1. No category

Dr. Peter Wipf
Chemistry 0310
1/18/2012
Sample Exam Solutions
A.
(30 points) (i) Compare and contrast the Lewis structures of nitromethane, CH3NO2,
and methylnitrite, CH3ONO. (ii) Write at least 2 resonance forms for each molecule. (iii) Based
on your examination of the resonance forms, what can you say about the polarity and bond
order of the two NO bonds in each substance?
Solution: Check problem 1/32 in Vollhardt & Schore!
B.
(30 points) What kind of a C-C bond length (in Å) would you expect for the dianion of
ethene, e.g. an [ethene]2- molecule where two additional electrons are "injected" into the πbond? Rationalize your conclusion graphically with an orbital diagram for [ethene]2-.
C.
(45 points) The following structure shows a natural product that my research group has
synthesized a few years ago. (i) Assign the hybridization at all carbon atoms and (ii) estimate
the bond distance of all unique bonds. (iii) What functional groups do you recognize?
1.54 Å
sp2
sp2
sp3
O
O
H
N
OH
N-H: 1.0 Å
O-H: 1.0 Å
1.54 Å
1.54 Å
O
1.43 Å
1.43 Å
H
N
1.20 Å
O
sp2
1.47 Å
1.20 Å
O
sp3
sp3
sp2
sp3
O
OH
1.34 Å
1.54 Å
FGs: ketone (carbonyl group), amide, alkene, alcohol, epoxide (oxirane)
CHEM 0310
D.
Page 2
(30 points) Which of the following structures violate the octet rule?
H
H N H
H
O
O
Cl
O
H
C
H
H
C
C
H
H
H
Br
P
Br
Br
H
H
H
H
H
E.
(40 points) Draw a Newman projection of the gauche conformation of butane and
explain the reasons for its energetic destabilization (by how much?) vs. the antiperiplaner
conformation.
The gauche interaction suffers from electron (orbital-orbital electron repulsion) as well as steric
destabilization due to the interaction of the two large methyl groups which are further
apart in the antiperiplanar orientation. The energy destabilization is about 0.9 kcal/mol.
F.
(45 points) (i) Determine whether each species in the following equations is acting as a
Brønsted acid or base, and label it. (ii) Indicate whether the equilibrium lies to the left or the
right. (iii) Estimate K for each equation using the Table copied on the next page.
Solution: Check problem 2/27 in Vollhardt & Schore!
CHEM 0310
G.
Page 3
(30 points) Use orbitals to explain the hyperconjugative stabilization of the ethyl radical.
hyperconjugation is the electron donation of a filled sp3 orbital of a neighboring C-H bond into
the partially filled p-orbital of the primary radical in the ethyl group.
H.
(50 points) Write in full the mechanism for monobromination of methane. Clearly
indicate initiation, propagation, and termination steps.