Addition Reactions of Furan - Online Tutor, Practice Problems & Exam Prep

Furan, a five-membered heterocyclic compound, exhibits diene-like behavior due to its low aromaticity. It can participate in Diels-Alder reactions, similar to cyclopentadiene, where its pi bonds interact with dienophiles. The reaction favors the endo product, resulting in specific stereochemistry. The relationship between aromaticity and electronegativity of heteroatoms is crucial; as electronegativity increases, aromaticity decreases. This inverse relationship influences the reactivity of furan compared to thiophene and pyrrole, highlighting the importance of understanding these concepts in organic synthesis.

Addition Reactions of Furan Concept 1

Hey, everyone. So in this video, we're going to talk about addition reactions of furane. Now we're going to say that pyrrole, furane, and thiophene can be considered as butadiene bridged by a heteroatom. Butadiene, so we have 1, 2, 3, 4 carbons, so buta and there's 2 double bonds, so that's a diene. We're going to say a five-membered heterocycle's ability to react like a diene depends on how aromatic it is.

Now, the relationship is, heterocyclic aromatic resonance stability is going to decrease as it decreases when the heteroatoms' electronegativity increases. So if we're taking a look here, we have aromaticity that's increasing as you go this way, and then we have electronegativity that increases this way. Notice the opposite relationship that these two have, the inverse relationship. As our aromaticity goes up, the electronegativity of the heteroatom is going down. So if we take a look here, we'd say that, in terms of electronegativity, sulfur would have the lowest one, followed by nitrogen, and then oxygen would be the most electronegative.

So from what we can see here, that as the electronegativity is increasing, furane with the most electronegative heteroatom, oxygen, but it has the lowest aromaticity. And thiophene sulfur, although it's the least electronegative of the 3 heteroatoms, would have the highest level of aromaticity. So, it would react very differently from furane. Alright. So just remember this inverse relationship between aromaticity and the electronegativity of the heteroatom.

Addition Reactions of Furan Concept 2

Now remember, we talked about the inverse relationship between aromaticity and the electronegativity of the heteroatom. We're going to say being the least aromatic 5-membered heterocycle, furan acts the most like a diene. I'm going to say furan can undergo the Diels-Alder reaction just like a cyclopentadiene can. So if we take a look here, we have cyclopentadiene. It's going to react as our conjugated diene and it's reacting with our dienophile.

Remember, a dienophile is an alkene or an alkyne that's further enhanced when it has a meta director attached to it. What's going to happen here? We have these pi bonds and these pi bonds that are going to interact. We can have this one here coming out and attaching we showed this one coming out and attaching here causing this to move here and causing this bond to move over here. Now, this carbon has to move up out of the way bend upwards out of the way.

If we're showing the 3D representation, this carbon here, which is bending up out of the way, would be here. And remember, the endo product is favored, so we would show these carboxylic acids which are cis right now both pointing down. And then we have this pi bond over here. So this will represent our final answer. Now these 2 are pointing down, so the hydrogens that are there will be pointing slightly up here and here.

Now if we take a look here, we have now furan, which again, remember, it's the least aromatic, so it acts most like a diene. Same process. We have our pi bonds that are going to be moving out of the way. So here we have this one come here, which causes this to come here and this one to come here. Now though, oxygen has to bend up out of the way.

So it would look like this. There goes the oxygen. Here goes our pi bond that moved here, and then these pi bonds became legs. This is still a cis alkene, so these carboxylic acids both point down out of the way. And then we have our hydrogens.

Right. So these would be our 2 deals, all the reactions. One that deals with cyclopentadiene, and one that deals with furan. Right? So it's the same process we've learned in earlier chapters.

Now we're using a heterocyclic compound to see how it happens.

Addition Reactions of Furan Example 1

Here in this example, it says, draw the structure of the major product for the following reaction. Here, we need to include stereochemistry. Alright. So again, we're going to say here is our dienophile. Only one of the pi bonds is going to interact in the Diels-Alder reaction.

This will come here and attach, causing this to move here, causing this to shoot out and move over here. Because our conjugated diene or actually our furan is cyclic, we're going to get a bicyclic compound. So that oxygen has to bend up out of the way. So we're going to get this. That oxygen is bending up out of the way.

There it goes right there. And it's bending up out of the way. Now we have stereo chemistry here. This has to orient itself in the indole formation here, so it will look more like this. And so just make sure it kind of orients down.

Okay. Remember these legs here came from the pi bonds, and there's still this one here that's unaffected, wasn't used. Not the best drawing, but remember, it's in the indole configuration, so it is kind of pointing down. It's below the plane. It's on the opposite side of this oxygen that's pointing straight up.

This here would be our final answer for this structure. And then remember we have our hydrogens. They'd be here too. Kind of wanted to show them. But this is pointing down out of the way.

Alright. So this would be our final 3D representation of the Diels-Alder reaction between our furane and this cyclic dienophile.

Write an appropriate synthetic sequence for the following compound starting from furan.

Hey, everyone. So, in this practice question, it says write an appropriate synthetic sequence for the following compound starting from furan. Now, if we take a look at this, it's a bicyclic product. And remember that these legs here originally belonged as pi bonds. And let's just imagine that these OHs weren't there. And before we had these OHs, we had a pi bond here. If we work backwards, this would move back here causing this to snap back over here and giving us this. What we had originally was our furan, and we know that because they say we're starting with furan. Our dienophile that we used was this one. Since both these carboxylic acids are pointing down, this was a cis alkene. And here goes the pi bond that we used. That originally gave us this structure, our bicyclic compound with this pi bond here. And if we go back to the original product, we can see those 2 OHs there. They're both pointing down. They're cis. We made a cis diol. So, going all the way back to our alkene addition reactions, we know a way to make a cis out of a cis diol is that we can use osmium tetroxide, and we can do it over sodium hydrogen sulfide. And then that would give us those 2 OHs being cis to one another. So this OH you would point down, and this one here would be down. Besides osmium tetroxide, you could also remember to use KMnO4 cold. That'd be yet another way to change an alkene into a cis diol. So, this must have been the sequence that we used in order to make this final product at the end. Right? So, keep that in mind when looking at questions like this that additional reactions of alkenes can play a role since we do have a pi bond present after a Diels-Alder reaction.

Here’s what students ask on this topic:

What is the relationship between aromaticity and electronegativity in furan?

The relationship between aromaticity and electronegativity in furan is inverse. As the electronegativity of the heteroatom increases, the aromaticity decreases. In furan, oxygen is the heteroatom, and it is highly electronegative. This high electronegativity reduces the aromatic stability of furan, making it less aromatic compared to other five-membered heterocycles like thiophene and pyrrole. This inverse relationship is crucial in understanding the reactivity of furan in various chemical reactions, including its behavior in Diels-Alder reactions.

How does furan participate in Diels-Alder reactions?

Furan participates in Diels-Alder reactions by acting as a diene due to its low aromaticity. In these reactions, the pi bonds of furan interact with a dienophile, which is typically an alkene or alkyne. The reaction mechanism involves the movement of pi bonds to form a new six-membered ring. The endo product is favored, resulting in specific stereochemistry. The oxygen atom in furan bends out of the way during the reaction, similar to how the carbon in cyclopentadiene behaves. This allows furan to form the desired product with the dienophile.

Why is furan considered less aromatic compared to thiophene and pyrrole?

Furan is considered less aromatic compared to thiophene and pyrrole because of the high electronegativity of its heteroatom, oxygen. The high electronegativity of oxygen reduces the delocalization of electrons within the ring, thereby decreasing its aromatic stability. In contrast, thiophene, which contains sulfur, and pyrrole, which contains nitrogen, have lower electronegativities, allowing for greater electron delocalization and higher aromatic stability. This difference in aromaticity influences the reactivity and chemical behavior of these heterocycles.

What is the significance of the endo product in Diels-Alder reactions involving furan?

The endo product in Diels-Alder reactions involving furan is significant because it is the thermodynamically favored product. The endo product results from the specific orientation of substituents during the reaction, where electron-withdrawing groups on the dienophile are positioned closer to the newly formed pi system. This orientation stabilizes the transition state through secondary orbital interactions, leading to the endo product. In furan, this means that substituents like carboxylic acids will be oriented in a way that minimizes steric hindrance and maximizes stability, resulting in the preferred stereochemistry.

How does the electronegativity of the heteroatom affect the reactivity of furan in addition reactions?

The electronegativity of the heteroatom in furan, which is oxygen, significantly affects its reactivity in addition reactions. High electronegativity reduces the aromatic stability of furan, making it more reactive and diene-like. This increased reactivity allows furan to participate readily in addition reactions such as the Diels-Alder reaction. The lower aromaticity means that the pi electrons are more available for interaction with electrophiles, facilitating the addition process. This contrasts with more aromatic heterocycles like thiophene, which are less reactive due to their higher aromatic stability.