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Midterm

chem_3351_third_hour_exam_key_fall_2015Exam


Department
Chemistry & Biochemistry
Course Code
CHEM 3351
Professor
All
Study Guide
Midterm

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Easily Legible Printed Name:______________________________
Page 1 of 8
CHEM 3351 (100), Fall 2015
Professor Walba
Third Hour Exam
November 17, 2015
scores:
1) 20
2) 20
3) 20
4) 20
5) 20
100
PLEASE read the questions very carefully!
PLEASE legibly print your name on each page of the exam.
H
He
Li
Be
B
C
N
O
F
Na
Mg
Al
Si
P
S
Ne
Cl
Ar
Br
I
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
35
53
1A
2A
3A
4A
5A
6A
7A
8A
CU Honor Code Pledge: On my honor, as a University of
Colorado at Boulder Student, I have neither given nor received
unauthorized assistance.
Signature: ______________________________________
Recitation TA Name: _____________________________
Recitation day and time: ___________________________
This is a closed-book exam. The use of notes, calculators, scratch
paper, or cell phones will not be allowed during the exam. You
may use models brought in a clear Ziploc bag. Please put all your
answers on the test in the appropriate place. Use the backs of the
pages for scratch (there are two additional blank scratch sheets
after the last page of the exam). DO NOT PUT ANSWERS ON
THE SCRATCH SHEETS.
Key

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Page 2 of 8 Total points for this page:
1 (20 pts) For this question, your explanations must fit in the boxes indicated. The answers do not need to be
complete sentences, and should be short phrases.
Circle the stronger base
in propanol solvent
O S
Explain why
O S O S
Circle the stronger nucloephile
in propanol solvent
Circle the stronger nucloephile
in propanol solvent
Explain why Explain why
OH3CO
Circle the stronger base
in propanol solvent
OH3CO
Circle the stronger base
in the gas phase
Explain why Explain why
Intentionally left blank
Intentionally left blank
DMF solvent
The thiolate is a weaker base = the thiol
is a stronger acid, due to the weak S-H bond
The alkoxide nucleophilicity is dramatically
lowered by hydrogen bonding, making the
thiolate the stronger nucleophile in propanol.
DMF is aprotic (no H-bond donor), so the
stronger base is the stronger nucleophile.
The methoxide is H-bonded, and the tert-
butoxide is less H-bonded (steric inhibition
of solvation.) This lack of H-bonding
destabilizes the tert-butoxide, and makes it
a stronger base than methoxide.
In the gas phase there is no H-bonding. The
tert-butoxide is stabilized by delocalization
of negative charge onto the CH3 groups,
making the tert-butoxide more stable, and
the methoxide a stronger base.
Weak S-H bond
H-bonding to the alkoxide Stronger base is stonger nucleophile
Steric inhibition of H-bonding Better delocalizaton of charge makes the
t-butoxide more stable
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Printed Name:
Page 3 of 8 Total points for this page:
2) (20 pts) Give the single major product for each of the following reactions, carefully showing
stereochemistry using wedges and dashes if appropriate. If a racemate is formed, show only one enantiomer
and label it “rac.” Assume chiral starting materials are single pure enantiomers unless they are labeled “rac.”
a)
b)
c)
d)
e)
CH2I2, Zn-Cu
ether
Br Br NaCN
DMF
Br CH3
H
Br O
OH
K
CH3ONa
CH3OH
H3C
Br a) Mg, ether
b) H2O
Br CN
CH3
H3C
Rac
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