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Problem Set
Acids and Bases
1. For the following pairs of reactants, write an acid-base reaction. Show electron dots and curved arrows that show the movement of electrons.
Write an equilibrium arrow that indicates the direction of the equilibrium. Identify the conjugate pairs in each completed equation.
O
a) H2O + H3CH2C C O H
O
H2O + H3CH2C C O
O
H3O + H3CH2C C O
e)CH3CH2NH
d) H-Br + F
g) H2C=CH2 + H-Cl
H3C-CH2 + Cl
H2O
j) CH3OH + H2SO4
k) HS
O
+ H3CH2C C O H
CH3OH
O
+ H3CH2C C O
q) HO
+ H3P
k) (CH3)2NH
+ Cl
+ HO
+ H2P
H2N
+ H-O-H
(CH3)2NH2
+ H-O
O
o) H-Cl + H3C C O
O
H3CH2C C O
Cl +
O
H3C C O H
H
r) CH3NH2 + CH3O
+ BF3
F
HO B F
F
Cl
+ H-O-H
i) H2N-H
+ H3C-Cl
(CH3)2NH2 +
O O
O C C O
CH3OH
O
n) (CH3)2NH + H3CH2C C O H
p) CH3NH2 + H-Cl
CH3NH3 +
+ CH3O
O
H3C C O H
+
HSCH3
CH3OH2 + HSO4
f) CH3O
O
+ H3C C O
h) H2O-H
HO
O O
O C C O H
+ CH3O-H
CH3CH2NH2
Br + H F
m) CH3O
O O
c) HO
+ H O C C OH
(excess)
O
+ H3CH2C C O H
b) HO
C H3NH2 + CH3OH
2. For the following groups of compounds, arrange in order of increasing acidity for the underlined hydrogen. EXPLAIN your choices.
a) CH3CH2OH (alcohol) < CH3COOH (carboxylic acid)
CH3COOH
b) CH4 < NH3 < H2O < HF
d) CH3COOH < CCl3COOH < CF3COOH
c) CH3COOH <
e) CH3CH2CH3 < CH3CH2NH2 < CH3CH2OH
3. Arrange the following in order of increasing basicity. Explain.
a) H2O < CH3COO- < CH3CH2O< F-
b) F- < HO- < NH2- < H3C-
c) I- < Br- < Cl-
d) CH3OCH3 < (CH3)3N < (CH3)3CH3N
e) Cl- < HS- < H2P- < H2N-
f) H2S < H2O <
4. Sodium hydride, NaH, is an ionic compound.
Na
H
a) Write the Lewis electron-dot structure for NaH.
b) If NaH is placed into water (a foolish thing to do), the hydride ion is converted to hydrogen gas (H2).
The resulting solution turns red litmus paper blue and has a pH
OH
Na>>7.H
+ H2O
H2
+ Na
Write a balanced equation showing the reaction of NaH with water.
c) Is the hydride ion an acid or a base? What is the relationship of hydrogen and hydride? They are conjugate acid and base.
d) Sodium hydride reacts with alcohols in a similar way. Write the reaction of NaH with ethanol (CH3CH2OH).
Na
H
+
CH3CH2OH
H2
+
Na
OCH2CH3
5. From the Ka values in the table, calculate the pKa for each compound. Using these data, arrange the compounds in
order of increasing acidity and explain the trend. (Hint: be sure to identify the hydrogen to which the Ka applies.)
Ka
Ka
pKa
pKa
2
CH3COOH
3
5
OH
CH3CH2NH2
1.75 x 10-5
4.75
1.0 x 10-10
10
1 x 10-36
36
CH3CH2OH
1 x 10-16
CH2COOH
CH3CH3
16
4
5.2 x 10-5 4.28 most acidic - 1
10-50
50
least acidic - 6
6. 1-Indanone (below) has three different types of hydrogen atoms (labeled A, B, and C in the structure). Removing the most acidic proton
forms an anion that is stabilized by resonance. Draw a structure for the anion and its resonance form(s).
Removal of either an A or a B hydrogen results in a resonance stabilized anion:
O
O
O
O
A
H
A
H
H
H
H
H
H H
H
H H
B
Removal of an A hydrogen yields an enolate
anion, particularly stable due to the resonance form
B
in which the negative charge resides on oxygen.
H
H
C
H
O
O
H
H
O
O
H
H
Removal of a B hydrogen results in an anion with MANY
resonance forms, including an enolate.
7. The conjugate acids of the following amines have the pKa values shown. Explain the trend.
CH3CH2CH2CH2NH2
CH3OCH2CH2CH2NH2
(CH3O)2CHCH2CH2NH2
NºCCH2CH2NH2
pKa of conjugate acid
10.60
The presence of electronegative groups
9.92
results
8.54
in a weaker base (a stronger conjugate acid).
7.80