CH 334
Second Midterm Exam
Monday, November 7, 2016
Name____________________________________________________________
You may use model kits but no other material with chemical information without
instructor approval. Tables of possibly useful data are included on the last page.
Please do not use any electronic gadgets (calculators, music players, phones, etc.).
1. (15 points) Draw correct structures for the following compounds.
a. trans-1,3-dichlorocyclobutane
b. cis-4-iodo-tert-butylcyclohexane
c. R-2-iodooctane
2. (20 points) Identify each molecule shown below as chiral (C) or achiral (A). Circle every
stereocenter and assign the configuration as R or S.
3. (20 points) The following reaction is accomplished using pig liver esterase enzyme.
Compound 2B is reported to be isolated in 86% ee from this reaction, and after
recrystallization the compound has a 99% ee and an optical rotation of -69°.
A. Is 2A chiral? Explain why or why not.
B. Does 2A have stereogenic carbons? Identify any that are present.
C. Is 2B chiral? Explain why or why not.
D. What property must pig liver esterase have to allow it to accomplish the stereochemical
outcome of this reaction? (Hint: this has nothing to do with the nuts-and-bolts chemistry of
the transformation.)
4. (25 points) This question deals with trans-1,4-dimethylcyclohexane.
A. Draw the two ring-flip isomers of the chair conformation of trans-1,4dimethylcyclohexane.
B. Which of these is more stable, and by how much? Explain your reasoning.
C. Five additional conformers and their energies are shown below (HC = half-chair; TB =
twist-boat). Construct an energy diagram showing the conversion of one ring flip isomer to
the other, showing the sequence of all 7 conformers and their relative energies versus the
lowest-energy chair form.
5. (20 points) 4-Methylcyclopentene is shown below.
A. Explain whether this compound is chiral and why you believe so.
B. This compound undergoes the following reaction. (AIBN is a radical initiator, and, as we
discussed in class, NBS is a source of low concentrations of Br2.)
This reaction results in two regioisomers that differ in where the bromine atom is connected.
Draw the mechanism, including electron-pushing arrows, for the chain-propagation steps.
Include a chemical rationale for the regioisomers formed. (You need not show initiation or
termination steps.)
(part C on the next page)
C. Choose one of the two regioisomers, and show all stereoisomers that form and describe
the relationship between each pair.
Bond strengths (kcal/mol):
F-F
Cl-Cl
Br-Br
I-I
H-F
H-Cl
H-Br
H-I
CH3-H
CH3CH2-H
(CH3)2CH-H
(CH3)3C-H
CH3-F
CH3-Cl
CH3-Br
CH3-I
CH3CH2-F
CH3CH2-Cl
CH3CH2-Br
CH3CH2-I
(CH3)2CH-F
(CH3)2CH-Cl
(CH3)2CH-Br
(CH3)2CH-I
(CH3)3C-F
(CH3)3C-Cl
(CH3)3C-Br
(CH3)3C-I
38
58
46
36
136
103
87
71
105
101
98.5
96.5
110
85
70
57
111
84
70
56
111
84
71
56
110
85
71
55
A values (kcal/mol)
Substituent
Me
Et
i
Pr
t
Bu
O-Me
Cl
Br
I
A
1.8
1.8
2.1
4.5
0.75
0.52
0.55
0.46