[orgo814]

Looking at the zwittterion of glycine (NH3+ and COO-), how does the carboxylate being an electron withdrawing group make it harder for NH3 to be deprotonated? I always thought an electron withdrawing group would make it easier but according to my book that's not the case. Any explanation welcome

[Babcock_Hall]

Consider the pK_a values of the ammonium group in glycine versus glycylglycine.

[orgo814]

Yes but my question is WHY

[Babcock_Hall]

One might use a model based on electrostatics.  The pK_a of glycine methylester is 7.75, whereas for glycine it is 9.78.  BTW the pK_a values of glycylglycine are 3.06 and 8.13 (another source give 8.25 for the second pK_a).

[rwiew]

Looking at the zwittterion of glycine (NH3+ and COO-), how does the carboxylate being an electron withdrawing group make it harder for NH3 to be deprotonated? I always thought an electron withdrawing group would make it easier but according to my book that's not the case. Any explanation welcome

The carboxylate makes it EASIER for NH₃⁺ to become deprotonated, exactly what you expected. I assume you compared the pKa's for glycine (9.6) to ammonium (9.3) ? That's deceptive, you must consider that the amine group has an alkyl on it in glycine as well, hence we must compare to a primary amine, methylamine for example (pKa = 10.6). NH₃⁺ in glycine has now a lower pKa and is easier to deprotonate.

[Babcock_Hall]

rwiew,

The ammonium group of glycine is a weaker acid than the ammonium group of glycine methylester, meaning that the ammonia group of glycine is a stronger base than the ammonia group of glycine methylester.  I think that the electrostatic attraction between the ammonium group and the carboxylate group is the most obvious explanation.  It also explains why the carboxylic acid group in glycine is a stronger acid than a typical carboxylic acid.

[AdiDex]

I think...(My logic , i haven't read it anywhere)
The answer is very simple..
Intramolecular Hydrogen bonding between H and O .
Think like this..
The N+ have more polarizing power then N (Neutral).
So N+ polarizes H to increase the efficiency and extent of Hydrogen bonding..
And if De-Protanation occurs, the extent of hydrogen bonding will decrease....
That why De-Protanation is slow....

[orgopete]

Looking at the zwittterion of glycine (NH3+ and COO-), how does the carboxylate being an electron withdrawing group make it harder for NH3 to be deprotonated? I always thought an electron withdrawing group would make it easier but according to my book that's not the case. Any explanation welcome

@butlerw2
I am in agreement with you on this. I suspect there may be something missing in this question taken from your textbook. As I would parse the question, "How does the carboxylate being an electron withdrawing group make it harder for R-NH3(+) to be deprotonated?" For me, I would draw an analogy of glycine with ethyl amine. An ethyl ammonium cation is a weaker acid than the ammonium group of glycine. I assume this was your thinking.

Reading the posts and since your textbook had the opposite answer, I should assume they are drawing a different analogy or this is an error in your textbook. They do occur. 

[AdiDex]

Just look at that hydrogen bonding...

[rwiew]

adidx, where is this figure from, particularly the numbers? You said you did not read about this anywhere, so where is it coming from?

I don't like this hydrogen binding argument, intramolecular H-bonding in a 6-membered ring no problem, but a 5-membered ring I think is too unfavorable deviation from the standard angles.

Babcock_Hall, I don't understand why are you making a comparison to the methyl ester of glycine and dismissing my comparison to methylamine? I believe the comparison to a primary amine reveals the inductive effect of the carboxylic group. Your comparison to the methyl ester is of course very useful as well - but I don't think electrostatics (why would the molecule want its ends pulled together? That might be enthalpically ok, but screams against the entropy) or hydrogen bonding play a role. Please prove me wrong, I will take some data for your models.

I have seen this numerous times in textbooks and in conversations with organic physical chemists - the inductive model is quite well discussed. Good you mentioned Gly-Gly as well - so the trend is Gly, Gly-Gly, Gly-OMe - acidity of the ammonium increases. Around the ammonium's pKa's the carboxylate in these molecules exists as: C(O)O^-, C(O)NHR, C(O)OMe. The electron density of the carboxylic moiety decreases in the same order (see IR data for the C=O stretch for confirmation) and hence the inductive electron withdrawing capabilities of the group increase - this leads to the destabilization of the ammonium and makes it a progressively stronger acid. Of course the increase in electron density is explained by a) the anionic O^- being the best donor for the reasons of delocalization of charge, b) amide nitrogen being a better lone pair donor than ester's oxygen because of the better overlap with the C=O π*.

The increased acidity is similarly explained by the inductive withdrawing effects of the NH₂ group.

What do you think?

[AdiDex]

@Rwiew

Yep, i didn't read about it , It just was a picture .

Sorry , I didn't think that it will be a 5 membered ring.

[Babcock_Hall]

Let's try arranging three compounds in order of decreasing pK_a values of the conjugate acid.  Methylamine (10.6); glycine (9.6); glycine ester (7.75).  This order means that the conjugate acid of methylamine is the weakest acid.  My initial hypothesis is that the methyl group stabilizes the positive charge on nitrogen by releasing electron density through induction.  The carbonyl functional group of glycine ester is the most destabilizing towards the positive charge by removal of electron density through induction.  Glycine itself is in the middle.  Possibly the reason is that two effects, induction and electrostatics, partially offset each other.  BTW Williams' Table of pK_a values lists glycine ester, but it is unclear what the esterifying group is.

[rwiew]

The carbonyl functional group of glycine ester is the most destabilizing towards the positive charge by removal of electron density through induction.  Glycine itself is in the middle.  Possibly the reason is that two effects, induction and electrostatics, partially offset each other. 

Yeah, I would just say that the carboxylic group of glycine itself is not as inductively electron withdrawing, because it is deprotonated by the time you get to the pH at which the NH3+ can be deprotonated. In the same way as the IR stretch of a (deprotonated) carboxylate is much lower than that of an ester. But maybe there could be some electrostatics going on - what exactly do you mean by that actually?

[Babcock_Hall]

The negative charge on the carboxylate group stabilizes the positive charge on the ammonium group, making the ammonium ion a weaker acid, all else held equal.  If the negative charge is moved farther away (glycylglycine) or removed altogether (glycine ethylester), then the ammonium ion becomes a stronger acid.

[rwiew]

Ok, I was just sceptical about a) whether hydration of the two charges won't be much more important and remove this electrostatic effect, b) any negative effects on the molecular geometry because of the charges attracting each other. Had a look around though and found this, which seems to be a good discussion of the argument you are using: http://pubs.acs.org/doi/pdf/10.1021/ja001749c and indeed they postulate that.