Show how octane-2,7-dione might cyclize to a cycloheptenone. E...

Question

Show how octane-2,7-dione might cyclize to a cycloheptenone. Explain why ring closure to the cycloheptenone is not favored.

Accepted Answer

All right. Hi, everyone. So this question is asking us to show the cy ization process of nonna 28 Dion to form a cyclo octone and discuss why this ring closure might be disfavored. So here I went ahead and drew the structure of the starting material on the screen. No name 28 Dion is a nine carbon chain with car bal on positions two and eight of the chain making it a die keto. So here our final product is cyclo octone which is a cyclic ketone with a double wand inside of the ring. Now, disaronno compounds are interesting in that they can undergo what's known as self condensation, which is ultimately a cycler reaction. And so what that means is that one carbonel is going to create an ulate and it's going to attack the carbon carbon of the other or the carbon of the other car bal I should say. And so ultimately, this is going to be known as an intra molecular Aldo condensation. And ultimately condensation is going to form an alpha beta unsaturated Carboni. And so here to form a cyclo octone, what that means is that a hydrogen from the end carbon will be removed and that's going to go ahead and attack the other car bottles carbon. So with that, let's go ahead and begin with the mechanism. And so first I'm going to go ahead and highlight one of the outermost carbons here in the parent chain just to showcase, which is going to be detonated here though there is a symmetry to the molecule, which means that you could do this on either side and get the same result. But with that being said here, this reaction is going to be catalyzed in the presence of some base to produce the Enola lets use hydroxide ion. So here we have the hydroxide ion detonating our alpha carbon adjacent to one of the car bottles which is going to result in an Enola. Now recall that in nates are remarkable in that they are stabilized via resonance, right. So now we have a negative charge on the carbon that had just been detonated. Now here, this is the resonance form that's ultimately going to create our eight carbon ring. However, it is worth at the very least showcasing the other resident or relevant residents for because this lone pair of electrons on the alpha carbon can delocalized with the car bal. This results in an intermediate in which there is a carbon carbon double bond and a single bond between carbon and oxygen with oxygen having a negative formal charge. However, as mentioned previously, it is the first resonance form that is going to ultimately create our ring. So here we have our car B and I on here on position one attacking the other car bottle. So here the second car bal specifically its pi bond is going to be displaced and oxygen is now going to have a negative charge. Recall that this is a reversible reaction, which is why I'm using equilibrium arrows. So now we have our eight carbon ring and this is where I definitely recommend labeling your carbons just so that you can keep track of substituents. Let's pause for a moment and go ahead and label our carbon chain here. Carbon number one is going to be the EOL late carbon and carbon number eight is going to be the carbon carbon that it attacked. So ont carbon number two or position number two of the ring is one car bottle and branching off of carbon number eight is one methyl group as well as the oxygen that currently has a negative formal charge. All right. So from here, I can go ahead and erase our labels now that our substituents have been addressed. But now as mentioned before, oxygen does currently have a negative formal charge. So water is available in solution to go ahead and proton need it. And so from here, we have a hydroxy group on position number eight of the ring. But our ultimate product is an alpha beta unsaturated compound. This means that our next step is going to be a dehydration type reaction in which water is ultimately removed from the molecule to produce an alkene. So here we notice that there is one carbon in between the car bal of position two of the ring and carbon number eight, which contains the hydroxy group. This position is an alpha carbon relative to a carbot. So with the hydroxide ion that we have in solution, it can be detonated to produce a resonance stable intermediate, another EOL ion. So here is a resonance stabilized anion. And from here, we've arrived to the last step of the mechanism because here the Enola can establish a double bond between the alpha and beta positions relative to the car bal, which displaces are hydroxy group. Now, it is worth mentioning that normally a hydroxy group is not a suitable leaving group because hydroxide is such a strong base. However, this scenario is an exception because there is already hydroxide ion in solution. And it's because of that, that this reaction proceeds as such. So now our mechanism is complete. So let's briefly discuss the last part of the question which is why this particular ring closure may not be favored. Now, the reason why this ring closure may not be favored is simply because of the size of the ring that's being produced here in order to create an eight carbon ring, both carbonel groups, both the nucleo file and the electro file that is have to be further apart. And that is going to decrease the probability of an intra molecular attack happening in the first place if a cyclist reaction produces a five or a six carbon ring, that would be more favorable but not necessarily an eight carbon ring. All right. So with that being said, our final answer is complete here is our mechanism for the self condensation reaction. And so to address the second part of the question, the answer reads as follows, it says that the reaction is not favored because the Nile and the electro are further apart and the probability of an intra molecular attack is decreased. And so if you watch this video all the way up to this point, thank you so very much for doing so. And I hope you found this helpful.

Show how octane-2,7-dione might cyclize to a cycloheptenone. Explain why ring closure to the cycloheptenone is not favored.

Key Concepts

Cyclization Reactions

Stability of Cyclic Compounds

Thermodynamics of Ring Closure

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