Chemistry 216
“Honors Organic Chemistry I”
Fall Semester 2001
Dr. Rainer Glaser
Examination #4
“Reactions of Alkenes.”
Friday, November 9, 2001, 9:00 - 9:50
Name:
Answer Key
Question 1. Halogenation Reactions. (VC #8)
20
Question 2. Alcohol Synthesis.
20
Question 3. Diol Synthesis. (News #8)
16
Question 4. Reductions & Oxidations. (VC #8)
24
Question 5. Cyclopropanation & Diels-Alder Reaction.
16
Question 6. Chlorine Dioxide. (Current News)
Total
4
100
—1—
Question 1. Halogenation Reactions. (20 points)
(a) Consider the electrophilic addition of HBr to propene. Indicate which of these products is the
Markovnikov product and which one is the anti-Markovnikov product. Indicate the major product.
Provide a detailed reaction mechanism for the formation of the products. (10 points)
Markovnikov or Anti-M.?
H3C–CH2–CH2–Br
H3C–CHBr–CH3
anti-M
M
Mark the major product.
major
Detailed reaction mechanism:
H3C–CH=CH 2 + H+
H3C–CH2–CH2+ (1°, minor)
H3C–CH=CH 2 + H+
H3C–CH+–CH3 (2°, major)
H3C–CH2–CH2–Br (minor)
H3C–CHBr–CH3 (major)
(b) B romination of cyclohexene . The bromide attacks the bromonium ion from the _________ (same,
opposite) side and the bromination is a ________ (syn, anti) addition that results in _________ (trans,
cis) 1,2-dibromocyclohexane. Draw complete Lewis structures (show all atoms, all lone pairs, formal
charges) of cyclohexene, of the bromonium ion, and of the product. Draw the product in that
conformation which is initially obtained in the addition; in that conformation one Br is ________ (axial,
equatorial) and the other Br is ________ (axial, equatorial). (10 points)
H
Br
H
Br
H
H
—2—
Br
Question 2. Alcohol Synthesis. (20 points)
(a) A molcular model of the borane-tetrahydrofuran complex is shown. Draw a Lewis structure for this
complex. Draw all lone pairs and clearly indicate any formal charges. The hybridization of B in free
BH3 is _______ (sp, sp2, sp3) and the hybridization of B in the borane-tetrahydrofuran complex is
_____ (sp, sp 2, sp3). The hybridization of O in free THF is _______ (sp, sp2, sp3) and the hybridization
of O in the borane-tetrahydrofuran complex is _____ (sp, sp 2, sp3). (6 points)
H
Lewis Structure
H
H
H
O
B
H
B
O
H
H
H
H
H
H
H
H
H
(b) 2-Methyl-2-butene is treated with H 3B•THF
in THF solvent and the resulting addition product
is oxidized with alkaline H2O2. The major
product is obtained in 98% yield. Draw the
structure of this product and provide a suitable
IUPAC name. (6 points)
(c) Oxymercuration/demercuration of 2-Methyl2-butene. The alkene is treated with the reagent
_____HgAc2_____ and then with aqueous acid to
form an organomercurial alcohol. Draw the
structures of the organomercurial alcohol and of
the alcohol obtained after reduction with NaBH 4.
(8 points)
H
H
Me
Me
Me
2-methyl-3-butanol
3-methyl-2-butanol
HO
Me
Me
HO
Me
Me
—3—
OH
H
Me
HgAc
H
H
Me
Question 3. Diol Synthesis. (16 points)
The article “NEW USES FOR OLD DRUGS” (Fort Worth Star-Telegram, Nov. 19, 1998) talked about
minoxidil. The drug is applied by rubbing a solution in ethanol and propylene glycol directly on the
body. Let’s consider some aspects of diols and specifically of “propylene glycol.” Propylene glycol is
another name for 1,2-propanediol, H3C-CH(OH)-CH2OH.
(a) Draw the structure of MCPBA and use curved arrows to show how MCPBA serves as the source of
an oxygen atom. (6 points)
Cl
Cl
O
O
H
O
O
H
O
O
(b) Draw the structure of the epoxide formed by reaction of propene with MCPBA and specify the
reaction conditions to convert the epoxide to 1,2-propanediol. (4 points)
O
H
H
Me
H
H+, H2O
diol
(c) Oxidation of propene with osmium tetroxide. Draw the structures of the cyclic osmate ester.
Provide the reagent for the oxidation of the cyclic osmate ester to the diol with simultaneous reformation
of osmium tetroxide. (6 points)
O
O
Os
O
H2O2
O
H
H
Me
H
diol
- OsO4
—4—
Question 4. Reductions & Oxidations. (24 points)
(a) Hydrogenations of alkenes require a catalyst and most hydrogenations are carried out as
heterogeneous reactions. Give an example of a heterogenous hydrogenation catalyst and briefly
describe how the catalyst works. (4 points)
Pt, Pd, Ni, Fe. Catalyst’s main function: Homolysis of hydrogen gas; H2 à 2 H•
Hydrogenation catalysts break H-H into 2 H, that is they give HOMOLYSIS of H-H. A
HETEROGENEOUS catalysts does not mean that HETEROLYSIS occurs. A solid catalyst is used
in a liquid; we are having different phases == a HETEROGENEOUS system. Frequently, Q4 was
answered assuming that HETEROGENEOUS catalysts cause HETEROLYSIS.
(b) Ozonolysis of 2-butene. Draw the structures of the primary ozonide, of the ozonide, and of the two
products obtained by reductive work-up. Specify the reducing reagent employed in the reduction of the
ozonide. (12 points)
O
O
O
O
O
CH
Me
Me
Me-S-Me
HC
Me
HC
CH
Me
O
or
Zn/acid
Me
2x
C
O
H
(c) Suggest a synthesis of the HOOC-CH2-CH2-CH2-CH2-COOH from cyclohexene.
structures of substrates and important intermediates and specify all reagents. (8 points)
O
O
O
COOH
O
COOH
O
O
Reagents: 1) Ozone; 2) hydrogenperoxide
—5—
Draw
Question 5. Cyclopropanation & Diels-Alder Reactions. (16 points)
(a) The bicyclic molecule shown, 1-methyl-bicyclo[5.1.0]octane, can be synthesized via a
cyclopropane synthesis using the Simmons-Smith reagent. Draw the alkene that you would use as the
starting material and specify what the Simmons-Smith reagent is. The reaction is carried out in diethyl
ether. (6 points)
H2CI2
CH3
CH3
Zn (Cu)
Note: H2CI2 is diiodomethane (no chlorine in that reagent); IZn-CH2-I is correct as well.
(b) What combination of diene and dienophile would you choose in order to prepare the cyclohexene
shown? Draw the two substrates and indicate the electron flow in the Diels-Alder reaction using curved
arrows. Name the product: _4,5-dicyano-cyclohexene_. (8 points)
CN
CN
CN
CN
(c) Suggest a way to reduce the cyclohexene shown in (b) to the corresponding cyclohexane without
reduction of the cyano groups. (2 points)
Any catalytic hydrogenation will do; heterogeneous or homogeneous.
Sample answer: Hydrogen gas and Pt catalyst.
—6—
Question 6. Chlorine Dioxide. (4 points)
The New York Times, November 3, 2001
Gas Will Be Used to Kill Bacteria in Senate Office Building. By DAVID E. ROSENBAUM
WASHINGTON, Nov. 2 — Workers from the Environmental Protection Agency finished sealing the
doors and windows of the Hart Senate Office Building today with sheets of plastic and duct tape to make
it airtight in preparation for a novel house-cleaning. Sometime next week, technicians will wheel in
machines that generate chlorine dioxide, a gas that kills germs, and the gas will be spread throughout the
anthrax-infected nine-story building using the ventilation system. …
The ClO2 Fact Sheet from http://www.clo2.com/factsheet/factindex.html
Chlorine dioxide (ClO2) is a synthetic yellowish-green gas with chlorine like odor. ClO 2 is unstable as a
gas and will undergo decomposition into chlorine gas (Cl2), oxygen gas (O2) and heat. However, ClO2
is stable and soluble in an aqueous solution. For example, solutions of approximately 1% ClO 2 (10 g/L)
may safely be stored if the solution is protected from light and kept chilled.
The instability of ClO2 has an important consequence. It negates the possibility of creating and
transporting cylinders or rail cars of the gas. Instead ClO 2 must be produced and used at the same
location. However, despite the care and safety considerations which must be taken when using ClO 2,
the potential advantages of using this chemical greatly outweigh the possible disadvantage of on site
production. When produced and handled properly, ClO2 is an extremely effective and powerful biocide,
disinfectant agent and oxidizer. Used extensively in the Pulp and Paper Industry, new applications for
ClO2 in other areas such as municipal water treatment, are increasingly being investigated.
Provide the correct resonance forms for chlorine dioxide: Accepted anything reasonable.
Cl
Cl
O
O
Cl
O
O
O
Cl
O
O
Cl
O
—7—
O
O
Scratch Page!
—8—