Some alcohols undergo rearrangement or other unwanted side
react...

Question

Some alcohols undergo rearrangement or other unwanted side reactions when they dehydrate in acid. Alcohols may be dehydrated under mildly basic conditions using phosphorus oxy-chloride (POCl3) in pyridine. The alcohol reacts with phosphorus oxychloride much like it reacts with tosyl chloride (Section 11-5), displacing a chloride ion from phosphorus to give an alkyl dichlorophosphate ester. The dichlorophosphate group is an outstanding leaving group. Pyridine reacts as a base with the dichlorophosphate ester to give an E2 elimination. Propose a mechanism for the dehydration of cyclohexanol by POCl3 in pyridine.

Accepted Answer

All right. Hi everyone. So this question looks like a mouthful. But the good news is that at the very least it gives us information that we need to solve it. So let's get started, alcohols can undergo dehydration when they are mixed with phosphorus oxy chloride or P. O. C. 03 impurity. In the reaction between an alcohol and phosphorus oxy chloride. The alcohol displaces a chloride ion from phosphorus to give an all killed by chloral phosphate ester just to jump in really quick. That is important because if the alcohol is displacing a chloride ion, that means that we have a nuclear feeling attack as our first step. Now keep that in mind, let's continue. Then the Dick laurel phosphate ester that forms from the previous step reacts with period in base to give an E. two elimination. So with this in mind we must suggest a mechanism for the dehydration of cyclops fentaNYL by P. O. C. L. Three impurity. This is a really interesting question. So let's go ahead and get started by drawing out our starting material. Cyclops fentaNYL is a five member ring with an O. H. Group attached. So this is our O. H. Group. And Now let's get started by redrawing our p. o. c. l. three. So I'm gonna redraw it over here, C. 03. Perfect. So now if the alcohol displaces the chloride ion from phosphorus, that means that it acts as a nuclear file and attacks phosphorus, therefore kicking out a chloride ion. So, in our intermediate step here we have the same five member ring. We have our O. H. But now the O. H. Is attached to P O. C. L. Two because one chlorine has been removed. But this is not the end of the story because the hydrogen is still attached to oxygen. The oxygen has a positive charge, which it's not a fan of. But the good news at the very least is that we have a base in solution, we have purity and period in, you can go ahead period in can go ahead and take this hydrogen from oxygen. So I'm actually gonna scroll down to give myself some space because this looks a little crowded. So I scrolled down to give myself space. And our next step paradigm has come in and save the day pretty much because now our oxygen here has a neutral charge, it has no charge, I should say. So now, just to scroll up for a quick second, Our Dick oral phosphate ester that we have is going to react with pirating again to give an E two elimination. So for an E two elimination recall that we need beta hydrogen. So this like oral phosphate ester is a fantastic leading group and it's on the alpha carbon. So the beta carbons therefore are going to be directly next to the alpha carbons and beta carbons have beta hydrogen attached to them. So for our next step period in, like we said before is going to come back in and it's going to take that beta hydrogen notice how for cyclops fentaNYL, the beta hydrogen are the exact same because there's symmetry, so it really doesn't matter which one you pick, the product will be the same. So let me just get rid of the alpha really quickly so that you can see how this pair of electrons holding the beta hydrogen is going to jump in between the alpha and the beta carbons, thus kicking out the dick laurel phosphate ester. And so this is going to yield our final product. It's cyclo Pantene. We have the same five member ring but with a double bond inside. And this is your answer right? Like the um like the text of the question was singing. We started off with cyclo sentinel that cyclops fentaNYL functioned as a nuclear file to go ahead and displace a chloride from P O. C. 03 and that therefore was then eliminated by period ng to generate cyclo painting at the end. So thanks so much for watching and I hope this was helpful

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