Chapter 8:Reactions of Alkenes

Electrophilic Addition to Alkenes (8-2)

ÚAddition is the most common type of reaction of alkenes. Since the alkene double bond is nucleophilic, the reactions are usually called electrophilic additions since an electrophile is

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electrophilic additions since an electrophile is first added to the molecule.

– A wide variety of FG can be formed by addition of suitable reagents to C=C

ÚAddition Characteristics (mechanism):In C=C:– pi electrons are loosely held to the nuclei– electrophiles (E+) have affinity for these

electrons– C=C acts as the nucleophile (Nu-) giving

electrons

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electrons

ÚFor example, the addition of HX (X=Cl, Br, I) follow this mechanism. Most reactions of alkenes follow this type of addition:

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– The addition reactions given by alkenes can be classified in 5 sub-classes:

Electrophilic additionReductionAddition of carbenesOxidative additions

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Oxidative additionsOxidative cleavage

Addition of hydrogen halide (8-3)The Markovnikov Rule

When the alkene is symmetrically substituted, the question of orientation of the addition is not relevant, ie the H goes to one carbon of the double bond, and X to the other.

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However, for unsymmetrically substituted alkenes, two possible products exist, one is formed preferentially.

HBrH or H

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H H Br Br H

this is the onlyproduct formedWHY?

ÚThe answer is found in the mechanism and is related to the stability of the carbocations.ØFor these products, the carbocations were:

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ÚRemember the stability of carbocations follows:

B

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H

HBr

HH Br

HBr H

this is the onlyproduct formed

HBrH

H+

secondarycarbocation

Br-

H+H

tertiarycarbocation

Br-

ÚObservation:H+ adds to the end of the double bond that is less substituted, i.e. generates the more highly substituted carbocation (more stable).

HHBr-

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H

HBr

H Br

HBr H

this is the onlyproduct formed

HBrH

H+

secondarycarbocation

H+H

tertiarycarbocation

Br-

Ú Markovnikov’s Rule

In an electrophilic addition to alkene, the electrophile adds in such a way as to generate the most stable intermediate (carbocation).

Ú Reactions following this rule as said to follow:Markovnikov orientation or to give Markovnikov

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Markovnikov orientation or to give Markovnikov product

HX (where X= Cl or Br or I) adds to give Markovnikov products. We say that the reaction is regioselective (one product is preferentially produced when more than one isomeric product is possible).

Practice QuestionÚ What is the major organic product obtained from the

addition of HBr to each of the following compounds?

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ÚDraw a mechanism to explain the formation of the blue product from the previous slide.

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Ú It is possible to change the orientation of the addition by forcing the reaction to proceed by a radical mechanism. In this case, the anti-Markovnikov (reverse orientation) product is obtained.

CH CH

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+ BrCH3 C

CH3

CH CH3 CH3 C

CH3

CH CH3

Br

tertiary radical (more stable)

C CH

H

H

CH3

Br

+ H Br C CH

H

H

CH3

Br H

+ Br

Examples

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Hydration (addition of H2O) (8-4)– Direct Method: (requires a strong acid)

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This is a reversible reaction…ie in the presence of a strongacid, alcohols are dehydrated to the corresponding alkenes

ÚWater and acid react to form the hydronium ion which can now be used as the electrophile.

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ÚThe hydration of alkenes follows a Markovnikov orientation, ie the most stable intermediate carbocation is produced during the protonation.

most stable carbocationis formed

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ÚReplacing the water by an alcohol will generate ethers rather than alcohols by a similar mechanism H2SO4

MeOH

MeO

Mechanism of Ether Formation from Alkenes

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H O+

HMe

+ HOMeO+Me H

HOMe OMe

Practice QuestionsGive the major organic product formed in the following acid-catalyzed reactions.

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H2SO4

EtOH

ÚProvide a detailed mechanism for the following dehydration of cyclohexanol.

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Ú Hydration (Indirect Methods): These methods will avoid problems associated with carbocation rearrangements since they will not be produced here.

Ø Oxymercuration/Reduction (8.5)Follows Markovnikov (best method to use to get Markovnikov hydration products)

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Markovnikov hydration products)

ÚAdvantage: does not involve a free carbocation, therefore rearrangement is not possible.

1) Hg(OAc)2, H2O

2) NaBH4

HO

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2) NaBH4

H2SO4, H2OOH

product of thedirect hydration

ÚThe intermediate in this reaction is not a carbocation, but the mercurinium ion.

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ÚThe electrophile Hg+OAc is the result of the natural dissociation of mercuric acetate which takes place in solution.

Mechanism of Oxymercuration/Reduction of Alkenes

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Ú In unsymmetrical alkenes, the attack of H2O is at the most substituted carbonsince it carries most of the positive charge. This is due to the inductive effect.

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Alkoxymercuration/Reduction (8-6)

This reaction is identical in terms of orientation of the substituents and mechanism. The only difference is that an alcohol is used rather than water, leading to an ether product. MarKovnikov products are

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an ether product. MarKovnikov products are obtained.

Practice QuestionHow could the following compound be synthesized from analkene? Give all the possibilities.

OH

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Hydration (indirect method)

Ú Hydroboration/oxidation (8-7)

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best method to obtain anti-Markovnikovalcohols from alkenes. It is a syn addition, ie both Nu/E are transferred to the double bond from the same side.

ÚSince boron is deficient in electron, it interacts with the most nucleophilic site of the alkene, i.e. the least substituted carbon and produce an alkylborane.

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ÚOxidation of the alkylborane produces the alcohol.

ÚSince the hydroboration is syn, in cyclic system where the stereochemistry may be obtained, the alcohol retains the stereochemistry of the borane.

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stereochemistry of the borane.

CH3CH3

H

BH2

CH3

H

OH

H2O2NaOH

BH3 THF

What products would be obtained from the hydroboration/oxidation of the following alkenes?

2-methyl-2-butene

Practice Question

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cyclohexene

Addition of Halogens to Alkenes (8-8)

The addition of halogen to alkenes leads to the formation of vicinal dihalides. The addition is anti and does not proceed via a carbocation. No

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proceed via a carbocation. No rearrangement is expected.

Mechanism of Haloganation of Alkenes

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Ú In cyclic alkenes, a trans product is usually obtained.

Br+

H H Br- Br

H

H

Br

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Br Br

Formation of Halohydrins (8-9)When this reaction (addition of X2 is performed in the presence of water, X2adds to give halohydrins instead of vicinal dihalides, because of the presence of another nucleophile (water) in the reaction mixture

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reaction mixture

Ú Mechanism: same as addition of X2, but in this case the nucleophile is water. Since water is also the solvent, there are not many chances of the halonium ion reacting with X-

Ú Orientation of the halohydrin formation follows Markovnikov’s rule, i.e. X is on the least substituted carbon and OH on the most substituted

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substituted

Catalytic hydrogenation of Alkenes (8-10)Also a syn addition of the two hydrogen atom across the double bond

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Reaction takes place at the surface of the metal via the absorption of H2 and alkene. The addition is “syn”: both hydrogen atoms add from the same sidesame side of C=C

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Practice QuestionsComplete the following reactions. Give the major organic product(s).

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Epoxidation of Alkenes (8-12)

The addition of an electrophilic oxygen to the C=C gives oxirane (epoxides), 3-membered cyclic ethers

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ÚRequires the use of a peracid (peroxyacid)

OO

H

O

peracetic acid

CO3H

Cl

CO3H

perbenzoic m-chloroperbenzoic

electrophilic oxygen

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peracetic acid perbenzoicacid

m-chloroperbenzoicacid (mCPBA)

Ú The addition is concerted and “syn” meaning that the alkene stereochemistry is preserved in the product

i.e. cis alkenes give cis epoxidestrans alkenes give trans epoxides

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Practice Question

How would you make these epoxide from an alkene?

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O

Syn Hydroxylation of Alkenes (8-14)As the name of the reaction suggests, the reaction is syn, meaning that the two hydroxyl group would be in a cis geometry on cyclic structures.

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ÚOsmic acid in hydrogen peroxide or potassium permanganate (KMnO4) in base can be used to accomplish this reaction. However, if KMnO4 is used, the solution must be diluted and cold.

OsO4OH

KMnO4OH

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OsO4

H2O2OH

cis diols are produced since the reaction is syn

KMnO4

NaOHOH

Ú Oxidative Cleavage of Alkenes (8-15)We just saw that reaction of alkenes with KMnO4 produce vicinal diols. This product is an alcohol and can be easily oxidized further, resulting in the cleavage of the original double bond. It is possible to carry out the cleavage of

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It is possible to carry out the cleavage of the double bond directly with KMnO4 ifthe solution is concentrated and warm.

Ú This is because KMnO4 is a strong oxidizerÚ For alkenes where the double bond has at least

one H atom attached to the C=C, carboxylic acid(s) is (are) produced (see below for increasing order of oxidation of carbon). If the alkene is not symmetrical, you must always write 2 productsfor this reaction.

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Ozonolysis

– Addition of ozone (O3) to the double bond is another method to produce carbonyl containing compounds by oxidative cleavage, it is a milder reaction and allows for the isolation of aldehydes under non-oxidative

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isolation of aldehydes under non-oxidative conditions.

Mechanism of Ozonolysis

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Practice QuestionGive the product of the following oxidative cleavage reactions.

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Study Problems

8-49 Show how you would synthesize the following compound using methylenecyclohexane as your starting material.

O

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make

ÚWrite a detailed mechanism for the following reaction.

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8-61α-Pinene has a formula C10H16. Using the following diagram, determine the structure of this compound as well as the products A-D.

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