CHEM 203
Midterm Exam 2
November 13, 2014
ANSWERS
Your name:
This a closed-notes, closed-book exam
You may use your set of molecular models
This document contains 7 pages
Time: 1h 30 min
1.
________ / 10
2.
________ / 15
3.
________ / 15
4.
________ / 20
5.
________ / 20
6.
________ / 20
TOTAL
________ / 100
This exam counts for 18.75% of your CHEM 203 final grade
Chem 203
midterm exam 2
p. 2 of 7
1. (10 pts.) The compounds shown below are similar – though clearly not identical – to
particular reagents that were discussed in class. On the basis of analogy, indicate the likely
use of each (write your answers in the appropriate boxes):
probably used as / for
N
N
N
radical initiator
(cf. AIBN; notes
of Oct 15)
probably used as / for
O
N
controlled release
of Cl2 (cf. NBS;
notes of Oct 22)
O
Cl
N
probably used as / for
probably used as / for
OH
Na
Si
N
Si
strong base (cf.
NaNH2; notes
of Oct 20)
radical inhibitor
(cf. BHT; notes
of Oct 15)
OCH3
2. (15 pts.) Inside the appropriate boxes, draw:
a. The structure of an alkane that is a poor substrate for radical chlorination, but a good one
for radical bromination, and one that is a poor substrate for radical bromination, but a
good one for radical chlorination:
poor for chlorination, good for bromination
poor for bromination, good for chlorination
e.g.,
e.g., CH3–CH3
b. The structure of an alkyl halide that is likely to react with H C C–Na by the SN2
mechanism, and one that is likely to react with the same reagent by the E2 mechanism.
likely to react by SN2
e.g.,
I
likely to react by E2
e.g.,
Br
Chem 203
midterm exam 2
p. 3 of 7
c. The structure of two carbonyl compounds that yield product A when treated with an
excess of CH3MgBr , followed by mild H3O+:
OH
CH3MgBr
O
O
or
e.g.,
OCH3
then mild
H3O+
A
d. The structure of a chiral alkyne that yields a chiral product when treated with H2 and
Lindlar catalyst, but an achiral product when treated with H2 and finely divided platinum:
e.g.,
e. The product obtained from alkene B upon reaction with HBr, followed by CH3Li and
CuBr:
1. HBr
2. CH3Li
CuBr
B
3. (15 pts.) Write an accurate mechanism for the following known reaction:
R–R
2R•
CH3S
CH3SH
CH3S •
radical
initiator
[ + CH3S •]
H SCH3
CH3S
•
=
CH3S
CH3S
•
•
H SCH3
Chem 203
midterm exam 2
p. 4 of 7
4. (20 pts.) Provide the structure of the major product expected from the following reaction
sequences. If no overall change is expected, answer "NO REACTION." Important:
compounds incorporating multiple stereogenic centers must be drawn with the correct
relative configuration.
Note: it is understood that chiral compounds will be obtained as racemic mixtures.
1. Br2, CH3OH
a.
H
2. NaN3
3. Zn, H+
b.
NH2
(or enantiomer)
CH3O
H
1. H2SO4, 180 °C
2. HI
OH
3. Na
4. Na, liq. NH3
CH3
1. HBr, radical
initiator
c.
2. CH3MgBr
CuBr
1. HCl
and
d.
2. CH3OH, heat
CH3
OCH3
1. NBS, hν
2. Mg
e.
3. CO2
4. mild H3O+
OH
O
(2 products)
Chem 203
midterm exam 2
p. 5 of 7
5. (20 pts.) Propose a method to achieve the transformations shown below. Indicate all the
reagents, in the correct order, that are required to induce each transformation. Present your
answer as a numbered list displayed above / below each reaction arrow. If a product appears
to be unavailable from the indicated starting material by any method known to you, write
"INACCESSIBLE" on the reaction arrow
Note: it is understood that chiral compounds will be obtained as racemic mixtures.
1. NBS, hν
OK
2.
a.
3. BH3
4. H2O2, aq. NaOH
OH
1. NaNH2, then CH3–I
2. Na, liq. NH3
b.
Br
H
3. Br2
H
Br
1. O3, then Zn, H+
2. CH3MgBr
c.
3.H2SO4, heat
1. Cl2, hν
2.
OK
d.
3. NBS, hν
4. CH3MgBr, CuBr
e.
OH
1. Br2
2. CH3SNa
O
SCH3
Chem 203
midterm exam 2
p. 6 of 7
6. (20 pts.) Propose a method for the preparation of compounds a. – e. below starting ONLY
with methane (CH4) and acetylene (H–C≡C–H) as the source of carbon atoms. You may use
any additional reagent that might be needed (e.g., borane, HCl, Mg, H2O2, potassium tertbutoxide, etc.). Present your answer as a clear flowchart that shows all intermediate steps and
products. Substances obtained in a prior sequence may be used in later sequences. It is not
necessary to draw mechanisms
H2O
OH
a.
**
H2
H2SO4
H
CH4
Na +
H
Cl2, hν
NaNH2
CH3–Cl
H
H2 **
O3, then
H
Na +
* Na / NH3 (leading to the trans-isomer) OK here
** BH3 then H2O2 and aq. NaOH OK for this step
CH3–Cl
O
b.
CH3–Cl
Lindlar
NaNH2
H
*
Zn / H+
Lindlar
Br
H2 *
H
H
Lindlar
Na +
CH2=CH2
HBr
Br
NaNH2
Na +
H
(part a.)
** Na / NH3 (leading to the trans-isomer) OK here
* Na / NH3 OK in this step
H
Chem 203
midterm exam 2
BH3, then
OH
c.
p. 7 of 7
H2O2 , aq. NaOH
HBr
Br
H2, Lindlar
(Na / NH3 OK, too)
Na +
H
(part a.)
(part a.)
Na, then
OCH3
d.
OH
O
(part b.)
H
CH3-Cl
(part a.)
MgBr
Mg
H2 *
HBr, rad.
Lindlar
initiator
H
(part b.)
(Li OK, too)
Br
* Na / NH3 OK in this step
CH3O
H2
Na +
CH3O
e.
Lindlar
(part a.)
CH3O
Br
Br2, CH3OH
NaOH
CH3–Cl
(part a.)
H2, Lindlar
CH3OH
(part a.)
(Na / NH3
OK, too)
(alternative answers may be acceptable)