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

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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

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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

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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.

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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!

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