Chemistry 211
Clark College
Homework 5 SOLUTION
Please put your answers on a separate sheet of paper.
Due Wednesday, 10/18, by 4 pm in my mailbox, in APH 203.
1. Assign absolute configuration to all stereocenters in the following biomolecules.
CHO
S
HO
H
S
HO
H
R
H
OH
R
H
OH
Mannose
H2N
R
O
OH
Aspartic Acid
HO
O
CH2OH
2. Identify the acid and the base out of the reactants in the following reactions. In addition, identify
each reaction as a more specific Brønsted-Lowry Acid/Base reaction, or as a more generic Lewis
Acid/Base reaction.
3. For 2(a) and 2(c), label each reaction with the terms “acid”, “base”, “conjugate acid” and “conjugate
base”
a) CH3OH + H3O+
B
CA
A
O
b)
+ TiCl4
B
Bronsted-Lowry
CH3OH2 + H2O
A
CB
O
TiCl4 CB
Lewis
CA
O
Na+
O
H
H + NaH
c)
H
B
CB
A
N
+ BH3
O
B
Fall 2006
Bronsted-Lowry
CA
CB
H
d)
+ H2
A
H BH3
N
CA
Lewis
O
Homework 4
Page 1 of 3
Chemistry 211
Clark College
4. Complete the following Brønsted-Lowery acid-base equations and give an order-of-magnitude
estimate for the equilibrium constants. Use the pKa values in Table 4.1 or Appendix A.2 to estimate
the values of the pKas of the acids.
Watch the transfer of protons, make sure you get your charges correct! Also, match the molecules
carefully, watch charge and significant structural features, such as aromatic (benzene) rings.
a)
CH3CH2O- Na+
+
OH
CH3CH2OH
NH2
+
NH3
(CH3)3NH
10
c)
H3CC CH
25
d)
O
C
H3C CH3
18
e) H C
3
0.5
Fall 2006
+
Na+ NH2-
(CH3)3N
4
H3CC C
+ NH3
38
+
O
S OH
O
O
C
CH3CH2O- K+
+ H2O
pKeq = 10-16 = -6
Keq = 106
O
16
10
b)
+
H3C CH2
H3C
Homework 4
pKeq = 10-4 = 6
Keq = 10-6
pKeq = 25-38 = -13
Keq = 1013
+ CH3CH2OH
16
O
S O + H3O+
O
-1.74
pKeq = 18-16 = 2
Keq = 10-2
pKeq = 0.5-(-1.74) = 2.24
Keq = 10-2.24
Page 2 of 3
Chemistry 211
Clark College
5. Answer the following questions, supporting your answers with diagrams or structures.
a) Which proton is the more acidic proton in the following diketone? Why?
O
O
O
O
By removing the end proton, one resonance structure can
be drawn to move the negative charge onto the oxygen.
When the central proton is
removed, two resonance structures
can be drawn. More resonance
structures, more acidic!
In addition to the resonance effect (which is the major component for increased acidity), an increased
inductive effect is also experienced by the “middle” hydrogen by the two carbonyl groups.
O
O
O
O
O
O
b) Which of the following substituted acids is more acidic? Why?
O
O
OH
OH
Cl
Cl
The chlorine that is closer to the formed anion can better withdraw electron density through
inductive effects.
6. For the following acid-base reactions, used curved-arrow notation to indicate the reaction that is
taking place.
When drawing arrows, they MUST originate from the base and point towards the acid. Also, when a
hydrogen ion is removed, the electrons from the bond must revert to the atom that the hydrogen was
originally attached to.
a)
O
O
H
H O H
+ H2O
H
H N H
b)
H
+
NH2
H
c)
H
Fall 2006
Cl
H Cl
H H
Homework 4
Cl
H H
Page 3 of 3