Meredith92

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Aug 29, 2012
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Two questions in this section really confused me (i appreciate you guys reading this- hopefully it will help you too!)

88. The carbon chain associated with aspartic acid is shorter than the one associated with glutamic acid, which results in a:
A stronger inductive effect, making aspartic acid's side chain more acidic
B weaker inductive effect, making aspartic acid's side chain more acidic
C stronger inductive effect, making aspartic acid's side chain less acidic
D weaker inductive effect making aspartic acids side chain less acidic

Answer: A
based on the pka values in table 1 (attached picture) aspartic acid (with a side chain pka of 3.88) is more acidic than glutamic acid (with a side chain pka of 4.32) this eliminates choice C and D (That makes sense to me). A shorter chain results in the electron withdrawing NH3+ group being closer to the side chain carboxylic acid group, which increases the acidity of the carboxylic acid group. this makes choice A the best answer

-->I originally chose B because i thought having a longer carbon chain would make glutamic acid have greater inductive effect (greater electron donation from more carbons) and therefore make glutamic acid less acidic and aspartic more acidic. It seems odd that the answer is based on the proximity to the electron withdrawing NH3+ group, when there is also a COO- group that is electron donating that it totally ignores. Can someone explain this?
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90. What is the normality of .5M glutamic acid in water?
A .5N
B1 N
C 1.5N
D 2 N

Answer: C
Normality is defined as the moles of equivalents per liter. because glutamic acid is triprotic it yields three acidic protons per molecule. for each mole of glutamic acid three moles of acidic protons can be generated. the normality in this case is the molarity multiplied by a factor of three.

-->Although this makes sense, I think the answer forgot to consider that water has a pH of 7. At a pH of 7, The carboxylic acid AND the R group (pKa 4.32) will be deprotonated. Therefore wouldnt there only be one equivalent of acidic hydrogen?

In general, with amino acids, in its neutral form the amino group is protonated and the carboxyllic group is deprotonated... so there is really only ONE equivalent of acidic hydrogen for an amino acid (ignoring the r group in this case). However, i think TBR would define it still as being diprotic. I think this is a really important point that I'm confused on, and may just have to do with the definition of normality. I would very much appreciate any help!!


Thank you guys so much!
 

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Meredith92

Meredith92

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Sorry this was such a long question- I guess one of my main questions ( to simplify it) is whether an amino acid with a neutral side chain is considered diprotic at physiological pH / in water. In this scenario the carboxylic group no longer has its protic hydrogen- so wouldnt the molecule only have 1 equivalent of hydrogen? (giving it a normality of 1x the concentration)

Thanks again!
 

baltimoreman

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Apr 21, 2012
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Sorry this was such a long question- I guess one of my main questions ( to simplify it) is whether an amino acid with a neutral side chain is considered diprotic at physiological pH / in water. In this scenario the carboxylic group no longer has its protic hydrogen- so wouldnt the molecule only have 1 equivalent of hydrogen? (giving it a normality of 1x the concentration)

Thanks again!
I don't think whether a compound is diprotic or not depends on the pH. You need to know how many sites can be protonated or deprotonated. An amino acid with a neutral side chain is diprotic. In this example, the amino acid is triprotic since it has a protic side chain, which gives normality=3xmolarity.
 

ToastyPremed

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Jul 26, 2013
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So, I stumbled into this question and was confused as well. I picked B thinking that it would be a trick question regarding pH, but when I was trying to justify how it was wrong, I was befuddled.

There's a conflict of understanding for me as to how protonation and deprotonation of amino acids work. pH of water should be 7. If that' the case, then the pH is less than the pKa of the amino group (8.8~) and thus, the amino group should remain in its protonated state, correct? If this is true, I thought that would imply a diprotic species.

Alternatively, the pKa of water is >14 but <20 (not sure if we're supposed to remember the exact value because I don't). As a result, water should be less acidic than the amino group...so water should pick up the proton? Then I guess it would be triprotic and C.

Or am I making a fundamental mistake by assuming any of this matters, and that we're just looking purely at how many acidic hydrogens could potentially exist? When does that pH greater or less than rule matter more than pKa in judging whether it'll be in a protonated or deprotonated state?

And to answer meredith's first question, though I dont think the original poster is still around, I think it's because the COO- functions as a EWG as well.
 

Heday

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88. The carbon chain associated with aspartic acid is shorter than the one associated with glutamic acid, which results in a:
A stronger inductive effect, making aspartic acid's side chain more acidic
B weaker inductive effect, making aspartic acid's side chain more acidic
C stronger inductive effect, making aspartic acid's side chain less acidic
D weaker inductive effect making aspartic acids side chain less acidic
So according to OP's picture, the aspartic acid is more acidic due to the smaller pKa. We can elminate C and D. Let's define induction as the polarization of sigma bonds. The acidic O--H is polarized already in both molecules. Now OP says that glutamic acid has greater electron donation(from more carbons), which is true. If there is greater electron donation from the extra carbon atom, then for the O--H bond, it becomes a little less polarized, correct? Therefore, glutamic acid actually experiences a weaker inductive effect (a weaker polarization of the sigma bond between O and the acidic H). This means that aspartic acid has a stronger inductive effect.

Now let's say they didn't give you the pKa. If aspartic acid has a stronger inductive effect (a stronger polarization of the sigma bond between O and the acidic H), then how would that affect acidity? If the charge difference between the two atoms is greater for aspartic acid, it's more likely for the bond to be longer (which means weaker), the bond has more ionic character, all of which point to aspartic acid being MORE acidic.

90. What is the normality of .5M glutamic acid in water?
A .5N
B1 N
C 1.5N
D 2 N
People ignore the pH of the solution. The normality depends only on the potential frequency of a given molecule in a solution. For example, HF is a weak acid, it wouldn't completely dissociate in water. If you had a 1M solution of HF, the normality would still be 1. Like wise with H3PO4, a 1M solution has a normality of 3. So here we have a 0.5M glutamic acid. It's triprotic--> 3*0.5=1.5 and that's it, don't make it more complicated than it needs to be.