[MorningStar]

Hi everyone,

I am wondering if my logic is correct on this problem.  I am asked to compare three molecules:
Conjugate acid of Isopropylamine
CH3CHCH3
      |
      NH3+  pKa 10.78

Propanoic acid
CH3CH2COOH  pKa 4.78

Conjugate acid of Alanine
CH3CHCOOH pKa 2.35
      |
      NH3+ pKa 9.87

These are the questions, followed by my responses.

1. How do you account for the fact that the --NH3+ group of the conjugate acid of alanine is a stronger acid than the --NH3+ group of the conjugate acid of isopropylamine?
My answer: The --NH3+ group of the conjugate acid of isopropylamine is adjacent to two methyl groups, which are electron donating, thus making it a weaker acid.  The --NH3+ group of the conjugate acid of alanine is adjacent to a carboxyl group, which is electron withdrawing, thus making it a stronger acid. 

2. How do you account for the fact that the --COOH group of the conjugate acid of alanine is a stronger acid than the --COOH group of propanoic acid?
My answer: The conjugate acid of alanine is a stronger acid because the --COO- group can easily deprotonate the --NH3+ group in an intramolecular reaction to stabilize the molecule. 

I am not very good at acid-base chemistry, so I appreciate the help.

[nj_bartel]

I think 1 and 2 should have very similar explanations.