The Knoevenagel condensation is a special case of the aldol co...

Question

The Knoevenagel condensation is a special case of the aldol condensation in which an active methylene compound reacts with an aldehyde or ketone, in the presence of a secondary amine as a basic catalyst, to produce a new C=C. Show the starting materials that made each of these by a Knoevenagel condensation.
(a)

(a)

Accepted Answer

Hi everyone, welcome back, Let's look at our next problem. It says noble Nagle condensation is a form of algal condensation. That involves the reaction between an active methylene group and an alga Haider ketone to produce a new CC double bond using an Amin base as a catalyst. Give the starting materials that are needed for this type of conversation to make the following compound. So we're given our final product and we're going to be kind of working our way backwards to the reactant since that participated in that nova nagel condensation. So first we want to think about what is the thing that's being produced in this reaction. And our problem tells us that that is a new CC double bond. So when we look at our product compound, we have only one CC double bond. So that's pretty easy. So I'll highlight that in yellow here and we know that to create that new bond, we had a reaction between an active methylene group and Ala Haider ketone. Let's recall what an active methylene group is. Space that definition here And it is a CH two between 2 electron withdrawing groups. So when we look at our two carbons that are participating in that double bond, we see that this carbon, that's part of the six carbon ring is indeed between two electron withdrawing groups is between two carbon eel groups. Um that being between those two groups makes the hydrogen on their very acidic. So that's what makes it participate in this reaction. So I'm gonna go ahead and highlight that carbon in red to help us keep straight, which is which. So that red carbon is going to be our active methylene group. Then that means that the other carbon in that double bond is part of the alga haider ketone. So we're gonna highlight that in blue. And if it were an al to hide we'd expect it to have a hydrogen attached to it, which it doesn't. Instead it has 22 carbons, two methyl groups attached to it. So therefore we know that it must be a ketone and it must have been acetone. It's just got to methyl groups there and then this blue carbon would have been part of a carbon will bond. So that part is pretty simple. Our Aldo haider ketone. So we'll just draw that hearing and it must be acetone. So there's our two methyl groups are carbon with its carbon bond and we'll go ahead and highlight that in blue to match it with the carbon on our final product. So now we just need to go back to what is the reactant that had our activate active methylene group. So let's just draw that out. So we have our six carbon ring. We have our two carbonell groups and then this carbon here which I'm going to highlight in red. It's a methylene group. It just has to hydrogen attached to it. So and result these would be the starting materials that we need for our nova Nagel reaction here's our active methylene group as part of the six carbon ring with two carbon groups. And here's our ketone, just plain old acetone. So if you want to keep watching to see the mechanism for this reaction, I'll go ahead and draw that out. So we know what we started with here. We know we have an Amin basis, a catalyst. So here's our the lambing here with our uh lone pair on the nitrogen. We have our activated methylene group, which as we discussed, means that the hydrogen on that methylene group is acidic due to those two surrounding electron withdrawing groups. So the first step here will be deep protein nation of are activated methylene. So the lone pair on the nitrogen here is going to snag this acidic hydrogen. And that means the electrons that were in this bond are going to pop down here form a double bond between these two carbons here, which will cause the electrons and the carbon will bond to hop up to the oxygen here. So that will result in this in a late intermediate which is stabilized by residents. You've got the one form here with the CC double bond and the negative charge on this oxygen. And that has this other resonance form where this lone pair comes here reforms that C O double bond. These electrons hop over here to make this a CC double bond with these electrons coming up to the oxygen. So we have this in a late which is residents stabilized with the two forms there. And our next step will be the nuclear filic attack of this in a late on our Aldo haider ketone in this case are acetone. So I'll just draw that out here. We'll have the electrons from this cc double bond in the final eight here are going to react with the carbon. Well, carbon that's going to form the first of those carbon carbon bonds that's going to lead to the carbon carbon double bond in the product. As a result, that's going to break that carbon ile C. O double bond. These electrons will come up here to the oxygen and now we have our new form here where the two compounds have been joined together. We have this new C. C bond. So we put the two pieces together and then we've got this negatively charged oxygen up here from that was originally from our carbon bond. This oxygen will then be protein ated. We recall, we have our Amin catalyst and some of it will be um in acidic form. So one of our lone pairs from are negatively charged oxygen here will react. Excuse me, we'll grab a proton here and I'm just gonna scroll down because I'm running out of space to draw the product here and as we look at this, we recall again with this carbon between the two ketone groups. This hydrogen here, that's here. I've drawn it in because it's about to react. We remember that. That's that acidic alpha hydrogen. So it's going to react with our Amin base. Yeah. In the final step here and we're going to see the lone pair on the nitrogen pick up this hydrogen. These electrons that were part of the C. H bond are going to come up here and form that second bond of the CC double bond. That's new in our product. And that's going to cause this hydroxy group to leave the molecule to give us our final form. That we saw at the beginning of our problem with the elimination of the oxygen from our original ketone and the formation of that new CC double bond. So again we can kind of briefly review our steps here. We had our d protein nation of are activated methylene leading to this resident stabilized in the late and then we had the nuclear filic attack of our in a late on the ketone to form the first of those carbon carbon bonds. Uh then followed by protein ation of the oxygen from that carbon carbon hell group. Excuse me. And then leading to our final steps here of our deep origination of the carbon a limb and elimination of the hydroxide to form that second carbon carbon bond. And we have our new carbon carbon double bond and our final product that we saw originally given to us scroll up again to remind us of the two starting materials we had in our nova angle condensation of our active methylene group within the six carbon ring and our acetone as being our ketone that reacted in a nova Nagel condensation reaction. See you in the next video.

Key Concepts

Knoevenagel Condensation

Active Methylene Compounds

Aldol Condensation

Watch next