Download Exam 2 SOLUTION

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Exam 2 SOLUTION
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1. Predict the product, or give the starting material for the following reactions: [12]
HCl
Cl
OH
H2O
H2SO4
(only this product)
HBr
Br
2. For each pair, circle which is the stronger acid, and give a brief reason for your choice. [12]
O
O
F
F
OH
F
H
CH3OH
Exam 2
Cl
Cl
OH
Cl
O
H
CH3NH2
The fluorines are more electronegative than the chlorines,
and can better inductively withdraw electron density from
the formed anion in the conjugate base.
The carbonyl group (C=O) provides a resonance
structure and an inductive effect to stabilize the anion.
The acidic proton is attached to a more electronegative
atom.
Fall 2005
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Chemistry 211
Clark College
3. Use the following energy diagram to answer the questions below: [8]
!H3
!H2
E
!H1
!H4
!Hrxn
Reaction Coordinate
a) How many steps are in the mechanism to this reaction?
There are 4 steps to this mechanism.
b) Which step is the rate-determining step?
The second step, with the highest activation barrier, is the RDS.
c) Label ∆Hrxn and the activation energies on the graph.
4. Using curved-arrow notation, give the mechanism for the reaction between ethanol and 3-methyl-2pentene, in the presence of an aqueous acid catalyst. [10]
OH
H O H
H
(+ H3O+)
H3O+
O
H
HO
O
H
O
H2O
Exam 2
Fall 2005
(you could also use an ethanol
molecule, which is slightly
more basic than water)
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Chemistry 211
Clark College
5. Using the appropriate pKa values, determine the value for Keq for the following reaction: [8]
H2PO4- + CH3CH2OH2+
CA
H3PO4 + CH3CH2OH
A
pKa values:
H3PO4 = 2.1
CH3CH2OH = 15.9
H2PO4- = 6.82
CH3CH2OH2+ = -2
pKeq = pKa(reactant) – pKa(product) = 2.1 – (-2) = 4.1
Keq = 10-pKeq = 10-4.1
Keq = 10-4.1
6. Nomenclature. [20]
a) Name the following molecules using the IUPAC system. Be sure to indicate stereochemistry,
where appropriate.
Cl
Z-5-chloro-2,3-dimethyl-1,4-heptadiene
Br
(2E, 4S)-4-bromo-3-ethyl-2-hexene
b) Draw the following molecules from the names given.
(2Z,4E)-2,5-difluoro-6-methyl-2,4-octadiene
F
F
1-bromo-3-propyl-2-hexene
Br
Exam 2
Fall 2005
Page 3 of 5
Chemistry 211
Clark College
7. The molecule shown below is imidazole, a building block of the amino acid histidine. When
imidazole is dissolved in water, it picks up a proton from water onto one of the nitrogens.
Determine which nitrogen is more basic – which one picks up the proton? Give
reasoning/structures to support your choice. [6]
H
N
N
H
N
N
+ H2O
OR
N
N
H
H H
H
H
H
N
N
N
?
N
N
N
H
H
H
You can draw three resonance structures for the first structure but NONE can be drawn for the
second structure – they all would have 5 bonds to Nitrogen (which is just as bad as 5 bonds to
Carbon!).
8. Many compounds behave as both Brønsted-Lowry bases and Lewis bases. For each compound
below, write an equation showing how the compound would react with H3O+ and with BF3. [12]
a) CH3OCH3
B-L reaction:
H
O
+
+ H2O
+
H
O
3
O
H3C
CH3
H3C
CH3
Lewis reaction:
H3C
O
BF3
O
H3C
CH3
CH3 + BF3
b) CH3CH2NH2
B-L reaction:
CH3CH2NH3 + H2O
CH3CH2NH2 + H3O+
Lewis reaction:
BF3
N H
H
CH3CH2NH2 + BF3
O
c)
B-L reaction:
O
+
O
H3O+
H
+ H2O
Lewis reaction:
O
Exam 2
+ BF3
Fall 2005
O
BF3
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Chemistry 211
Clark College
Multiple Choice. Circle the choice that best answers the question.
9. Which carbocation represents the lowest energy (most stable) intermediate? [4]
A
B
C
It’s resonance stabilized!
D
10. Which of the following molecules represents a “Z” alkene? [4]
Cl
F
A
B
C
D
11. How many isomers of 1,6-heptadiene exist? [4]
a) 1
b) 2
c) 3
d) 4
Both alkenes are terminal – no isomers!
Exam 2
Fall 2005
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