Propose a mechanism for each of the following reactions: b.

Question

Chemical reaction showing tautomerization with H2SO4 catalyst, converting a ketone to an alcohol.

Accepted Answer

Alright. Hi, everyone. So for this question, it's proposed a suitable mechanism for the reaction given below. And our starting material has a ketone as part of a six member ring with an isopropyl and an ethyl substitue in on the same carbon. And in our final product, we have a benzene ring with an O H group instead of a carbonnel and our isopropyl and ethel substitue ints have shifted over right there on different carbons. Now. Mhm. Now, the fact that we have our isopropyl and methyl substitution on different carbons in the final product is going to indicate that there has been a carbon Acadian rearrangement in particular via and alcohol shift, right. So our intermediate is going to be a carbon cat ion in our reaction mechanism and that carbon cat ion is going to rearrange itself to go from a less stable form to a more stable one. So in order to show that right, our first step is going to be the pro donation of our carbon oxygen via our H two S 04. So I'm gonna go ahead and draw out our H two S so that we can get started, right. So this is an expanded structure Of H two S 04. So one of the lone pairs of electrons on oxygen is going to snatch up a proton from it and we're going to end up with a carbonnel. But this time the oxygen is going to have a formal charge of plus one. So here's our carbonnel oxygen and now, like I mentioned before, we have an extra hydrogen, which gives our oxygen here a positive charge or rather a formal charge of plus one, right? So notice how this O H plus group in our intermediate step kind of looks like the O H group of our product, right? The only difference is that the O H group here is positive because of this car bottle, but that can be fixed if the pi electrons of our car bottle shift upwards towards oxygen. Right? Mhm But at the same time that that's happening, one of the pi bonds inside of our ring structure is going to shift over to prevent this carbon carbon from becoming positive, right? So what we end up with is a neutral, a wage group instead of that car bomb. So here's our neutral O H and now like I mentioned before, one of our pi bonds has shifted over which creates a carbo cat ion right next to the carbon that has our isopropyl and methyl substitutes, right? So this carbon copy on here is secondary but it can rearrange to be a more stable tertiary carbon copy, meaning that either are isopropyl group or ethyl group are going to move over. Right. And in order to explore both possibilities, let's recall really quickly that carbon cat ions in this case, are going to be stabilized via hyper conjugation. Right. And hyper conjugation is the overlap of a carbo cat ions empty P orbital with adjacent sigma bonds. So in particular, the number of hyper congregations that stabilize a carbon cat ion is going to be equal to the number of sp three carbon hydrogen bonds that you count. So To explore one possibility, I'm gonna go ahead and redraw intermediate on the side here. Mhm And let's say for example, that our isopropyl group moves over, right, that's going to create a carpool cat ion associated to our ethyl group. Now, this ethyl group has two adjacent sp three carbon hydrogen bonds which can stabilize this um this carbon cat eye on here. So that's one possibility to keep in mind. But what if it's our ethyl group that moves instead? So let me erase this. And what if it's the fo group that moves instead? Right. That means that our carbon cat ion is going to be stabilized by the isopropyl group it's attached to. And this isopropyl group has just one adjacent sp three carbon hydrogen bond. So because the ethyl substitue in has more sp three carbon hydrogen bonds, it is better capable of stabilizing this carbon cat eye on via hyper conjugation. So in our mechanism, that's what's going to happen, right. Our isopropyl group is going to move over and our tertiary carbon cat ion is going to be stabilized via the two hyper congregations of the associated ethyl group. Yeah. So now we have a more stable carbon cat ion, but we have to get rid of it, right? Because our final product is neutral. So that can be remedied by the deep throat nation of our carbon that has our isopropyl substitution. So recall that in this case, water can behave as a base here and it can get rid of that hydrogen. It can get rid of that hydrogen to create a double bond in between those carbons that have our isopropyl and ethos substitue ints. Mhm. So now here's our fully conjugated benzene ring, here's our isopropyl and here's our ethyl. Mhm. And now here's R O H group. So just as a quick recap, right? If I scroll up to the top, then our first step was the proto nation of my carbon oxygen with sulfuric acid, creating a positively charged car bottle oxygen which turned into a neutral wage group. It turned into a neutral O H group and created a carbo cat ion in the process which could rearrange itself from a secondary to a more stable tertiary one. And from there, the deportation of one of our ring carbons created a pi bond or rather an additional pi bond to create that benzene ring in our final product. So this corresponds to option B in the multiple choice. And I wrote that in blue, which I didn't want to, but that corresponds to option B within the multiple choice. And so if you stuck around to the end, thank you very much for watching and I hope you found this helpful.

Propose a mechanism for each of the following reactions: b.

Key Concepts

Tautomerization

Acid-Catalyzed Reactions

Mechanism of Tautomerization

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