Crazy Berkeley Review Chem Questions

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t1me

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Berkeley Review Chem ch4 passage13 q88

aspartic-acid-and-glutamic-acid.gif


"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."


Would the shorter carbon chain (less hydrogens thus less electron density) produce a weaker inductive effect, or would the closer protonated amino group [(NH3+)- ] (more electronegative nitrogen) produce a stronger inductive effect? Which one is more important in this situation and why?!

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Berkeley Review Chem ch4 passage13 q90

"What is the normality of 0.50 M glutamic acid in water?

B. 1.00N H3N+CH(CH2CH2CO2H)CO2H(aq)
C. 1.50N H3N+CH(CH2CH2CO2H)CO2H(aq)"

Passage info on glutamic acid: pKa1=1.8-2.6 pKa2=4.32 pKa3=8.8-10.6

The BR answer is c because glutamic acid is triprotic. However, wouldn't water (a neutral pH solvent) only deprotonate the acid twice? Thus the acid would only be diprotic "in water" as the question asked, right?

Please help!! These crazy questions are driving me... CRAZY!! :scared:

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I think aspartic acid would be more acidic due to stronger inductive effect from the amino group over shorter distance. NH3+ is a strong electron withdrawing group.

As far as normality goes, for acids it's molarity of acid times the number of protons it can give up regardless of the solvent or the pH. I've seen many definitions and uses of normality, but I think that's what the mcat wants you to know.

Besides, if you had glutamic acid in water, you wouldn't even get two moles of proton per formal concentration of the acid since they are weak acids. That is, we say that any weak acid HA whose concentration is 1M is 1N even though the concentration of the proton will be much less.
 
"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."

I believe the answer is A because the EWGs (mainly the protonated -NH3+) would be closer to the acidic hydrogen in aspartic acid because of its shorter length, creating a stronger inductive effect. The other hydrogens have little effect on acidity.

"What is the normality of 0.50 M glutamic acid in water?

B. 1.00N H3N+CH(CH2CH2CO2H)CO2H(aq)
C. 1.50N H3N+CH(CH2CH2CO2H)CO2H(aq)"

Passage info on glutamic acid: pKa1=1.8-2.6 pKa2=4.32 pKa3=8.8-10.6

The pKa of water is around 15.7, which is higher than the all three pKas listed here. Thus water is less acidic than any of of the protons on glutamic acid and all three acidic hydrogens would be deprotonated in water.
 
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@Rabolisk

Thank you so much for the reply on normality. It makes sense to me now. I didn't have a good enough understanding of the term.

Also about the first question, I guess what I'm trying to ask is:

Does the size of an alkyl group affect its inductive effects? and would that difference be negligible compared to the inductive effects of another EWG or EDG (in this case -NH3+)?
 
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