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.
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