January 21, 2022, 06:55:16 PM
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Topic: Make arrow pushing representation of hydrolysis of an alkyl hydrogen sulfate?  (Read 5207 times)

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Offline Agent-X

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Hi, everyone.

I'm trying to figure out how to make an arrow pushing representation of the hydrolysis of an alkyl hydrogen sulfate. So far, I cannot figure out how exactly to do it. I'm under the impression that the hydrolysis is perhaps a two-step reaction mechanism? I'm not sure, and I was hoping someone here could help me figure that out.

Attached is an image describing what I've found on the Internet, but I don't find it good enough.
It doesn't really say how the insertion of water breaks the sulfate bond; it gives the impression that the sulfate bond breaks itself. Furthermore, it seems as though that water then adds to the carbocation that remains.

The things represented in the attachment make me think that the hydrolysis of an alkyl hydrogen sulfate is two-step, but I have been told that it is a one-step, biomolecular reaction mechanism. The professor offered no drawing, though.

Could someone please draw a more accurate representation or an argument for one version or the other?
Thanks.

Diagram 1 = Overall reaction
Diagram 2 = Process 1 = addition of sulfuric acid
Diagram 3 = Process 2 = hydrolysis of alkyl hydrogen sulfate
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Offline KritikalMass

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There are a couple of errors in your drawing that may be throwing you off.

For the overall reaction (Diagram 1) you drew 2,3-dimethyl-2-butene, but in Diagram 2 you drew  2-methyl-2-butene. Also, your hydrogen sulfate anion at one point has 5 oxygen in it. Think it would be helpful to you if you drew out the structure of the hydrogen sulfate. That would probably clear things up for you since you may have to do some arrow pushing there.

Actually, the more I look at it am starting to think that the acid just acts as a catalyst and doesn't even add across the DB. Think the water adds to the carbocation and then the hydrogen sulfate anion comes in and pulls a hydrogen off of the -(OH2)+

Wish I had a program to draw it out for ya. Maybe someone will come along and be able to show you what I am trying to say or maybe you will understand what I'm talking about.

Offline Agent-X

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...

I didn't make this diagram. I grabbed it from a .doc I found on the Internet, and tried meshing it together.

Anyway, yeah, the molecule in diagram one is not representative of those in diagrams 2 and 3. I should have done something about that. Imagine that diagram 1 shows 3-methyl-2-butene.
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Offline macman104

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I agree with KritikalMass, the acid is a catalyst to create the carbocation.  Then water attacks (the sulfate anion is pretty  stabilized and non-nucleophilic), and the sulfate anion pulls off a hydrogen from OH2+ to recreate the acid.

Offline KritikalMass

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I agree with KritikalMass, the acid is a catalyst to create the carbocation.  Then water attacks (the sulfate anion is pretty  stabilized and non-nucleophilic), and the sulfate anion pulls off a hydrogen from OH2+ to recreate the acid.
This makes me feel much better after posting a horrible answer in a TLC thread a few days ago. I botched that thing up royally! In fact, my answers were so wrong it was very embarrassing- I was wrong about every single thing I posted.

However, it was a learning experience because the Ochem Gods came in with the right answer and really cleared things up. Even though I still looked like a fool, I still learned a thing or two.

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