Alkene reaction exercises v3.2

Prepared by Bill Weigel and Dr. Laurie Starkey [Version 3.2]

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Reaction Reagents Added Groups Regioselectivity Stereoselectivity

HydrohalogenationsHBr -H -Br Markovnikov Mixed

HBr, ROOR -H -Br Anti-Markovnikov Mixed

Halogenation Br2 -Br -Br N/A Anti

Halohydrin Formation Br2, H2O (or ROH) -Br -OH (or OR) "Markovnikov" Anti

Acid Catalyzed Hydration H2SO4, H2O -H -OH Markovnikov Mixed

Oxymercuration-Demercuration1) Hg(Oac)2, H2O

-H -OH Markovnikov Anti2) NaBH4

Hydroboration-Oxidation1) BH3·THF, H2O

-H -OH Anti-Markovnikov Syn2) H2O2, NaOH

Hydroxlyation KMnO4 or OsO4 -OH -OH N/A Syn

Epoxidation mCPBA (RCO3H) Epoxide Ring N/A Syn

Hydrogenation H2, Ni/Pt/Pd cat. -H -H N/A Syn

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For each transformation shown, propose an acceptable reaction mechanism. Be sure to use proper arrow pushing, and include all lone pairs and formal charges.

A)

C) D)

B)

BA C D

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1

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A)

C) D)

B)

BA C D

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2

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Predict the product in each of following reactions.

A)

C) D)

B)

B DCA

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3

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A)

C)D)

For additional practice, determine if the products shown are chiral and if so, propose a separate mechanism for the formation of its enantiomer.

B)

BA C D

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A) B)

C) D)

(±)(±)

(±)(±)

BA C D

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Predict the major product(s) in each of the following reactions.

A)

B)

C)

E)

F)

D)

BA C D

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E F

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D)

For each transformation shown, propose an acceptable reaction mechanism accounting for the products shown. Be sure to use proper arrow pushing, and include all lone pairs and formal charges.

Acid-Catalyzed Hydration

A)

B)

C)

BA C D

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A)

B)

C)

D)

E)

F)

Predict the major product in each of the following reactions.

BA C D

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E F

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Complete the mechanism pathway for the oxymercuration of propene by filling in the missing arrows, lone pairs, and formal charges.

The reductive demercuration step involves a more complex mechanism pathway in which the acetomercury

group is displaced by a borohydride hydrogen. (You do not need to show this)

Oxymercuration-Demercuration

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This reaction can be used to achieve “Markovnikov” additions of H2O to an alkene without the possibility of rearrangements.

Explain why this is.

9

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A)

B)

C)

D)


BA C D

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Hydroboration-Oxidation of an alkene is a 2 step process as shown in the reaction above. Fill in the missing intermediate compounds for the hydroboration portion of this reaction below. Use arrows to account for the formation of the intermediates.

Hydroboration-Oxidation

Speculate as to why the successive intermediates become increasingly more regioselective for the anti-markovnikov orientation.

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A)

B)

D)

C)

E)

BA C D

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E

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A)

B)


C)

D)

BA C D

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13

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For each transformation shown, propose an acceptable chemical mechanism. Be sure to use proper arrow pushing, and include all lone pairs and formal charges.

A)

B)

C)

D)

BA C D

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14

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A)

B)


C)

D)

BA C D

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15

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A)

B)

C)


D)

E)

F)

BA C D

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E F

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BA C E

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D

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BA C D

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E F

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Predict the reagent(s) needed to accomplish the following transformations.

BA C D

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E

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The next slides contain only the answers. There are no more questions from this point forward.

Hydrohalogenation of Alkenes (I)Exercise A

For each transformation make sure your mechanism correctly accounts for the Markovnikov addition of HX.

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Hydrohalogenation of Alkenes (I)(Exercise B)


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Hydrohalogenation of Alkenes (I)(Exercise C)


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Hydrohalogenation of Alkenes (I)Exercise D


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Hydrohalogenation of Alkenes (II)(Exercise A)

Each of the following mechanisms first involves homolytic cleavage of a peroxide and the subsequent creation of a bromine radical:

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Hydrohalogenation of Alkenes (II)(Exercise B)


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Hydrohalogenation of Alkenes (II)(Exercise C)


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Hydrohalogenation of Alkenes (II)(Exercise D)


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Hydrohalogenation of Alkenes (III)(Exercise A)

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Hydrohalogenation of Alkenes (III)(Exercise B)

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Hydrohalogenation of Alkenes (III)(Exercise C)

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Hydrohalogenation of Alkenes (III)(Exercise D)

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Reactions Involving Halonium Ion Intermediates (I)(Exercise A)

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Reactions Involving Halonium Ion Intermediates (I)(Exercise B)

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Reactions Involving Halonium Ion Intermediates (I)(Exercise C)

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Reactions Involving Halonium Ion Intermediates (I)(Exercise D)

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Reactions Involving Halonium Ion Intermediates (II)(Exercise A)

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Bromide and water have lone pairs that in theory can allow either to act as a lewis base. However, solvents are typically present in large excess compared to reagents. In terms of probability, what would be the more likely base, the solvent (H2O) or a reagent (Br-)?

When proposing deprotonations, also take into consideration the relative strengths of any proton acceptors in solution. What will be more likely to perform deprotonation, water (a weak base) or bromide (the conj. base of a strong acid)?

Reactions Involving Halonium Ion Intermediates (II)(Exercise B)

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Reactions Involving Halonium Ion Intermediates (II)(Exercise C)

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Reactions Involving Halonium Ion Intermediates (II)(Exercise D)

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Be mindful of the creation of chiral products and when this is the case, indicate so by either drawing out both enantiomers or indicating the presence of the enantiomer.

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A)

Reactions Involving Halonium Ion Intermediates (III)(Exercise A)


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B)

Reactions Involving Halonium Ion Intermediates (III)(Exercise A)


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C)

Reactions Involving Halonium Ion Intermediates (III)(Exercise C)


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D)

Reactions Involving Halonium Ion Intermediates (III)(Exercise D)


E)

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Reactions Involving Halonium Ion Intermediates (III)(Exercise E)


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F)

Reactions Involving Halonium Ion Intermediates (III)(Exercise F)

The first step installs two leaving groups (the halogens) and the second step involves E2 between the base and each alkyl halide (two individual eliminations for each Cl).

Additions of H2O to Alkenes (I) (Exercise A)

Be sure to correctly apply Markovnikov’s Rule in your mechanism. Ask the question “How can a proton be added to the alkene in such a way that the resulting carbocation is in the more stable location?”

Note how the acid-catalyzed mechanism begins with the consumption of the acid and ends with its regeneration


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http://en.wikipedia.org/wiki/Markovnikov's_rule

Additions of H2O to Alkenes (I) (Exercise B)




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Additions of H2O to Alkenes (I) (Exercise C)




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Additions of H2O to Alkenes (I) (Exercise D)

Each if the three products involves a rearrangement of the carbocation intermediate. The answer below shows the rearrangement steps using green arrows for clarity.


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Additions of H2O to Alkenes (II)(Exercise A)


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A)

Additions of H2O to Alkenes (II)(Exercise B)


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B)

Additions of H2O to Alkenes (II)(Exercise C)


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C)

Additions of H2O to Alkenes (II)(Exercise D)


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D)

Additions of H2O to Alkenes (II)(Exercise E)


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E)

Additions of H2O to Alkenes (II)(Exercise F)


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F)

(Ring Expansion Product)

Additions of H2O to Alkenes (III)Oxymercuration-Demercuration

Rearrangements are not observed because no carbocation is ever generated in the mechanism and therefore no carbocation shifts can occur. The carbons in this case only develop partial charges and the water attacks the more substituted location (similar to bromonium ion) that results in the final markovnikov orientation.

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Additions of H2O to Alkenes (IV)(Exercise A)


Consider both regiochemistry and stereochemistry. Remember to indicate stereoisomerism when appropriate.

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A)

Additions of H2O to Alkenes (IV)(Exercise B)



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B)

Additions of H2O to Alkenes (IV)(Exercise C)



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C)

Additions of H2O to Alkenes (IV)(Exercise D)



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D)

Additions of H2O to Alkenes (V)Hydroboration-Oxidation

As successive alkyl groups are added, steric bulk increases and increasingly favors a transition state that minimizes steric forces. The anti-markovnikov orientation is thus preferred; the more substituted alkene carbon across from the smaller borohydride H.

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Additions of H2O to Alkenes (VI)(Exercise A)


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You should consider both regio and stereo selectivity when predicting the products. Be mindful of any chiral products that are generated.

A)

Additions of H2O to Alkenes (VI)(Exercise B)


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B)

Additions of H2O to Alkenes (VI)(Exercise C)


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C)

Additions of H2O to Alkenes (VI)(Exercise D)


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D)

Additions of H2O to Alkenes (VI)(Exercise E)


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E)

Epoxidation Reactions(Exercise A)

A)

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Epoxidation Reactions(Exercise B)

B)

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Epoxidation Reactions(Exercise C)

C)

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Epoxidation Reactions(Exercise D)

D)

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Acid/Base Catalyzed Ring Openings of Epoxides (I)(Exercise A)

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Acid/Base Catalyzed Ring Openings of Epoxides (I)(Exercise B)

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Acid/Base Catalyzed Ring Openings of Epoxides (I)(Exercise C)

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Acid/Base Catalyzed Ring Openings of Epoxides (I)(Exercise D)

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Acid/Base Catalyzed Ring Openings of Epoxides (II)(Exercise A)

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A)

Acid/Base Catalyzed Ring Openings of Epoxides (II)(Exercise B)

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B)

Acid/Base Catalyzed Ring Openings of Epoxides (II)(Exercise C)

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C)

Acid/Base Catalyzed Ring Openings of Epoxides (II)(Exercise D)

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D)

A)

Oxidative Cleavage via Ozonolysis(Example A)

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*NOTE* The reagents used in the second step of these reactions (Zn and DMS) are said to be “reductive” workups for their ability to leave 1° carbons as aldehydes. However there are other reagents (H2O2) that that convert 1° carbons to carboxylic acids and are thus termed “oxidative” workups.

The terms “reductive” and “oxidative” in this case are relative to each other; both workups still yield products that are more oxidized than the alkene staring material.

Oxidative Cleavage via Ozonolysis(Example B)

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B)



Oxidative Cleavage via Ozonolysis(Example C)

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C)



Oxidative Cleavage via Ozonolysis(Example D)

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D)



Oxidative Cleavage via Ozonolysis(Example E)

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E)



Oxidative Cleavage via Ozonolysis(Example F)

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The pi-bonds within the phenyl ring are less reactive (due to aromatic properties) and are thus resistant to ozonolysis.

Predict the Products (I)(Exercise A)

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A)

Predict the Products (I)(Exercise B)

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B)

Predict the Products (I)(Exercise C)

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C)

Predict the Products (I)(Exercise D)

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D)

Predict the Products (I)(Exercise E)

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E)

Predict the Products (II)(Exercise A)

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A)

Predict the Products (II)(Exercise B)

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B)

Predict the Products (II)(Exercise C)

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C)

Predict the Products (II)(Exercise D)

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D)

Predict the Products (II)(Exercise E)

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E)

Predict the Products (II)(Exercise F)

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F)

Predict the Reagents(Exercise A)

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A)

Predict the Reagents(Exercise B)

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B)

Predict the Reagents(Exercise C)

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C)

Predict the Reagents(Exercise D)

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D)

Predict the Reagents(Exercise E)

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E)

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Alkene reaction exercises v3.2

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