Predict the relative pK_a values of cyclopr...

Question

Predict the relative pK_a values of cyclopropene and cyclopropane.

Accepted Answer

Hello, everyone. Let's do this problem. It says suggest the relative P K A values of the following compounds. We have two fused bicyclic rings. Compound A is benzo cyclo protein and compound B is by cyclo 410, he the 24 dye. All right. Let's get into the problem. So we want to find the relative P K A values of the following compounds. Well, what does that mean? We basically just want to say which compound has a higher P K A, which one has a lower P K A? So what does P K A mean? Exactly? Well, P K A is the acid dissociation constant which basically tells us how likely that compound is to give up a proton or dissociate, right. So the lower the P K A value, the more acidic that compound is which makes it more likely to donate a proton. And if a compound is more likely to give up a proton, that means it's going to have a higher stability as a conjugate base. So we want to find the compound that is more stable as a conjugate base or rather whose conjugate base is more stable, right? So both of these compounds I see have several pie bonds, they are cyclic. So we can talk about stability in terms of aromatic. So the ranking for stability by different types of aromatic is aromatic compounds are the most stable then non aromatic compounds. And lastly anti aromatic compounds are the least stable of the three. So what are the criteria for each of these types of aromatic? Let's quickly review them. So the criteria for aromatic compounds known as huckle rule R as follows. So first, the compound needs to be cyclic and cleaner or flat, then the compound must be fully conjugated. And lastly, the number of pi electrons has to equal the expression four N plus two. All right. What about anti aromatic compounds? How do we know if we have an anti aromatic compound? This set of rules is called Brelo's rule. And it's very similar to the aromatic rules in that we need a cyclic planar compound. It must be fully conjugated. But the difference is that the number of pi electrons satisfies a different expression. It satisfies the expression four times in. All right. And lastly, what about non aromatic compounds? Well, any compound that fails any of the criteria listed before for aromatic or anti aromatic compounds is going to be non aromatic. So if the ring or the compound is not cyclic and planar or if it's not fully conjugated, it will be a non aromatic compound. So let's get into the structures that were given to us starting with structure a benzo cyclo propine. So we have a benzene ring that is fused with a three membered carbon ring. So is this compound aromatic anti aromatic or non aromatic. Well, this smaller fused ring is not conjugated, right? It has no pi bonds. So this structure is not going to be fully conjugated. So this is non aromatic because it fails that criteria. So what happens if we lose a proton? Right, we're talking about which compound is more acidic. And we said we wanted to compare the conjugate base. So we would do that by giving up a proton and seeing how stable that conjugate base is. So when we give up a proton, now we have a lone pair and a negative charge on this smaller ring, right? So now is this compound aromatic anti aromatic or non aromatic. I have 2468 pi electrons and it's fully conjugated now because of that lone pair. So this is anti aromatic because the eight electrons satisfies the four N expression. So this is anti aromatic. All right, let's draw the conjugate base. For compound B, we have a six member ring with two double bonds on carbons, two and four. And then we have our smaller fused ring. So what is the aromatic of this compound? Well, again, this compound is not fully conjugated. So it is non aromatic. And if this compound gives up a proton to form the conjugate base. Now we will have a long pair and negative charge on that smaller ring again. So what is the aromatic of this compound? The conjugate base is this compound cyclic and planar. Yes. Is it fully conjugated? No? Right? There are carbons in between these pi bonds and the lone pair. So this is going to remain non aromatic. So donating a proton does not change the aromatic of this compound. So compound A is anti aromatic compound. B is non aromatic, which one is going to be more stable, the non aromatic compound right, more stable. So the anti aromatic compound is less stable. So what does that mean in terms of P K A, the more stable compound we said will have a lower P K A? Right? Because it's able to donate that proton more easily since the conjugate base is more stable and the less stable compound will have a higher P K A making it a worse acid. So in summary, to answer this question, A has a higher P K A value than B. That's it for this problem. I hope this video was helpful. Thank you for watching.

Predict the relative pK_a values of cyclopropene and cyclopropane.

Key Concepts

pKa and Acidity

Cyclopropene vs. Cyclopropane Structure

Resonance Stabilization

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Predict the relative pKa values of cyclopropene and cyclopropane.

Key Concepts

pKa and Acidity

Cyclopropene vs. Cyclopropane Structure

Resonance Stabilization

Watch next

Predict the relative pK_a values of cyclopropene and cyclopropane.