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Author Topic: Organic Synthesis.. I bet you can't explain why?!!!!!!  (Read 5611 times)

Offline Gregorian

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My professor is trying to make para-benzenedithiol, HSC6H4SH, starting from the diamine,
H2NC6H4NH2, by doing two diazonium reactions with NaSH. It never worked,
always gave him the quinone instead.

The thing is he was able to get the ortho and the meta products, but it did not work for the para!!!


Can you explain why?!



Btw he is from UK, and I recall that he told me that he knows only one person (from UK) who can have an answer for this incident! or something like that


 

Offline Chemistry4me

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Organic Synthesis.. I bet you can't explain why?!!!!!!
« Reply #1 on: 20/03/2009 07:57:46 »
The funny thing is, the EXACT same question has been asked in a chemistry forum. :)
 

Offline lancenti

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Organic Synthesis.. I bet you can't explain why?!!!!!!
« Reply #2 on: 20/03/2009 09:21:28 »
It looks to me like a Nucleophilic Aromatic Substitution Reaction. (I could be wrong about the second step because I've no idea what the steps/intermediates look like.)

It would be nice to know what steps you tried, since that says a lot about where the reaction is going or if there's something you overlooked.

What I can say is that Thiophenol is an activating group so it stabilizes positive charges at the para and ortho positions. The Diazonium ion is reactive because it his highly nucleophilic.

Unfortunately, the diazonium at the para-position is in a sense 'deactivated' leading to no further substitution by SH-. I have no idea how you get the quinone from this, but I suspect the HNO2 to be responsible

The Meta Position is unaffected substantially by this and so reaction can go. (if you go draw the canonical structures)

The Ortho Position I find interesting. The presence of such an electron-rich S atom (which is huge) and the bulky N2+ group in such close proximity makes me believe that there should be a lot of steric strain around this area. I would go so far as to think that an S-N bond will form but obviously that doesn't happen since we're supposed to get our 1,2. At this point I'm guessing. Electrons don't stabilize the 2-position and thus still allow for a nucleophilic attack at the point? The steps would really help in drawing mechanisms to see what might actually be going on.
 

Offline Gregorian

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Organic Synthesis.. I bet you can't explain why?!!!!!!
« Reply #3 on: 20/03/2009 13:57:54 »
The funny thing is, the EXACT same question has been asked in a chemistry forum. :)

That is because I was the one who asked it on chemicalforums.com :)
You can also find the same question on yahoo answers :D
 

Offline Gregorian

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Organic Synthesis.. I bet you can't explain why?!!!!!!
« Reply #4 on: 20/03/2009 14:26:41 »
It looks to me like a Nucleophilic Aromatic Substitution Reaction. (I could be wrong about the second step because I've no idea what the steps/intermediates look like.)

It would be nice to know what steps you tried, since that says a lot about where the reaction is going or if there's something you overlooked.

What I can say is that Thiophenol is an activating group so it stabilizes positive charges at the para and ortho positions. The Diazonium ion is reactive because it his highly nucleophilic.

Unfortunately, the diazonium at the para-position is in a sense 'deactivated' leading to no further substitution by SH-. I have no idea how you get the quinone from this, but I suspect the HNO2 to be responsible

The Meta Position is unaffected substantially by this and so reaction can go. (if you go draw the canonical structures)

The Ortho Position I find interesting. The presence of such an electron-rich S atom (which is huge) and the bulky N2+ group in such close proximity makes me believe that there should be a lot of steric strain around this area. I would go so far as to think that an S-N bond will form but obviously that doesn't happen since we're supposed to get our 1,2. At this point I'm guessing. Electrons don't stabilize the 2-position and thus still allow for a nucleophilic attack at the point? The steps would really help in drawing mechanisms to see what might actually be going on.

I haven't done orgo for so long.. so I need to refresh on it.. but why do you say this ""the diazonium at the para-position is in a sense 'deactivated' leading to no further substitution by SH-"""?? I don't recall learning anything like that!


Quinone forms because the reaction is done in water


Quote
The thing is he was able to get the ortho and the meta products, but it did not work for the para!!!

oups I think I put "meta product" in there accidentally
« Last Edit: 20/03/2009 14:28:53 by Gregorian »
 

Offline Raghavendra

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Organic Synthesis.. I bet you can't explain why?!!!!!!
« Reply #5 on: 21/03/2009 08:43:05 »
 [:-'(] tooo easy ......   all our memembers are right,, you should search in google
 

Offline Chemistry4me

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Organic Synthesis.. I bet you can't explain why?!!!!!!
« Reply #6 on: 21/03/2009 12:48:36 »
Too easy? You really think so? :)
 

Offline lancenti

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Organic Synthesis.. I bet you can't explain why?!!!!!!
« Reply #7 on: 22/03/2009 16:08:42 »
The Thiol Group is electron donating, so it pushes electron density into the aromatic ring and more so into the Para and Ortho positions.

For the SNAr reaction, you need a Strong Withdrawing Group Para or Ortho (1,4 or 1,2) to the leaving group to form a strong partial positive charge. So the presence of a Strong Activating Group (-SH) might actually counteract that.

But then again Wikipedia says diazonium compounds substitute following a different (SN1) mechanism. So maybe the positive charge in the stabilized intermediate isn't enough to attract SH-. The added stability could also be used to explain a lower EA and thus more favourable forward reaction, so it's not really anywhere near a nail in the coffin. Alternatively, it may actually make the C-N bond stronger than it should be preventing it from leaving in the way that it should under the SN1 mechanism.
« Last Edit: 23/03/2009 12:55:18 by lancenti »
 

Offline Raghavendra

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Organic Synthesis.. I bet you can't explain why?!!!!!!
« Reply #8 on: 23/03/2009 10:49:07 »
Too easy? You really think so? :)

 What you think... it is
 

Offline Robert Lee

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Organic Synthesis.. I bet you can't explain why?!!!!!!
« Reply #9 on: 24/03/2009 15:57:55 »
That is because I was the one who asked it on chemicalforums.com :)
You can also find the same question on yahoo answers :D

Sorry to give you false hope by bumping the thread, as I really have no idea whatsoever as to the answer; but I'd just like to say could you please post the responses of yahoo answers?  Not intending to be snobbish or such, but I generally find them quite entertaining.
« Last Edit: 24/03/2009 16:04:11 by Robert Lee »
 

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Organic Synthesis.. I bet you can't explain why?!!!!!!
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