Acids and Bases: Use of the pKa Table
Exercise Solutions

a. The circled hydrogen is part of an amine functional group.  The pKa table reveals that NH3 (ammonia; also an amine) has a pKa of 38.  Thus we estimate the pKa of H2NCH3 (methylamine) to be approximately 38.  Note that while ammonium ions may be somewhat similar to amines, the best match for methylamine is ammonia, not either of the ammonium ions on the pKa table.


b. The circled hydrogen is part of an alcohol functional group.  The only other alcohol on the pKa table is CH3CH2OH (ethanol), pKa 15.9.  Thus we estimate the pKa of (CH3)3COH (tert-butyl alcohol) as 16.  Acetic acid (CH3CO2H) also has an OH group, but the functional group of this carboxylic acid (CO2H) is significantly different from an alcohol functional group (OH).
c. The circled hydrogen is part of a phenol (OH attached to a benzene ring) functional group.  The only other compound in the pKa table with this functional group is phenol, pKa 9.95.  Thus we estimate the pKa of our more complex phenol as approximately 10.
d. Identify the functional groups, and then estimate their pKa values from the pKa table:

From this analysis, we conclude that the carboxylic acid proton has the lowest pKa, and so it is the most acidic proton in this molecule.


e. Recall that the equilibrium favors the weakest acid/base pair.  Use the pKa table to evaluate pKa values.  The pKa of C6H5CO2H (benzoic acid) is 4.19, and the pKa of H3PO4 (phosphoric acid) is 2.1.  Benzoic acid is a weaker acid than phosphoric acid, so the equilibrium favors benzoic acid and thus lies to the right.
f. This molecule is not in the pKa table so estimates must be made.  The closest match to the ammonium end of glycine is CH3NH3+ (methylammonium ion, pKa 10.64).  The closest match to the carboxylic acid end of glycine is CH3CO2H (acetic acid, pKa 4.76).   Methylammonium ion is a weaker acid than acetic acid, so the ionic form of glycine is favored and the equilibrium lies to the left.
g. To analyze this case, we must use the pKa of CH3CH2OH2+ (an oxonium ion), not CH3CH2OH (ethanol), because CH3CH2OH is serving as a base (proton acceptor), not as an acid in this reaction.  The closest match on Table 2.1 to the structure of CH3CH2OH2+ is H3O+ (hydronium ion, pKa -1.74).  Because the two acids are very close in pKa, this equilibrium will not significantly favor either side.

Link to: