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Offline Yeffer

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Most acidic hydrogen
« on: May 20, 2009, 11:30:45 AM »
Hello,
I recently took a certain well known Orgo end of year exam.
There was a question on it that bore no relation to what I learned in class, just one of many really.

It went something like this. What is the most acidic hydrogen?

Ph-CH2-CH=CH-C=-(triple)CH

I assumed it was the hydrogens at the CH2, because they were less 'tethered' if you will by nearby electrons, whose negative charge would attract. In addition, when a CH3 is on a benzene ring, it's an electron donator, so if its e- were inclined to abandon the carbon that would be all the more reason why the two H's would pop off.

However the correct answer was the H at the end, next to two pi bonds and therefore lots of negative charge.

Can anyone explain simply why this is?

Thanks!

Offline StarvinMarvin

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Re: Most acidic hydrogen
« Reply #1 on: May 20, 2009, 11:46:56 AM »
terminal alkyne hydrogen is the most acidic simply due to the nature of the triple C-C bond. Since it's the shortest bond the electron density is also the highest in that region. Therefore, the C-H bonding electron pair is shifted towards the carbon atom. That makes this particular site the easiest to deprotonate. Right?

Offline lavoisier

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Re: Most acidic hydrogen
« Reply #2 on: May 20, 2009, 01:37:42 PM »
The pKA of an alkyne should be around 25.
A diarylmethane (that you might assume to be a decent approximation of PhCH2-vinyl-vinyl - or could you?) around 34.

So apparently there is no question, the alkyne is more acidic.

But I have to admit I had to look it up, that's the kind of thing you forget if you only study it at school and then don't apply in practice.

You can't really derive it from pure reasoning, because OK, they tell you the alkyne has a strong 's character', therefore its carbons are very electronegative, therefore the proton is very acidic, but at the same time you have an extensive conjugated system that should provide very good stabilization for the anion formed on that CH2.
That's all qualitative information, but how do you quantify it? It's one of those things you need to learn by heart, full stop.

Therefore, a rather malicious question in an exam, I'd say.

Offline Yeffer

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Re: Most acidic hydrogen
« Reply #3 on: May 20, 2009, 09:47:58 PM »
Thanks folks.

I agree it was a rather malicious test question, but then it was the comprehensive exam from a well known organization (that one).

Two days before the exam I heard there were official practice questions available. I looked at them as did other students and we were mostly mystified...

FWIW, we used the McMurry book.

Offline azmanam

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Re: Most acidic hydrogen
« Reply #4 on: May 20, 2009, 09:52:46 PM »
one minor correction:

You're asking for the most acidic proton, not hydrogen.
Knowing why you got a question wrong is better than knowing that you got a question right.

Offline 408

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Re: Most acidic hydrogen
« Reply #5 on: May 20, 2009, 10:31:46 PM »
That is not a malicious question.  Intro O chem spends significant time talking about pkas of various CH protons and the related acid-base chemistry.

Offline lavoisier

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Re: Most acidic hydrogen
« Reply #6 on: May 21, 2009, 09:13:09 AM »
408,
I only meant to say that it wasn't a typical case, like acetylene vs toluene, and the presence of an unusual, very extended conjugated system might have induced the examinand to think that the acidity of that benzylic proton was higher than usual.

Then if you really need to humiliate Yeffer by telling him that actually it was a very simple question and he should have known the answer, I'm in no position to stop you.

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