chapter 4 reactions of alkenes
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Chapter 4 Reactions of Alkenes. Adapted from Profs. Turro & Breslow, Columbia University and Prof. Irene Lee, Case Western Reserve University. Electrophilic Additions: Alkenes. Addition of Hydrogen Halides. What is the product?. Carbocation formation is the rate-limiting step. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 4
Reactions ofAlkenes
Adapted from Profs. Turro & Breslow, Columbia University and Prof. Irene Lee,Case Western Reserve University
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Electrophilic Additions: Alkenes
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Addition of Hydrogen Halides
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What is the product?
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Which carbocation is more stable?
Carbocation formation is the rate-limiting step
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Carbocation Stabilities
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Alkyl groups decrease the concentration of positive charge in the carbocation
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Delocalization of Electrons
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Molecular Orbital Diagram in a Hyperconjugation System
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Hammond postulate: the transition state will be moresimilar to the species that it is closer to energetically
Exergonic reaction: early transition state resembles reactants (I).
Endergonic reaction: late transition state resembles products (II).
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I: early transition state(Like reactants)
II: mid-transition state III: later transition state(Like products)
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In a regioselective reaction, one constitutional isomer is the major or the only product.
Markovnikov’s RuleThe electrophile adds to the sp2 carbon that is bonded to the greater number of hydrogens
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Explained by the intermediates, for example:tert-butyl cation is formed faster and it is more stable
than isobutyl.
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Regioselectivity ofRegioselectivity ofHydrogen Halide Addition:Hydrogen Halide Addition:
Markovnikov's RuleMarkovnikov's Rule
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When an unsymmetrically substituted When an unsymmetrically substituted alkene reacts with a hydrogen halide, alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that the hydrogen adds to the carbon that has the greater number of hydrogen has the greater number of hydrogen substituents, and the halogen adds to substituents, and the halogen adds to the carbon that has the fewer hydrogen the carbon that has the fewer hydrogen substituents.substituents.
Markovnikov's RuleMarkovnikov's Rule
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acetic acidacetic acidBrBr
CHCH33CHCH22CHCHCHCH33
Example 1Example 1
Markovnikov's RuleMarkovnikov's Rule
CHCH22CHCH33CHCH22CHCH HBrHBr
(80%)(80%)
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CHCH33
CHCH33
CHCH33 CC
BrBr(90%)(90%)
CC CC
Markovnikov's RuleMarkovnikov's Rule
Example 2Example 2
acetic acidacetic acid
HBrHBrCHCH33
CHCH33
HH
HH
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0°C0°CCHCH33
ClCl
CHCH33
(100%)(100%)
Markovnikov's RuleMarkovnikov's Rule
Example 3Example 3
HClHCl
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Mechanistic BasisMechanistic Basisforfor
Markovnikov's RuleMarkovnikov's Rule
Protonation of double bond occurs in Protonation of double bond occurs in direction that gives more stable of two direction that gives more stable of two
possible carbocations.possible carbocations.
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BrBr
CHCH33CHCH22CHCHCHCH33
Mechanistic Basis for Markovnikov's Rule:Mechanistic Basis for Markovnikov's Rule:Example 1Example 1
CHCH22CHCH33CHCH22CHCHacetic acidacetic acid
HBrHBr
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BrBr
CHCH33CHCH22CHCHCHCH33
Mechanistic Basis for Markovnikov's Rule:Mechanistic Basis for Markovnikov's Rule:Example 1Example 1
CHCH22CHCH33CHCH22CHCH
HBrHBr
CHCH33CHCH22CH—CHCH—CH33 + Br + Br – – ++
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BrBr
CHCH33CHCH22CHCHCHCH33
Mechanistic Basis for Markovnikov's Rule:Mechanistic Basis for Markovnikov's Rule:Example 1Example 1
CHCH22CHCH33CHCH22CHCH
HBrHBr
CHCH33CHCH22CH—CHCH—CH33 + Br + Br – – ++
CHCH33CHCH22CHCH22—CH—CH22
++
primary carbocation is less stable: not formedprimary carbocation is less stable: not formed
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ClCl
CHCH33
CHCH33
HH
Mechanistic Basis for Mechanistic Basis for Markovnikov's Rule:Markovnikov's Rule:Example 3Example 3
0°C0°C
HClHCl
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ClCl
CHCH33
CHCH33
HH
CHCH33
HH HH
++ Cl Cl ––
Mechanistic Basis for Mechanistic Basis for Markovnikov's Rule:Markovnikov's Rule:Example 3Example 3
HClHCl
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ClCl
CHCH33
CHCH33
HH
CHCH33
HH HH
++
++
HH
CHCH33
HH
Cl Cl ––
Mechanistic Basis for Mechanistic Basis for Markovnikov's Rule:Markovnikov's Rule:Example 3Example 3
HClHCl
secondary secondary carbocation is carbocation is less stable: less stable: not formednot formed
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Carbocation Rearrangements in Carbocation Rearrangements in Hydrogen Halide Addition Hydrogen Halide Addition
to Alkenesto Alkenes
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HCl, 0°CHCl, 0°C
CHCH33CHCCHCHH(CH(CH33))22 ++
CHCH33CHCCHCHH(CH(CH33))22
ClCl (40%)(40%)
CHCH33CHC(CHCHC(CH33))22 ++
HH
CHCH33CHCH22C(CHC(CH33))22
ClCl(60%)(60%)
Rearrangements sometimes occurRearrangements sometimes occur
HH22CC CHCCHCHH(CH(CH33))22
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Rearrangement of Carbocation1,2-hydride shift a more stable
carbocation
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a more stable carbocation
Rearrangement of Carbocation
1,2-methyl shift
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Carbocation Rearrangement
a more stablecarbocation
Ring Expansion
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Carbocation does not always rearrange …
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Addition of Halogens to Alkene
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Addition of Water to Alkene(alcohols)
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Acid-Catalyzed Addition of Alcohol(ethers)
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Addition of Halogens in the Presence of Water
(halohydrins)
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Oxymercuration and Mercuration of Alkene
(alcohols w/o carbocation rearrangement)
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Addition of BoraneHydroboration–Oxidation
Anti-Markovnikov’s rule in product formation(less substituted alcohols)
Vs. Markovnikov’s rule in product formation(more substituted alcohols)
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Formation of Alkyl BoranesAnti-Markovnikov
AdditionBoron adds to least hindered carbon
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Anti-MarkovnikovAddition
Boron adds to least hindered carbonand is replaced w/ -OH by oxidation
MarkovnikovAddition
Formation of the moststable carbocation
(A type of pericyclic reaction; important reaction and mechanism in directing reactions both regio- and stereo- selectively.)
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Examples of Anti-Markovnikov Addition of an OH Group
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Carbene: another reactive intermediateReaction with an Alkene
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Synthesis of Bromobutane Isomers
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Generation of Free RadicalsUsing 1/2 arrows for the movement of one electron
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Addition of Radicals to AlkenesInitiation Propagation Termination
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Relative Stabilities of Alkyl Radicals
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Addition of Hydrogen to Alkenes
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Catalytic Hydrogenation of an Alkene