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CHEMISTRY 313-01
MIDTERM # 2 – answer key
March 12, 2009
Statistics:
• Average: 72 pts (72%);
• Highest: 98 pts (98%); Lowest: 21 pts (21%)
• Number of students performing at or above average: 22 (50%)
• Number of students performing at or below 55%: 8 (18%)
1.
2.
3.
(8 pts) Mark as true (T) or false (F) the following statements. Do not explain!
• (T) All E1 reactions involve formation of carbocations;
• (T) More stable carbocations are generated faster;
• (T) Carbocations are electrophiles;
• (T) Carbocations are electron deficient;
• (T) Free radicals are electron deficient;
• (T) Alcohols are Brønsted bases;
• (F) The rate-determining step is always the fastest elementary step;
• (F) More stable alkenes have larger heats of hydrogenation;
Circle ALL that apply:
A. (3 pts) Free-radical brominations:
a. Are chain reactions;
b. Occur via carbocations;
c. Require an initiation step;
d. Are selective for tertiary positions;
B. (3 pts) E2 reactions:
a. Require strong bases;
b. Occur faster at lower temperatures;
c. Are subject to stereoelectronic control;
d. Generate alkyl halides;
C. (3 pts) The following reactions can be used to prepare alkenes:
a. Free-radical chlorination of alkanes;
b. Hydroboration – oxidation of alkenes;
c. Dehydration of alcohols;
d. Hydrogenation of alkenes;
(3 pts) Provide an acceptable name for each of the following molecules:
OH
F
4-fluoro-1-butanol
trans-cyclooctene
4.
OH
2-cyclohexenol
(5 pts) Provide structural formula for each of the following molecules:
Br
Cl
5.
(E)-2-Penten-1-ol
Methylenecyclopropane
Vinyl chloride
I
OH
OH
Trans-2-bromocyclopentanol
Allyl iodide
(5 pts) Assign E or Z configuration to each of the following alkenes. Do not explain!
F
O
O
HO C
CH3
C
NH2
Br
CH2OH
H3CO C
N
Z
HO C
HC O
HC CH2
O
E
E
E
6.
(3 pts) For the species below, complete the Lewis structure and draw two more valid resonance structures. Clearly show, using
the curved arrow notation, the shift of electron pairs. Rank the resonance structures.
O
N
O
1
7.
O
N
O
O
N
O
1
2
(4 pts) Arrange the following alkenes in order of increasing stability.
A. Cis-2-pentene;
B. Trans-2-pentene;
C. 2-Methyl-2-butene;
D<A<B<E<C
D. 1-Pentene;
E. 2-Methyl-1-butene;
8.
(2 pts) Select the alcohol which is expected to have the fastest reaction with hydrobromic acid.
A. 1-Pentanol;
B. Neopentyl alcohol
C. 3-Pentanol;
D. 2-Methyl-2-butanol;
9.
(20 pts) Predict the organic product in each of the following reactions. If more than one product is formed, indicate the major
component.
CH3
CH3
H2
OH
H3PO4
+
CH3
Ni
CH3
Δ
Br
NaOC2H5
HBr
C2H5OH
ROOR
Br
1) O3
1) BH3.THF
-
2) H2O2, OH
O + O
2) H2O, Zn
OH
Cl
1 eq. Cl2
HCl
Cl
hν
HO
Br
OH
Br
100% H2SO4
+ 2 PBr3
CH3
Br2
H3C
hν, Δ
Br
CH2
CH3
NaOCH3
+
CH3OH
major
OH
SOCl2
pyridine
Cl
NaOC2H5
C2H5OH
+
OSO3H
10. (4 pts) In each of the following cases, suggest the appropriate starting material for the indicated transformation.
MCPBA
CH2Cl2
1) O3
O
O
O
2) (CH3)2S
11. (4 pts) Each of the following carbocations can rearrange to a more stable one. Propose structures for the likely rearranged
species. In each case clearly indicate the type of rearrangement (i.e. hydride shift, methyl shift, etc.).
methyl shift
hydride shift
12. (6 pts) Provide one example of each of the following:
A. An anti-addition reaction to an alkene;
Cl
Cl2
H2O
OH
B. A concerted reaction;
MCPBA
CH2Cl2
O
C. An E1 reaction;
OH
H3PO4
Δ
13. (2 pts) Provide the structure of a 5-carbon alkyl iodide, which yields a single alkene upon reaction with NaOC2H5/C2H5OH.
I
14. (4 pts) For each of the reactions below, select the appropriate starting alkyl bromide that would yield the indicated alkene as the
exclusive product.
Br
NaOCH3
Br
CH3OH
NaOCH3
CH3OH
15. When trans-2-methylcyclohexanol is subjected to acid-catalyzed dehydration, the major product is 1-methylcyclohexene.
However, when trans-1-bromo-2-methylcyclohexane is subjected to dehydrohalogenation with NaOCH2CH3/CH3CH2OH, the
only product is 3-methylcyclohexene.
A. (4 pts) Provide equations for the above outlined reactions;
CH3
CH3
OH
H2SO4
CH3
CH3
Br
heat
NaOC2H5
C2H5OH
major
only
B. (4 pts) Account for the difference in products in the two reactions (Show explicit structures; I will not accept a
verbal only explanation!! More structures, fewer words!).
Dehydration: It occurs via E1 mechanism. A carbocation is formed as an intermediate and it generates the alkene products
according to the Zaitsev Rule, yielding predominantly the most substituted, 1-methylcyclohexene.
CH3
CH3
H3O
+
minor CH
3
CH3
CH3
OH
OH2
major
H-shift
CH3
CH2
minor
Dehydrobromination: It occurs via E2 mechanism (i.e. a concerted process). The type and quantity of product(s) is subject
to stereoelectronic control. Abstraction of proton can occur only from Cβ – H bonds that are coplanar to the C – Br bond
(best is anti-coplanar, or at least syn-coplanar). The starting material can exist in two conformations, of which only the
diaxial has ONE Cβ – H bond, which is anti-coplanar to the C – Br bond. Only this particular proton can be abstracted by the
base, yielding a single product: 3-methylcyclohexene.
Br
OC2H5
H3C
Br
no coplanar
C - H bonds
H3C
CH3
CH3 H
ONE anti-coplanar C - H bond
16. (2 pts) Provide the structure of an octane isomer, which yields only one monochlorinated product upon free-radical chlorination.
17. (4 pts) The addition of HCl to 1-isopropylcyclohexene yields a rearranged product as shown below. Offer a detailed mechanism
to account for the reaction outcome.
+
HCl
Cl
H
H-shift
+
H Cl
+
Cl
Cl
Cl
18. Suggest synthetic sequences for each of the following transformations:
A. (3 pts) Conversion of 1-propanol to 2-propanol.
H3PO4
OH
Δ
H2O
H3O+
OH
O .
B. (4 pts) Conversion of cyclopentane to
Cl
Cl2
NaOCH3
MCPBA
hν
CH3OH
CH2Cl2
O
19. (2 pts) BONUS PROBLEM (In order to receive credit for this problem, it has to be solved entirely!!). There are other
reagents, besides HBr, that can add to alkenes following a free-radical mechanism, in the presence of peroxides. Suggest a
plausible mechanism and predict the product of free-radical addition of methanethiol (CH3SH) to propene.
+
CH3SH
ROOR
Δ
??
Initiation:
1)
RO OR
2) RO
+
2 RO
Δ
ROH
H SCH3
+
SCH3
Propagation:
1)
+
2)
SCH3
SCH3
SCH3
+
SCH3
H SCH3
Overall:
+
CH3SH
SCH3
ROOR
Δ
+
SCH3