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Author Topic: how to identify acidic protons  (Read 28225 times)

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hpl912

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how to identify acidic protons
« on: October 11, 2008, 04:17:53 PM »

How do you know which protons are the acidic ones in a compound?
like for carbonyl compounds, the acidic protons are attached to the alpha position carbon?
but how about for any other compounds except carbonyl ones? are they at the alpha position of the functional group too?
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spirochete

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Re: how to identify acidic protons
« Reply #1 on: October 11, 2008, 05:37:06 PM »

The two biggest influences on acidity are electronegativity and resonance.  Both influences are ultimately derived from the concept that a strong acid has a stable conjugate base.   

If loss of a proton places a negative charge (or more generally an extra lone pair) on a more electronegative atom than that proton will be more acidic.

Secondly, if loss of a proton allows a pair of electrons to be delocalized by resonance this increases the acidity of that proton.  This second one is more tricky and must be taken on a case by case basis.  The first step to understanding is having a solid ability to draw resonance structures.

For example look at the molecule methyl ethanoate: http://en.wikipedia.org/wiki/Methyl_acetate

It has two sets of protons, one of which is much more acidic.  Could you guess which is more acidic and why?
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hpl912

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Re: how to identify acidic protons
« Reply #2 on: October 12, 2008, 01:19:58 AM »

thanks for the explanation :)

you said a strong acid has a stable conjugate base, how would you know it's a strong or not the conjugate base? and to get the conjugate base, you take out an acidic proton right?

about the methyl ethanoate, i would say the protons at the left side of the molecule are the more acidic ones because they will give a negative charge to the oxygen where as the right side protons will give a positive charge to the right side oxygen, is that right?
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nielsgeode

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Re: how to identify acidic protons
« Reply #3 on: October 12, 2008, 01:51:03 AM »

you are right   ;D
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spirochete

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Re: how to identify acidic protons
« Reply #4 on: October 12, 2008, 05:55:38 AM »

thanks for the explanation :)

you said a strong acid has a stable conjugate base, how would you know it's a strong or not the conjugate base? and to get the conjugate base, you take out an acidic proton right?

about the methyl ethanoate, i would say the protons at the left side of the molecule are the more acidic ones because they will give a negative charge to the oxygen where as the right side protons will give a positive charge to the right side oxygen, is that right?

I think you are on the right track but I'm not sure if your logic is 100% correct.  Removing a proton results in a negative charge, or loss of a positive charge.  So there's no additional positive charge to keep track of in the conjugate base like you were saying.  I would think about the new "free" electron pair that's created which may have a chance to be involved in resonance.

   Here's a brief explanation
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spirochete

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Re: how to identify acidic protons
« Reply #5 on: October 12, 2008, 08:14:14 AM »

where can I link from? 
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nielsgeode

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Re: how to identify acidic protons
« Reply #6 on: October 12, 2008, 09:33:55 AM »

where can I link from? 

if it is a jpg picture (or gif) you can upload it on www.tinypic.com
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spirochete

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Re: how to identify acidic protons
« Reply #7 on: October 12, 2008, 10:45:58 AM »

  We'll see if this one works.

One thing I should add is that the stability of the of the lower molecule may be enhanced slightly more by induction than the other, but resonance is typically a much stronger effect and therefore we can assume that the conjugate base on the top is more stable.

Induction is simply the ability of an electronegative atom to pull some electron density toward it through a sigma (aka single) bond.  This effect stabilizes a molecule with a negative charge by spreading it out slightly over the rest of the molecule.
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hpl912

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Re: how to identify acidic protons
« Reply #8 on: October 12, 2008, 12:17:04 PM »

i don't get it how come no resonance structures can be drawn for the bottom one? wouldn't the negative charge with its pair of electrons move onto that sigma bond next to it resulting in the right side oxygen getting a positive charge with a pi bond and sigma bond?
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nj_bartel

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Re: how to identify acidic protons
« Reply #9 on: October 12, 2008, 12:45:43 PM »

If you were to move that lone pair onto the ether oxygen, that oxygen would be negatively charged and have 10 valence electrons.
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hpl912

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Re: how to identify acidic protons
« Reply #10 on: October 12, 2008, 03:02:14 PM »

can you guys check if i'm on the right track and if i'm missing something... the order, the first is the most acidic compound.



#4 the most acidic because it has 3 more electronegative than oxygen atoms delocalizing the positive charge. no acidic protons
#1 the most resonance structures can be drawn from the ring and the carbonyl oxygen. acidic protons on the ring
#3 negative charge on carbonyl oxygen. acidic proton on the methyl.
#2 negative charge on carbonyl oxygen. acidic proton on methyl and the t-butyl group electron withdrawing?
#5 negative charges on the carbonyl oxygen and positive on the ether oxygens destabilized. no acidic protons
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spirochete

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Re: how to identify acidic protons
« Reply #11 on: October 12, 2008, 04:34:49 PM »

Does your school have a tutoring center?  A lot of this stuff would be much easier and faster to explain with step by step drawings on paper or a blackboard. 

IMO that question is very difficult for somebody starting out just learning about acids and bases.  Some of those molecules are not directly comparable and I'd have to look up the pKa's myself since I haven't studied them in a while.
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Re: how to identify acidic protons
« Reply #12 on: October 13, 2008, 02:53:43 AM »

From most acidic to least acidic: (i'll give you a short reasoning, but it would be up to you to go into your book and read more about each reason.  Like spirochete said, to fully explain all of these would be better 1-on-1)

MOST
5 - see malonic esters, lots of resonance structures
2 - ketones are more acidic than esters
3 - general ester - oxygen pushes electron density into the enolate, raising the pka of the conjugate acid
1 - no acidic alpha protons (and aromatic protons are extremely non-acidic), zero resonance structures
LEAST

#4's not really a fair question.  It will decompose into dichlorocarbene* almost immediately after deprotonation.  But we do that reaction with sodium alkoxides (deprotonated alcohols), so it must be a stronger acid than an alcohol. Thus #4 must be the most acidic, with a pka less than 15, although I don't like including that in this list.  The pka is not listed in Evans' pka table, Bordwell's pka table or the Merck index.

*
http://en.wikipedia.org/wiki/Dichlorocarbene
Evans - http://www2.lsdiv.harvard.edu/labs/evans/pdf/evans_pKa_table.pdf
Bordwell - http://www.chem.wisc.edu/areas/reich/pkatable/index.htm

in general:
http://www.cem.msu.edu/~reusch/VirtTxtJml/react1.htm#rx1b
http://www.cem.msu.edu/~reusch/VirtTxtJml/suppmnt2.htm#top4
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