here and put that as position
5, that's fine right?
But what if my drawing is a little off and I exaggerated and put it over there?
Is that still correct?
YES!
It's not drawn very pretty but it's correct.
What if I now take carbon 5 and put it all the way up here?
It doesn't look right but as long as Carbon 5 is attached to carbon 1 and to carbon 4,
it's technically correct.
Now recognize the bonds can bend that way because carbons 1 and 4 are sp2 hybridized,
meaning they're trigonal planer and flat.
But for the sake of this reaction, I'm just drawing it out of the way, I'm exaggerating
it so that I can make my diene look like the standard form.
For the dienophile, I can rotate the molecules that I used either of the two pi bonds.
It doesn't matter because we have the symmetrical starting molecule.
So when I set it up, I make one of the pi bonds face the diene and label that A and
B. Add some heat to get the reaction going and that's it.
Now remember I showed you that you cover your starting molecules, you don't wanna get confused
by all those bonds.
Instead, you want to go for your known product of a cyclohexene with the pi bond between
carbons 2 and 3.
Before we'll look back at the reactants, we want to number it following the pattern, 1,2,3,4,
a, and b.
Now that we have the skeleton for the product, let's go back and see what we want to add.
Carbons 1 and 4 are attached to that funky looking carbon 5 so i'll put carbon 5 up there
and attach it to 1 and 4.
For a and b, we have the ring continuing so let's call that c, d, and e where e fuses
back to a.
So if we have b, let's go with c, d, and e which fuses back to a.
And last but not the least we have the pi bond between c and d so we'll put that in.
And that's our product.
It's not very pretty, it would've been hard to get if you tried it directly with the mechanism.
But by breaking down the core reaction to look like our diels alder template and then
filling it the missing pieces, the product just pops into place.
When you have a bicyclo product, it can be represented in different ways so make sure
that no matter how you see it, you recognize it.
And how do you recognize it?
because of the numbers.
So let's simplify that bridged molecule.
If you have a molecule that looks like this, where we had that number 5 on top, numbered
it as follows, 1,2,3,4,a and b and that's number 5.
You may have also seen the bread basket version of this molecule, where instead of having
the hexagon, both sides are kinda pointed down, we still have the pi bonds here and
there's our funky 5 up there.
Let's prove it, 1,2,3,4,5, a and b.
You may even see this thing upside down so that it looks like this but we still have
5 on top and still have the pi bond.
Ones again 1,2,3,4,5, a and b.
They all represent the same molecule, they're just drawn slightly differently.
Now let's see how to undo a bicyclo compound.
So here we're given a bicyclic product and we're asked to find the starting molecules
that underwent the diels alder reaction to form this product.
We'll have our trick template to refer to and first thing identify the pattern.
I see the pi bonds and that means carbon 2 and 3 and my cyclohexene product and we'll
label it 2 and 3 which means here we have 1 and 2, a and b.
Let's number and label everything else so we figure out where to go with it.
Attach to carbons 1 and 4, we have carbon 5.
Attached to carbon a and b we have the carboxylic acid.
So for b we'll call that one c and then we'll call the lower one d.
I can use the pattern of breaking between 4 and a, 5 and 1 or go back to the other trick
of break make, break make, break make.
Starting with the pi bonds between carbons 2 and 3 we are going to break a bond, make
a bond, break a bond, make a bond, break a bond and make a bond.
Then we draw it exactly as we see it so we have 1, 2,3 and 4.
With the new pi bond between 1 and 2, another one between 3 and 4 and carbon 5 attached
somewhere on top.
For the other side, we have a pi bond a and b and then attached to a and b of carboxylic
acid.
I want you to recognized that the carboxylic acid is in the up position on carbon b in
the down position on carbon a.
If they are facing opposite directions and they're attached to a pi bond that means the
starting molecule has to be a trans pi bond.
If they were facing up, they're both facing down, then that would be a cis starting molecule.
It doesn't matter which one you put on top and which one you put on the bottom meaning
do I put this one , do I make a trans like this or trans like that, because this molecule
does have symmetry so as long as it's trans you're good to go.
So we have a carboxylic acid trans to the other carboxylic acid and these are our starting
molecules.
Now what happens if your starting molecules are Asymmetrical, how do you know how to orient
the starting molecules when fitting it into your pattern and how do you know what to put
up and what to put down on your product.
That's exactly what we'll cover on the next video which you can find along with the diels
alder reaction guide and practice problem set by visiting my website leah4sci.com/dielsalder.
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