State University of New York at Albany State University of New York at Albany

Reactions of AlkenesReactions of Alkenes

Some Reaction TypesSome Reaction Types::

C X + Y C Y + X

C CC C

X Y

+ X Y

C C X Y+ C C

X Y

Addition Addition

Elimination Elimination

SubstitutionSubstitution

Addition ReactionsAddition Reactions

The vast majority of the reactions of The vast majority of the reactions of alkenes are additions.alkenes are additions.

When the addition reaction is initiated by When the addition reaction is initiated by attack of the pi bond electrons of the double attack of the pi bond electrons of the double bond on an electrophile to form a bond on an electrophile to form a carbocation, the reaction is known as an carbocation, the reaction is known as an electrophilic additionelectrophilic addition..

Addition of HX to alkenesAddition of HX to alkenes In the addition of H-X to alkenes, H bonds to the In the addition of H-X to alkenes, H bonds to the

carbon of the double bond with the greater number carbon of the double bond with the greater number of hydrogens. This is called of hydrogens. This is called Markovnikov’sMarkovnikov’s rule. rule.

C C + H X C

H

C

X

CH2CH3 CH + H Br CH3 CH

Br

CH2

H

Examples:

General Reaction:

HCl

Cl

CH2CH3 CH + H CH3 CH CH2

H

CH3 CH CH2

H

BrCH3 CH

Br

CH2

H

Mechanism:

1.

2.

The reaction is The reaction is regiospecificregiospecific, and the product obtained, and the product obtainedis called the Markovnikov product. It is formed becauseis called the Markovnikov product. It is formed becausethe reaction proceeds to form the most stable carbo-the reaction proceeds to form the most stable carbo-cation intermediate.cation intermediate.

CH2CH3 CH + H Br CH3 CH

H

CH2

Br

Example:ROOR

When HBr is added in the presence of peroxides,When HBr is added in the presence of peroxides,the the anti-Markovnikovanti-Markovnikov product is formed. product is formed.

The reason for the change in The reason for the change in regiochemistryregiochemistry is isthat the reaction proceeds via a free radical that the reaction proceeds via a free radical mechanism rather than a carbocation mechanism rather than a carbocation mechanism. The production of free radicals ismechanism. The production of free radicals isinitiated by the presence of the peroxide.initiated by the presence of the peroxide.

RO ORheat

R O + O R

R O + H Br R OH + Br

CH2CH3 CH Br+ CH2CH3 CH

Br

CH2CH3 CH

Br

H Br CH2CH3 CH

BrH

+ Br

MechanismMechanism::

Note that reaction proceeds to form the most stableNote that reaction proceeds to form the most stableradical.radical.

3. Addition of Water—Hydration of Alkenes3. Addition of Water—Hydration of Alkenes

C C + C

H

C

OH

H2OH

General Reaction:General Reaction:

CH2CH3 CH + CH3 CH

OH

CH2

H

Example:

H2OH

This reaction follows Markovnikov’s rule.This reaction follows Markovnikov’s rule.

H OCH2CH3 CH + CH3 CH CH2

H

CH3 CH CH2

H

CH3 CH

O

CH2

H

Mechanism:

1.

2.

H

H

OH2

+ H2O

HH

CH3 CH

O

CH2

HHH

3. CH3 CH

OH

CH2

H

+ H3O

4. Addition of Sulfuric Acid4. Addition of Sulfuric Acid

C C + C

H

C

OSO3H

General Reaction:

HH2SO4

alkyl hydrogen sulfate

CH2CH3 CH + CH3 CH

OSO3H

CH2

Example:

H2SO4

H

The alkyl hydrogen sulfate can be converted The alkyl hydrogen sulfate can be converted to an alcohol by boiling in water.to an alcohol by boiling in water.

C

H

C

OSO3H

H2OC

H

C

OH

+ HSO4

The reaction proceeds by an SThe reaction proceeds by an SNN1 mechanism 1 mechanism

in which the OSOin which the OSO33H acts as a leaving group.H acts as a leaving group.

CH3CH3 CH CH3 CH CH2

H

CH3 CH CH2

H

CH3 CH

O

CH2

H

Mechanism:

1.

2.OH2

+

HH

CH3 CH

O

CH2

HHH

3. CH3 CH

OH

CH2

H

+ H3O

OSO3H

O S OH

O

O

good leaving group

5. Oxymercuration-Demercuration5. Oxymercuration-Demercuration

C C + C

OH

C

HgOAc

General Reaction:

H2OHg(OAc)2

mercuric acetate

NaBH4C

OH

C

H

Example:

1. Hg(OAc)2/H2O

2. NaBH4

CH3

OH

The addition is anti, and Markovnikov The addition is anti, and Markovnikov orientation is observed.orientation is observed.

O

O

Hg O

O

O

O

Hg O

O

+

O

O

Hg

Hg

OAc

Hg

OAc

OH2

CH3

O

HgOAc

H

H

mercuronium ion

CH3

O

HgOAc

H

HOH2

NaBH4CH3

OH

Mechanism:Mechanism:

6. Alkoxymercuration-Demercuration6. Alkoxymercuration-Demercuration

Example:

1. Hg(OAc)2/CH3OH

2. NaBH4

CH3

OCH3

When mercuration takes place in an alcohol When mercuration takes place in an alcohol solvent, ethers are formed. The mechanism solvent, ethers are formed. The mechanism is the same as that for oxymercuration-is the same as that for oxymercuration-demercuration, except that the mercuroniumdemercuration, except that the mercuroniumion is attacked by the alcohol instead of water.ion is attacked by the alcohol instead of water.

7. Hydroboration of Alkenes—anti-Markovnikov7. Hydroboration of Alkenes—anti-Markovnikov addition. addition.

C C + C

H

C

BH2

General Reaction:

C

H

C

OH

B2H6

H2O2

alkyl borane

Example:

1. BH3/THF

2. H2O2/OH

H

CH3

OHOH

H

=

BH

H

H

BH3

H

CH3

BH2

The reaction is anti-Markovnikov. The The reaction is anti-Markovnikov. The addition is syn and the reaction is addition is syn and the reaction is stereospecific.stereospecific.

Mechanism for hydroboration step:Mechanism for hydroboration step:

8. Catalytic Hydrogenation:8. Catalytic Hydrogenation: This reaction is formally a reduction, with HThis reaction is formally a reduction, with H22 adding adding

across the double bond to give an alkane. The across the double bond to give an alkane. The process usually requires a catalyst containing Pt, process usually requires a catalyst containing Pt, Pd, or Ni. The addition is syn.Pd, or Ni. The addition is syn.

C C + C

H

C

H

General Reaction:

H2

Pt

Example:H

CH3

H

H2/Pt

9. Addition of Halogens to Alkenes9. Addition of Halogens to Alkenes

C C + C

X

C

X

General Reaction:

X2

Example:CH3

Br

Br

Br2

Mechanism:Mechanism:

Br

Br

bromonium ion

Br Br+

BrCH3

Br

Br

+ Br

+

The reaction is stereospecific and anti.The reaction is stereospecific and anti.

10. Halohydrin formation.10. Halohydrin formation.

Br

Br

bromonium ion

CH3

O

Br

H

HOH2

Br Br+

CH3

O

Br

OH2H

H

CH3

OH

Br

+

Br+

H3O+

11. Epoxidation of Alkenes.11. Epoxidation of Alkenes.

An epoxide (oxirane) is a three membered An epoxide (oxirane) is a three membered ring cyclic ether.ring cyclic ether.

C C + C C

General Reaction:

RCOOOH

Operacid

+ RCOOH

epoxide

Example:O

RCOOOHRCOOH+

Mechanism.Mechanism.

O

O

H

O

R

O

HO

OR

O

+ RCOOH

This is a concerted electrophilicThis is a concerted electrophilicaddition. The addition is syn.addition. The addition is syn.One of the peracids that is oftenOne of the peracids that is oftenused is “meta-chloroperoxy-used is “meta-chloroperoxy-benzoic acid (MCPBA). If youbenzoic acid (MCPBA). If yousee MCPBA as a reactant, thissee MCPBA as a reactant, thismeans that you are using a permeans that you are using a peracid.acid.

12. Acid Catalyzed Epoxide Ring Opening.12. Acid Catalyzed Epoxide Ring Opening.

Epoxides react with water in the presence of aEpoxides react with water in the presence of acatalytic amount of acid to form diols. The orientationcatalytic amount of acid to form diols. The orientationis of the -OH groups is anti.is of the -OH groups is anti.

C C

O

H3OC C

OH

OH

General Reaction:

a glycol

Mechanism.Mechanism.

CH3

O

OH

H

HOH2

CH3

OH

OH

OH

O

H

O

H

OH2

CH3

O

OH

H

H

trans 1,2-cyclopentane diol

13. Syn Hydroxylation of Alkenes.13. Syn Hydroxylation of Alkenes.

Hydroxylation is the addition of an OH group to eachHydroxylation is the addition of an OH group to eachend of a double bond to form a glycol. The reactionend of a double bond to form a glycol. The reactioncan be accomplished with OsOcan be accomplished with OsO44/H/H22OO22, or KMnO, or KMnO44/OH/OH--..

C C

General Reaction:

OsO4/H2O2C C

OH OH

or KMnO4/OH

Example:OsO4/H2O2 OH

CH3

OH

Mechanism for OsOMechanism for OsO44 Hydroxylation. Hydroxylation.

O

Os

O

O O

osmate ester

O

O O

O

H2O2

O

O O

O

OH

OH

OsO4+

Mechanism for MnOMechanism for MnO44 Hydroxylation. Hydroxylation.

O

Mn

O

O O

OH/H2O

O

O O

O

OH

OH

MnO2+O

O O

O

14. Oxidative Cleavage of Alkenes w/ Permanganate.14. Oxidative Cleavage of Alkenes w/ Permanganate.

C C

H

R'

R

R

General Reaction:

KMnO4 (warm, conc)R C C H

OH

R R'

OH

O

R

R

+ O

H

R'

HO R'

O

acid

KMnO4

warm/conc.

O

HO

O

+

Example of oxidative cleavage withExample of oxidative cleavage withpermanganate.permanganate.

15. Ozonolysis.15. Ozonolysis.

Ozone cleaves double bonds to give ketonesOzone cleaves double bonds to give ketonesand/or aldehydes.and/or aldehydes.

C C

H

R'

R

R

General Reaction:

O

R

R

+ O

H

R'O3

O O

O R'R

R H

CH3S

ozonide

1. O3

2. (CH3)2S

O

O

=

CHO

CHO

Example

16. Simmons-Smith Reaction.16. Simmons-Smith Reaction.

C C

General Reaction:

+ ICH2ZnI

CH2

+ ZnI2(or CH2I2/ZnCuCl)

Example:

CH2I2

Zn, CuCl

17. Addition of Carbenes to Double Bonds.17. Addition of Carbenes to Double Bonds.

When CHBrWhen CHBr33 or CHCl or CHCl33 is treated with aqueous KOH, is treated with aqueous KOH,

a carbene is formed.a carbene is formed.

CHBr3 + KOH CBr2 + Br + H2O

dibromocarbene

Carbenes react at double bonds to form cyclopropanes.Carbenes react at double bonds to form cyclopropanes.

Example:

CHBr3

NaOH, H2O

BrBr