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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!
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?
----------------------------------
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!