Lecture 7 - Alkenes & Alkynes
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Transcript of Lecture 7 - Alkenes & Alkynes
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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.
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Chapter 4Chapter 4
Alkenes & Alkynes
Lecture 7
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Reactions of Alkenes:Reactions of Alkenes:Addition ReactionsAddition Reactions
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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
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Hydrogenation of AlkenesHydrogenation of Alkenes
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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
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ExampleExampleExampleExample
HH22, Pt, Pt
(73%)(73%)
HH33CC
HH33CC CHCH22
HH33CC
CHCH33
HH33CCHH
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ProblemProblemProblemProblem
What three alkenes yield 2-methylbutane on What three alkenes yield 2-methylbutane on catalytic hydrogenation?catalytic hydrogenation?
HH22, Pt, Pt
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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
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H
CC CCAA
BB
XX
YY
H H H
Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.
Just for general knowledge, will not be tested on
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H
H H HCCCC
AABB
XXYY
Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.
Just for general knowledge, will not be tested on
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H
H H HCCCC
AABB
XXYY
Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.
Just for general knowledge, will not be tested on
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H H
H
CCCC
AABB
XXYY
H
Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.Figure 6.1Figure 6.1
Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.Figure 6.1Figure 6.1
Just for general knowledge, will not be tested on
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Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.Figure 6.1Figure 6.1
Mechanism of catalytic hydrogenation.Mechanism of catalytic hydrogenation.Figure 6.1Figure 6.1
H HH
CCCC
AABB
XXYY
H
Just for general knowledge, will not be tested on
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Stereochemistry of Stereochemistry of Alkene HydrogenationAlkene Hydrogenation
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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
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synsyn-Addition versus -Addition versus antianti-Addition-Additionsynsyn-Addition versus -Addition versus antianti-Addition-Addition
syn additionsyn addition anti additionanti addition
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Example of syn-AdditionExample of syn-AdditionExample of syn-AdditionExample of syn-Addition
HH22,, Pt PtCOCO22CHCH33
COCO22CHCH33
COCO22CHCH33
COCO22CHCH33
HH
HH
(100%)(100%)
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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
Just for general knowledge, will not be tested on
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Electrophilic Addition of Electrophilic Addition of Hydrogen Halides to Hydrogen Halides to
AlkenesAlkenes
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++ EE——YY ––
CC CC EE YYCC CC
General equation for electrophilic additionGeneral equation for electrophilic additionGeneral equation for electrophilic additionGeneral equation for electrophilic addition
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++ 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
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CHCH33CHCH22 CHCH22CHCH33
HH HH
CHCH33CHCH22CHCH22CHCHCHCH22CHCH33
BrBr
(76%)(76%)
CHClCHCl33, -30°C, -30°CCC CC
ExampleExampleExampleExample
HBrHBr
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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
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XXHH
HH
CC CC
MechanismMechanismMechanismMechanism
....
.... ::
CC CC++
....
.... ::XX::––
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XXHH
HH
CC CC
MechanismMechanismMechanismMechanism
....
.... ::
CC CC++
....
.... ::XX::––
HHCC CC........XX::
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Regioselectivity ofRegioselectivity ofHydrogen Halide Addition:Hydrogen Halide Addition:
Markovnikov'sMarkovnikov's Rule Rule
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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.
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acetic acidacetic acidBrBr
CHCH33CHCH22CHCHCHCH33
Example 1Example 1Example 1Example 1
Markovnikov's RuleMarkovnikov's RuleMarkovnikov's RuleMarkovnikov's Rule
CHCH22CHCH33CHCH22CHCHHBrHBr
(80%)(80%)
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CHCH33
CHCH33
CHCH33 CC
BrBr
(90%)(90%)
CC CC
Markovnikov's RuleMarkovnikov's RuleMarkovnikov's RuleMarkovnikov's Rule
Example 2Example 2Example 2Example 2
acetic acidacetic acid
HBrHBrCHCH33
CHCH33
HH
HH
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0°C0°C
(100%)(100%)
Markovnikov's RuleMarkovnikov's RuleMarkovnikov's RuleMarkovnikov's Rule
Example 3Example 3Example 3Example 3
HClHClCHCH33
ClCl
CHCH33
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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%)
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Addition of Sulfuric Acid Addition of Sulfuric Acid to Alkenesto Alkenes
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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
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++
slowslow
++
fastfast
MechanismMechanismMechanismMechanism
CHCH33CHCH CHCH22 HH....
SOSO22OHOH....
++CHCH33CHCH CHCH33
....SOSO22OHOH
....::––
OO
OO
CHCH33CHCHCHCH33
OSOOSO22OHOH:: ....
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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
++
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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
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Acid-Catalyzed Hydration of AlkenesAcid-Catalyzed Hydration of Alkenes
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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
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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
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CHCH22
(80%)(80%)
Follows Markovnikov's RuleFollows Markovnikov's RuleFollows Markovnikov's RuleFollows Markovnikov's Rule
50% H50% H22SOSO44
50% H50% H22OO
CHCH33
OHOH
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Addition of HalogensAddition of Halogensto Alkenesto Alkenes
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++ 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
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CHCH33CHCHCHCHCH(CHCH(CH33))22
BrBr BrBr
(100%)(100%)
CHClCHCl33
0°C0°C
ExampleExampleExampleExample
CHCHCH(CHCH(CH33))22CHCH33CHCHBrBr22
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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
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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
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BrBr
BrBr
––
Formation of bromonium ionFormation of bromonium ionFormation of bromonium ionFormation of bromonium ion
Electrons flow Electrons flow from alkene toward Brfrom alkene toward Br22
++++
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BrBr
BrBr––
++
Formation of bromonium ionFormation of bromonium ionFormation of bromonium ionFormation of bromonium ion
electrons of alkeneelectrons of alkenedisplace Brdisplace Br–– from Br from Br
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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