Lecture 7 - Alkenes & Alkynes

General

Organic ChemistryTwo credits

Second Semester 2009

King Saud bin Abdulaziz University for Health Science

Reference Book: Organic Chemistry: A Brief Course, by Robert C. Atkins and Francis A. CareyThird Edition

Instructor: Rabih O. Al-Kaysi, PhD.

Chapter 4Chapter 4

Alkenes & Alkynes

Lecture 7

Reactions of Alkenes:Reactions of Alkenes:Addition ReactionsAddition Reactions

Reactions of AlkenesReactions of AlkenesReactions of AlkenesReactions of Alkenes

The characteristic reaction of alkenesThe characteristic reaction of alkenes is addition to the double bond. is addition to the double bond.

+ + AA——BBCC CC AA CC CC BB

Hydrogenation of AlkenesHydrogenation of Alkenes

Hydrogenation of ethyleneHydrogenation of ethyleneHydrogenation of ethyleneHydrogenation of ethylene

+ H—H+ H—H

catalyzed by finely divided Pt, Pd, Rh, Nicatalyzed by finely divided Pt, Pd, Rh, Ni

CC CC HH CC CC

HH HH

HH HH

HH

HH

HH

HH

HH

ExampleExampleExampleExample

HH22, Pt, Pt

(73%)(73%)

HH33CC

HH33CC CHCH22

HH33CC

CHCH33

HH33CCHH

ProblemProblemProblemProblem

What three alkenes yield 2-methylbutane on What three alkenes yield 2-methylbutane on catalytic hydrogenation?catalytic hydrogenation?

HH22, Pt, Pt

H H CC CCAA

BB

XX

YY

Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.

H H

Just for general knowledge, will not be tested on

H

CC CCAA

BB

XX

YY

H H H



H

H H HCCCC

AABB

XXYY



H

H H HCCCC

AABB

XXYY



H H

H

CCCC

AABB

XXYY

H

Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.Figure 6.1Figure 6.1





H HH

CCCC

AABB

XXYY

H


Stereochemistry of Stereochemistry of Alkene HydrogenationAlkene Hydrogenation

Two spatial (stereochemical) aspects ofTwo spatial (stereochemical) aspects ofalkene hydrogenation:alkene hydrogenation:

Two spatial (stereochemical) aspects ofTwo spatial (stereochemical) aspects ofalkene hydrogenation:alkene hydrogenation:

• syn addition of both H atoms to double bondsyn addition of both H atoms to double bond• hydrogenation is stereoselective, hydrogenation is stereoselective,

corresponding to addition to less crowded corresponding to addition to less crowded face of double bondface of double bond

synsyn-Addition versus -Addition versus antianti-Addition-Additionsynsyn-Addition versus -Addition versus antianti-Addition-Addition

syn additionsyn addition anti additionanti addition

Example of syn-AdditionExample of syn-AdditionExample of syn-AdditionExample of syn-Addition

HH22,, Pt PtCOCO22CHCH33

COCO22CHCH33

COCO22CHCH33

COCO22CHCH33

HH

HH

(100%)(100%)

Example ofExample ofstereoselective reactionstereoselective reaction

Example ofExample ofstereoselective reactionstereoselective reaction

HH22, cat, cat

H2 adds to bottom face of double bond.

H2 adds to bottom face of double bond.

HH33CC CHCH33

HH33CC

HH

CHCH33HH33CC

HH33CCHH

HH

HH


Electrophilic Addition of Electrophilic Addition of Hydrogen Halides to Hydrogen Halides to

AlkenesAlkenes

++ EE——YY ––

CC CC EE YYCC CC

General equation for electrophilic additionGeneral equation for electrophilic additionGeneral equation for electrophilic additionGeneral equation for electrophilic addition

++ HH——XX ––

CC CC HH XXCC CC

When EY is a hydrogen halideWhen EY is a hydrogen halideWhen EY is a hydrogen halideWhen EY is a hydrogen halide

CHCH33CHCH22 CHCH22CHCH33

HH HH

CHCH33CHCH22CHCH22CHCHCHCH22CHCH33

BrBr

(76%)(76%)

CHClCHCl33, -30°C, -30°CCC CC


HBrHBr

Electrons flow from the Electrons flow from the system of the system of thealkene (electron rich) toward the positivelyalkene (electron rich) toward the positivelypolarized proton of the hydrogen halide.polarized proton of the hydrogen halide.

MechanismMechanismMechanismMechanism

XXHH

HH

CC CC


....

.... ::

CC CC++

....

.... ::XX::––

XXHH

HH

CC CC


....

.... ::

CC CC++

....

.... ::XX::––

HHCC CC........XX::

Regioselectivity ofRegioselectivity ofHydrogen Halide Addition:Hydrogen Halide Addition:

Markovnikov'sMarkovnikov's Rule Rule

Markovnikov's RuleMarkovnikov's Rule

When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents.

acetic acidacetic acidBrBr

CHCH33CHCH22CHCHCHCH33

Example 1Example 1Example 1Example 1

Markovnikov's RuleMarkovnikov's RuleMarkovnikov's RuleMarkovnikov's Rule

CHCH22CHCH33CHCH22CHCHHBrHBr

(80%)(80%)

CHCH33

CHCH33

CHCH33 CC

BrBr

(90%)(90%)

CC CC



acetic acidacetic acid

HBrHBrCHCH33

CHCH33

HH

HH

0°C0°C

(100%)(100%)



HClHClCHCH33

ClCl

CHCH33

Addition of HBr with a regiochemistry oppositeAddition of HBr with a regiochemistry oppositeto Markovnikov's rule can also occur whento Markovnikov's rule can also occur wheninitiated with light with or without added peroxides.initiated with light with or without added peroxides.

CHCH22 + HBr+ HBrLightLight CHCH22BrBr

HH

(60%)(60%)

Addition of Sulfuric Acid Addition of Sulfuric Acid to Alkenesto Alkenes

follows Markovnikov's rule:follows Markovnikov's rule:yields an alkyl hydrogen sulfateyields an alkyl hydrogen sulfate

follows Markovnikov's rule:follows Markovnikov's rule:yields an alkyl hydrogen sulfateyields an alkyl hydrogen sulfate

CHCH33CHCHCHCH33

OSOOSO22OHOH

Addition of HAddition of H22SOSO44Addition of HAddition of H22SOSO44

CHCH33CHCH CHCH22

HOSOHOSO22OHOH

IsopropylIsopropylhydrogen sulfatehydrogen sulfate

++

slowslow

++

fastfast


CHCH33CHCH CHCH22 HH....

SOSO22OHOH....

++CHCH33CHCH CHCH33

....SOSO22OHOH

....::––

OO

OO

CHCH33CHCHCHCH33

OSOOSO22OHOH:: ....

Alkyl hydrogen sulfates undergo Alkyl hydrogen sulfates undergo hydrolysis in hot waterhydrolysis in hot water

Alkyl hydrogen sulfates undergo Alkyl hydrogen sulfates undergo hydrolysis in hot waterhydrolysis in hot water

H—OHH—OH

HO—HO—SOSO22OHOH++

CHCH33CHCHCHCH33

OO SOSO22OHOH

heatheat

CHCH33CHCHCHCH33

OO HH

++

1. H1. H22SOSO44

2. H2. H22O, heatO, heat

(75%)(75%)

Application: Coversion of alkenes to alcoholsApplication: Coversion of alkenes to alcoholsApplication: Coversion of alkenes to alcoholsApplication: Coversion of alkenes to alcohols

OHOH

Acid-Catalyzed Hydration of AlkenesAcid-Catalyzed Hydration of Alkenes

Acid-Catalyzed Hydration of AlkenesAcid-Catalyzed Hydration of AlkenesAcid-Catalyzed Hydration of AlkenesAcid-Catalyzed Hydration of Alkenes

H—OHH—OH

reaction is acid reaction is acid catalyzed; typical catalyzed; typical

hydration medium is hydration medium is 50% H50% H22SOSO44-50% H-50% H22OO

reaction is acid reaction is acid catalyzed; typical catalyzed; typical

hydration medium is hydration medium is 50% H50% H22SOSO44-50% H-50% H22OO

CC CC ++

OHOHCC CCHH

Follows Markovnikov's RuleFollows Markovnikov's RuleFollows Markovnikov's RuleFollows Markovnikov's Rule

(90%)(90%)

50% H50% H22SOSO44

50% H50% H22OO

HH33CC

HH33CC CHCH33

HH

CC CC

OHOH

CC CHCH22CHCH33CHCH33

CHCH33

CHCH22

(80%)(80%)

Follows Markovnikov's RuleFollows Markovnikov's RuleFollows Markovnikov's RuleFollows Markovnikov's Rule

50% H50% H22SOSO44

50% H50% H22OO

CHCH33

OHOH

Addition of HalogensAddition of Halogensto Alkenesto Alkenes

++ X X22 XX XX

electrophilic addition to double bondelectrophilic addition to double bondforms a vicinal dihalideforms a vicinal dihalideelectrophilic addition to double bondelectrophilic addition to double bondforms a vicinal dihalideforms a vicinal dihalide

CC CCCC CC

CHCH33CHCHCHCHCH(CHCH(CH33))22

BrBr BrBr

(100%)(100%)

CHClCHCl33

0°C0°C


CHCHCH(CHCH(CH33))22CHCH33CHCHBrBr22

Limited to ClLimited to Cl22 and Br and Br22

FF22 addition proceeds with explosive violence addition proceeds with explosive violence

II22 addition is endothermic: addition is endothermic:

vicinal diiodides dissociate to an alkene and Ivicinal diiodides dissociate to an alkene and I22

Limited to ClLimited to Cl22 and Br and Br22

FF22 addition proceeds with explosive violence addition proceeds with explosive violence

II22 addition is endothermic: addition is endothermic:

vicinal diiodides dissociate to an alkene and Ivicinal diiodides dissociate to an alkene and I22

ScopeScopeScopeScope

Formation of bromonium ionFormation of bromonium ionFormation of bromonium ionFormation of bromonium ion

BrBr

BrBr

Mutual polarizationMutual polarizationof electron distributionsof electron distributionsof Brof Br22 and alkene and alkene

BrBr

BrBr

––


Electrons flow Electrons flow from alkene toward Brfrom alkene toward Br22

++++

BrBr

BrBr––

++


electrons of alkeneelectrons of alkenedisplace Brdisplace Br–– from Br from Br

attack of Brattack of Br–– from side opposite from side opposite

C—Br bond of bromonium ion givesC—Br bond of bromonium ion gives

anti additionanti addition

++

––BrBr:: ::....

....

BrBr....

....

BrBr

BrBr....

::....

....::::

....

StereochemistryStereochemistryStereochemistryStereochemistry

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Lecture 7 - Alkenes & Alkynes

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