ORGANIC CHEMISTRY I
PRACTICE PROBLEMS FOR BRONSTED-LOWRY ACID-BASE CHEMISTRY
1. For each of the species below, identify the most acidic proton and provide the
structure of the corresponding conjugate base. You might want to draw detailed Lewis
formulas in some cases.
HF
CH
3
CH
2
OH
H
3
O
H
2
O
CH
3
CH
3
CH
3
CN
HC CH
H
2
CH
3
OH
2
RNH
3
2. For each of the species below, identify the basic atom and provide the structure of the
corresponding conjugate acid. You might want to draw detailed Lewis formulas in some
cases.
CN
SO
4
CH
3
CH
2
OH
H
2
O
NH
2
O O
CH
3
CH
2
O
OH
PhCOO
Br
2
_
3. Fill in the reactants or products for the following acid-base reactions. Keep in mind
that in line-angle formulas hydrogens are not shown. When in doubt, write complete
Lewis structures. Abbreviations used: Ph=phenyl (a benzene ring attached to a carbon
chain)
HNO
3
+ H
2
O
F + H
3
O
H
3
C CH
3
O
+ OH
PhCOO + HCl
H
2
CO
3
+ OH
+ H
2
O
OH
+ H
NH
3
+
HC C
4. Arrange the substances by order of acidity or basicity as indicated. You may use pKa
tables, periodic tables, or any other tools available to you (except for your cell phone).
H
2
Se H
2
O
H
2
S
weakest acid
strongest acid
strongest base
weakest base
strongest acid
weakest acid
weakest basse
strongest base
CH
3
CH
2
OH CH
3
COOH HF Benzene
weakest acid
strongest acid
Br F I Cl
CH
4
HI PH
3
H
2
Se
F OH NH
2
CH
3
CH
2
CH
2
CH
2
5. Circle the side favored by equilibrium in the following acid-base reactions.
CN + NH
3
HCN + NH
2
OH
+
H
3
C CH
2
O
O
+
H
3
C CH
3
O
H
2
O +
NH
3
H
3
O +
NH
2
H
2
CO
3
+ H
2
O
HCO
3
+ H
3
O
CH
3
CH
2
CH
2
CH
3
+
CH
2
CH
3
CH
2
CH
2
CH
2
+
CH
3
6. The conjugate acid of ammonia, NH
3
, is:
A) NH
4
+
B) NH
2
OH C) NH
2
-
D) none of the above
7. Methanesulfonic acid, CH
3
SO
3
H, has a pK
a
of -7 while ethanol, CH
3
CH
2
OH, has a pK
a
of
15.9. Which is the stronger acid and what accounts for this large difference in relative
acidity?
8. Would you predict trifluoromethanesulfonic acid, CF
3
SO
3
H, to be a stronger or weaker
acid than methanesulfonic acid, CH
3
SO
3
H? Explain your reasoning.
9. Consider the species CH
3
O
-
, NH
2
-
, and CH
3
COO
-
. Rank these ions in order of
increasing basicity, and explain your rationale.
10. The K
a
of formic acid is 1.7 x 10
-4
. The pK
a
of formic acid is __________.
A) 1.7 B) 10.3 C) 4.0 D) 3.8 E) -2.3
11. Provide the structure of the conjugate base of phenol (shown below) and all its
resonance forms.
12. Rank the following in order of increasing acidity: CH
3
OH, HCl, NH
3
, and CH
4
.
13. Rank the following in order of increasing basicity: CH
3
O
-
, H
2
N
-
, H
2
O, and NH
3
.
14. When methanol (CH
3
OH) acts as a base, its conjugate acid is __________.
A) CH
4
OH B) CH
3
OH
2
+
C) CH
4
O
+
D) CH
3
O
-
E)
-
CH
2
OH
15. Which of the following pairs of bases lists the stronger base first?
A) H
2
O > HO
-
B) H
2
N
-
> CH
3
COO
-
C) CH
3
COO
-
> HO
-
D) I
-
> Cl
-
E) HO
-
> H
2
N
-
16. Draw the structure of the conjugate acid of acetone (CH
3
COCH
3
).
ANSWERS
1.
HF
CH
3
CH
2
OH
H
3
O
H
2
O
CH
3
CH
3
CH
3
CN
HC CH
H
2
CH
3
OH
2
RNH
3
F
CH
3
CH
3
O
H
2
O
OH
RNH
2
CH
3
CH
2
CH
2
CN
HC C
H
CH
3
OH
2.
CN
SO
4
CH
3
CH
2
OH
H
2
O
NH
2
O O
CH
3
CH
3
O
OH
PhCOO
Br
2
_
HCN
HSO
4
CH
3
CH
2
OH
2
O O
H
3
O
CH
3
CH
2
OH
H
2
O
PhCOOH
HBr
NH
3
3.
HNO
3
+ H
2
O
F + H
3
O
H
3
C CH
3
O
+ OH
PhCOO + HCl
H
2
CO
3
+ OH
+ H
2
O
OH
+ H
NH
3
+
HC C
H
3
O
+
NO
3
HF + H
2
O
H
3
C CH
2
O
+ H
2
O
PhCOOH + Cl
HCO
3
+ H
2
O
+ OH
NH
2
+
HC CH
O
+ H
2
4.
weakest acid
strongest acid
strongest base
weakest base
strongest acid
weakest acid
weakest basse
strongest base
weakest acid
strongest acid
F
H
2
O H
2
SeH
2
S
<
<
Cl
Br
I
>
>
>
HI
H
2
Se PH
3
CH
4
>
> >
F OH NH
2
CH
3
CH
2
CH
2
CH
2<
<
<
Benzene
CH
3
CH
2
OH
CH
3
COOH
HF
<<
<
Periodic trend
Periodic trend
Periodic trend
Periodic trend
pK
a
trend
5. The numbers represent approximate pK
a
values for the substances acting as acids.
CN + NH
3
HCN + NH
2
OH
+
H
3
C CH
2
O
O
+
H
3
C CH
3
O
H
2
O +
NH
3
H
3
O +
NH
2
H
2
CO
3
+ H
2
O
HCO
3
+ H
3
O
CH
3
CH
2
CH
2
CH
3
+
CH
2
CH
3
CH
2
CH
2
CH
2
+
CH
3
38
9
17
20
4 -1.7
-1.7
6.4
48
40
6. A
7. Methanesulfonic acid is the stronger acid. The lower the pKa, the stronger the acid. A
lower pKa is associated with a larger Ka which signifies greater dissociation. The large
relative difference in acidity in this case can be most easily seen by gauging the relative
basicities of the conjugate bases. The weaker the base, the stronger the corresponding
conjugate acid. Methanesulfonate, CH
3
SO
3
-
, is considerably stabilized by resonance
delocalization which is not found in ethoxide, CH
3
CH
2
O
-
. This effect greatly reduces the
basicity of methanesulfonate relative to ethoxide. Draw the Lewis formula for
methanesulfonate and the resonance forms for practice.
8. Trifluoromethanesulfonic acid is a stronger acid. Compare the strengths of the
conjugate bases and remember that the weaker the base, the stronger the conjugate
acid. Both bases are stabilized by resonance, but in the case of the trifluoro derivative,
the presence of the highly electronegative fluorine atoms serves to delocalize the
negative charge to an even greater extent by the inductive effect of fluorine. This
additional delocalization makes trifluoromethanesulfonate a weaker base.
9. CH
3
COO
-
< CH
3
O
-
< NH
2
-
first factor to consider is the nature of the atom which bears the negative charge. The
more electronegative the atom that bears the negative charge, the more stable the
anion. Stable anions are less reactive and are hence weaker bases. Since O is more
electronegative than N, the NH
2
-
is the strongest base in the set. In the remaining two
species, the negative charge is on the O, but in the case of CH
3
COO
-
, the negative
charge is also delocalized by resonance.
10. D
11. See class notes, or p. 417 of the Wade textbook, for all the resonance structures of
the phenoxide ion shown below.
O
(see p. 429 for resonance structures)
12. CH
4
< NH
3
< CH
3
OH < HCl (periodic trend)
13. H
2
O < NH
3
< CH
3
O
-
< H
2
N
-
Negatively charged ions are stronger bases than
neutral counterparts. A table of pK
a
values will further aid in deciding the final order of
basicity.
14. B
15. B
16.
H
3
C CH
3
OH
4 more resonance structures