Name _________________________
Exam 1 SOLUTION
Please work in the space provided. Box your answers to set them off. The last page may be torn off
and used for scratch paper. Bring all materials up when you are finished with the exam. Good luck!
1. Nomenclature. Name the following molecules, using IUPAC and common naming rules. Indicate
stereochemistry where appropriate. [12 pts]
a.
b.
cis-1-isopropyl-4-methylcyclohexane
(isopropyl groups are alphabetized on the ‘”i”)
Because the molec is symmetric, there are no R/S isomers!
F
4-tert-butyl-3-fluoro-2,6-dimethyloctane
(t-butyl group are alphabetized on the “b”)
CH3
c.
OH
(2S, 3S)-3-methyl-2-pentanol
2. Draw the Lewis Structure for N2O (N is in the middle). Include formal charges and all valid
resonance structures, and draw curved arrows to show the movement of electrons. Indicate which
resonance structure is preferred. [8 pts]
N
N
O
N
N
O
This structure is preferred as
the negative charge is on the
more electronegative element.
3. Assign absolute stereochemistry to each chiral center in the following molecules. [12 pts]
CH3
H
Exam 1
CH3
CH2CH2Br
F
F
CH2CH2Br
S
CH3
H2N
H
H
R
SH R
CH3
Fall 2005
Page 1 of 4
Chemistry 211
Clark College
4. Draw both chair conformations of the following substitute cyclohexanes. Indicate the more stable
chair conformer for each pair. [10 pts]
H
H
H
Cl
Cl
CH3
H
Cl
CH3
H
CH3
H
H
H
Br
Br
H3C
Br
H
H
5. Rank the following molecules in order of stability, relative to enthalpies of combustion. 1 = least
stable molecule, 4 = most stable molecule. [4 pts]
3
1
4
2
Stability increases as branching increases!
6. Find all chiral centers in the molecule Penicillin G, and mark them with an asterisk*. What is the
maximum number of isomers that can be formed? [6 pts]
H
N
N
O
O
Exam 1
S
* *
3 stereocenters;
23 = 8 stereoisomers
*
OH
O
Fall 2005
Page 2 of 4
Chemistry 211
Clark College
7. Use Fisher projections to draw all stereoisomers of 2-bromo-3-iodopentane. You do not need to
assign absolute configurations. Number your unique isomers, and give the relationships between
them (enantiomers, diastereomers, etc…) [12 pts]
1
2
3
CH3
H
Br
H
I
H
H
CH3
CH3
Br
H
I
H
H
H
CH3
CH3
H
Br
I
H
H
H
CH3
4
CH3
Br
H
H
I
H
H
CH3
1&2 are enantiomers.
3&4 are enantiomers.
1&3, 1&4, 2&3, 2&4
are diastereomers.
Short answer. Briefly answer the following questions. Use drawings or figures, when appropriate.
8. What is the relationship between 1,2,3-trimethylcyclopropane, methylcyclopentane, and
cyclohexane? [6 pts]
These are all structural/constitutional
isomers. They have the same molecular
formula, but different connectivity.
9. tert-Butylcyclohexane is almost always found with the t-butyl group in the equatorial position.
Explain this observation. [6 pts]
H
CH3 H
H3C
C
H
H
H
H
In an axial position, the t-butyl
groups interacts with other axial
hydrogens, causing steric strain.
H
This interaction does not
occur in the equatorial
position, as the t-butyl group
points away from the ring.
10. Draw Newman projections representing the anti and the gauche configurations of butane, and label
them. Which is the more stable configuration? Explain. [6 pts]
H
CH3
H
CH3
Anti
H
H
H
H
CH3
H
Gauche
CH3
H
The anti conformation is more stable. The hydrogens on the
two CH3 groups in the gauche configuration can interfere
with each other in space, resulting in torsional strain.
11. Use a Fisher projection to draw meso-2,3-difluorobutane. Explain why it is a meso compound. [6
pts]
CH3
H
F
H
F
CH3
Exam 1
meso-2,3-difluorobutane has an internal mirror plane that
bisects the molecule, making the top half of the molecule
identical to the bottom half. When a molecule possess this
trait, its mirror image is actually the exact same molecule.
Fall 2005
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Chemistry 211
Clark College
Multiple Choice. Circle the choice that best answers the question.
12. Which of the following molecules has a net molecular dipole? [4 pts]
H
F
F
F
H
H
F
F
F
H
H
H
H
H
F
F
1
2
3
4
All other molecules have bond dipoles that oppose each other, so they cancel out.
13. What is the approximate C-C-C bond angle in propene? [4 pts]
a. 90°
b. 109.5°
c. 120°
d. 180°
14. Which of the following represents the most stable conformer of 2-methylbutane? [4 pts]
H
H3C
CH3
CH3
H
H
1
Molecule #3 is NOT 2-methylbutane!
Exam 1
H3C
H
CH3
H
H3C H
H
H3C
2
CH3
H
H
3
Fall 2005
CH3
H3C
H3C
CH3
H
H
H
4
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