January 24, 2022, 05:42:43 AM
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Topic: Different kinds of reactions...What are the nonreactive cmpds there for...?  (Read 3516 times)

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Offline Lo.Lee.Ta.

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There are many different kinds of reaction, but it seems evey one of them has these extra compounds on the reaction arrow (:rarrow:) that don't even do anything...
They say they are catalysts or whatever, but what are they even doing???

For instance, I was writing out what the product is from 1-hexyne with H20 and H2SO4 and HgSo4.

They call this sort of reaction a keto-enol tautomerism reaction. 

So first it produces an enol, and then it rearranges to form a keto tautomer.
But really, the H2O is the only thing here that actually reacted to form a new product. The O and the 2 H'S that got added were only from the H2O.

So what is the point of the other stuff in the reaction? I don't see how the H2SO4 or the HgSO4 did anything here... I guess this reaction would not have taken place without them, but WHY NOT???

Thanks for responding! You're awesome!  :)

Offline discodermolide

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The reaction may be overall addition of O and 2H's that got added, but how did they get there?
The acid and the Hg salt are doing something, what?
If the were not needed then they would not be there!
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Offline Lo.Lee.Ta.

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Yeah, that's what I'm wondering.
How does the Hg salt and the acid even affect the reaction?
I know that it does affect it; otherwise, it would not be added.
But HOW is it doing anything?

Offline discodermolide

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Well I know why they are there, but forum rules say you must attempt to find out for yourself.
So I'll give you a hint: oxymercuration.
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Offline Lo.Lee.Ta.

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Thank you for responding and the hint! Obviously, I did not understand the entire process of what goes on here!

Alright! So it all starts off with an oxymurcuration reaction.
CH3CH2CH2CH2C≡CH --(H20, H2SO4, HgSO4)-->

Two electrons from the triple bond pull over the H2SO4 and bond with it.

Now the other carbon is lacking 2 electrons, so it becomes a carbocation.

The molecule of water decides it will  supply its two electron and bond with the carbocation.

Now, apparantly, another molecule of water comes along and sees the water (having a plus + chaarge) is not very happy being bonded to the molecule,  so comes and takes off one of the H's one the water.

Now the water that was bonded to the molecules is happy because it is now an O bonded to an H with 2 lone pairs.

The other molecule that took the H has now become an H3O+.

Now there is a switch of subtituents that takes place. The HgSO4 bonded to the molecule leaves and gets replaced by an H, which left the newly-formed H3O+.

So now we have:
CH3CH2CH2CH2C(OH)=CH2 <--- This is the enol!

And NOW the Keto-enol tautomerization takes place!

The 2 electrons from the double bond now grabs away the H that was attached to the OH and bonds to it.
Now the O has an abundance of electrons, so it forms a double bond with the carbon it is attached to.

The final result is: CH3CH2CH2CH2C(=O)-CH3

Offline discodermolide

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I like your flowery description of molecules deciding to do things and molecules being happy or unhappy entities.

Basically you are correct. For a more "scientific description" see:
http://faculty.sheltonstate.edu/~rsaylor/221test3/RxnMechAlkynes.pdf
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Offline Lo.Lee.Ta.

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Haha, you're funny! Thanks! :)

Still, though, it seemed that we used 2 H2O molecules and one HgSO4 molecule.
But we never used the H2SO4...
Why is that? Shouldn't we have used everything?
And how did we get to use 2 H2O molecules when it seems we can only use 1?

Thank you so much for helping me!!! :)

Offline discodermolide

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Here is perhaps a better description of the mechanism, involving the acid.

http://chem.348.wsu.edu/files/Lectures/lecture%2016.pdf
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Offline orgopete

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Internal acetylenes can react directly with H2SO4 and water without mercury, but terminal acetylenes may react vey slowly or not at all. Remember, carbon is a better electron donor than a proton, that is why internal acetylenes react faster with electrophiles. Mercury first coordinates with the triple bond. Water then adds to give a mercury enol intermediate.

This addition should only require a catalytic amount of mercury as the mercury cation is regenerated.
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Offline Lo.Lee.Ta.

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Oh, WOA. Okay! So is it that the H2SO4 in this particular problem acts just as the second molecule of water that I mistakenly added in the reaction I wrote out?

(I was following what happened in the oxymercuration reaction that is written in my chem book. But wouldn't there be the same enol product either way - if it was a water molecule or H2SO4?

It looks like an H breaks off from the H2SO4 and the resulting HSO4- takes the H from the water molecule, which is bonded to the large overall molecule. (By the way- is there a name for the large overall molecule?  ??? thanks!)

Then the H (that was separated from the H2SO4) switches places with the HgSO4.

So now we have an enol: CH3CH2CH2CH2C(OH)=CH2

Now the 2 electrons from the double bond snatch away the H from the OH, making the O have an abundance of electrons. The O then forms a double bond with the carbon it is bonded to.

The final result is: CH3CH2CH2CH2C(=O)-CH3  +  HgSO4  +  H2SO4

Wow! After this long process, the water is the only thing that is used up!
So is this really right?

Thanks for helping me! :)



Offline discodermolide

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Re: Different kinds of reactions...What are the nonreactive cmpds there for...?
« Reply #10 on: November 21, 2012, 02:56:02 AM »
That is effectively the case.
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