Propose a mechanism for the reaction of methyl isocyanate with...

Question

Propose a mechanism for the reaction of methyl isocyanate with 1-naphthol to give Sevin® insecticide.

Accepted Answer

All right. Hi, everyone. So for this question, let's draw a suitable mechanism for the reaction between pheno and methyl isocyanate to produce a urethane compound. So recall really quickly, right, that another way of referring to a urethane compound like the one we have in our product here is a carbonate ester, right? And carbonate ester or urethane compounds are compounds that have both an Aloxi group and an amino group attached to a car bottle. So one way in which we can prepare carbonate ester or urethane compounds for that matter is by adding an alcohol or a phey to an isocyanate compound. So if you go ahead and analyze our starting materials and our products, what you'll notice is that there is a new bond between the hydroxy oxygen of our phey and the carbon in our society because notice how the carbon in between nitrogen and oxygen in my isocyanate is what's going to create our carbonyl at the very end, right. So for the purposes of our mechanism, I've gone ahead and I've redrawn methyl isocyanate and pheno on the bottom here. So the first step if you recall is going to involve the nucleic attack of the phenolic oxygen to the carbon of our isocyanate. Because that way we start off by creating the new bond between our pheno and our isocyanate because the carbon of our isocyanate is going to form the carbon. So here the phenolic oxygen is going to attack my isocyanate carbon. And now as a consequence, the pi electrons in between carbon and oxygen in my isocyanate are going to move towards oxygen. So that gives me the following intermediate. Here is my benzene ring and here is my phenolic oxygen. So now here's my carbon. And now you can see is that both compounds are now combined together. So here's the rest of my isocyanate. So not only does the the oxygen from my isocyanate have a negative charge, the oxygen from my pheno is going to have a positive charge because it has an extra covalent bond. And what's worth mentioning here is that the intermediate compound that I have on the screen after my first step is stabilized via resonance because here, right, a lone pair of electrons on my negatively charged oxygen can't form a carbon. And as a consequence of this, the pi electrons between carbon and nitrogen can shift towards nitrogen. So this is a resonance, right? So I'm going to draw the appropriate brackets. And as a result, we end up with the following. Here's my benzene ring, my phenolic oxygen is still going to have a positive charge for now. So let me go ahead and draw that. But now, right in my isocyanate or rather the part of my intermediate that comes from my isocyanate. I'm going to have a double bond in between oxygen and carbon and a single bond between carbon and nitrogen. However, right, because of the movement of these electrons between carbon and nitrogen in my resonance nitrogen is going to have a negative charge. So now let me go ahead and close these brackets and I'm actually going to move it over a bit before I proceed. There we go. So notice how two things need to occur. In order for our final urethane compound to form, right. Number one nitrogen has to gain a proton. And number two, our phenolic oxygen has to lose the proton that it has, right? So that both of these atoms can have neutral charges. So here, right, two steps have to occur. Recalled a molecule of phenol was protend actually no, excuse me, it wasn't pronated, but we did react with one molecule of pheno in the first step. However, when it comes to having pheno in solution, we don't have just one molecule, right. So for this step, we can go ahead and introduce a second molecule of pheno in this next step. So let me go ahead and move some space like so right. So here, here's my second molecule pheno and I'm going to actually leave it like this. So here my second molecule of phenol can go ahead and react as a base due to the lone pairs of electrons on oxygen. So here my second molecule of phenol can go ahead and dep proin, it can go ahead and detonate the phenolic proton in my intermediate. So that solves one problem, right. And as a consequence of this acid based reaction, we're also going to see the conjugate acid of pal because here pheno has gained an extra proton as a result. So that extra proton is of importance to my negatively charged nitrogen here. So in this step, right, my negatively charged nitrogen can go ahead and take a proton from the conjugate acid of phenyl. And I'm going to go ahead and actually rearrange these compounds around. So you can better see what's happening, right. I'm going to go ahead and move pal to the top of my arrow, whereas its conjugate acid is going to go on the bottom instead, there we go. So once again, right. Pheno is going to take my proton here and then my negatively charged nitrogen is going to remove the proton from the conjugate acid of pheno. And that is going to yield my final product, which is my neutro urethane compound. So here's my carbonyl coming from my isocyanate are nitrogen and our methyl group, right. So there you have it and here is our final reaction. So just as a quick recap, let's go ahead and go through each of the following the first step involved the nucleic attack of the phenolic oxygen onto the carbon atom of our isocyanate to produce a resonant stabilized intermediate. No, as a result of resonance, right, we generated a car bottle and we also created a negative charge on the nitrogen from isocyanate. So to facilitate the formation of the neutro urethane compound, the transfer of a proton through the solvent. In this case, Phey is what yielded the final neutral urethane compound. So a second molecule of pheno derotated, the phenolic hydrogen from are intermediate and then the negatively charged nitrogen detonated the conjugate acid of phey. So there you have it. And with that being said, thank you so much for watching and I hope you found this helpful.

Propose a mechanism for the reaction of methyl isocyanate with 1-naphthol to give Sevin® insecticide.

Key Concepts

Isocyanate Reactivity

Nucleophilic Aromatic Substitution

Formation of Carbamate

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