chapter 8 alkenes: reactions - · pdf filechapter 8 alkenes: reactions name type 1. halo ......
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Chamras Glendale Community College
Organic Chemistry 105
Chapter 8
Alkenes: Reactions Name Type
1. Halo-hydrogenation (Ionic & Radical) Electrophilic Addition 2. Hydration (Acid-Catalyzed) Electrophilic Addition 3. Oxymercuration-Demercuration Electrophilic Addition 4. Alkoxymercuration-Demercuration Electrophilic Addition 5. Hydroboration-Oxidation Electrophilic Addition 6. Hydrogenation Catalytic Addition 7. Cyclopropanation Carbene Addition 8. Halogenation Oxidative Addition 9. Vicinal Halohydrin Formation Oxidative Addition 10. Epoxidation Oxidative Addition 11. Anti Hydroxylation Oxidative Addition 12. Syn Hydroxylation Oxidative Addition 13. Oxidative Cleavage by Ozonolysis Oxidative Cleavage 14. Oxidative Cleavage by Potassium Permanganate Oxidative Cleavage
The Common Theme in Addition Reactions of Alkenes
First step: The C=C π-bond act as a source of available electrons, attacks electron-deficient species.
Y X
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Hydro-Halogenation Reaction (Addition of hydrogen halides)
General Equation: General Comments:
• Functional Group Transformation:
• Rearrangement? • Variety of solvents are used: C6H6, C5H12, CH3COOH, CHCl3, etc…
• Reactivity of Halides: HF << HCl < HBr < HI
The Reason:
• Regiochemistry of addition: (Reaction is Regioselective)
H X+XH
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Example: Mechanism:
Markovnikov’s Rule:
HBr+Acetic Acid
Br
H Br
intermediate
+ Br –
H
Regioselectivity of the first addition
Facial selectivity ofthe second addition
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Markovnikov Addition vs. Anti-Markovnikov Addition: Example: Write the mechanism & predict the product: (Possibility for rearrangement?)
+ HCl
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***Special Case for Hydro-Halogenation of Alkenes*** In Presence of PEROXIDES, when the alkene is hydrobrominated, the reaction yields the opposite regiochemical outcome: Example: Mechanism: Regiochemistry of hydrohalogenation reactions of alkenes: (Reaction is Regioselective)
+ HBr
CH3
O
O
H3C
Br
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Hydration (Acid-Catalyzed)
General Equation: General Comments:
• Functional Group Transformation:
• DILUTE (50%) sulfuric acid is required for this reaction.
• Rearrangement?
• Regiochemistry of addition: (Reaction is Regioselective)
• Mechanistic Example:
H2O+OHH
H+
H2O+H+
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Hydration by Oxymercuration-Demercuration
General Equation: General Comments:
• Functional Group Transformation:
• Rearrangement?
• Regiochemistry of addition: (Reaction is Regioselective)
Hg(OAc)2(aq)HgOAcHO
NaBH4HHO
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• Mechanistic Example:
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Alkoxymercuration-Demercuration
General Equation: General Comments:
• Functional Group Transformation:
• Rearrangement?
• Regiochemistry of addition: (Reaction is Regioselective)
• Mechanistic Example:
Hg(OAc)2HgOAcRO
NaBH4HRO
ROH
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Hydroboration-Oxidation General Equation: General Comments:
• Functional Group Transformation:
• Rearrangement
• Regiochemistry of addition: (Reaction is Regioselective)
• Stereochemistry of addition: (Reaction is stereospecific)
Mechanistic Example: 1. Hydroboration:
R
H
H
H
R
H
H
H
OHH
BH3 . THF
3 BR
H H
HH
R
H
H
H
HR
H
H
H
H
H2O2
OH– 33
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Stereochemistry of Hydroboration: (A relevant issue for cyclic alkenes) 2. Oxidation: Exercise: Propose reaction conditions that would achieve the following transformations:
OH
OH
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R1
R2
R3
R4
H2, Cat.H H
R3
R4
R1
R2
Hydrogenation
General Equation: General Remarks:
• Functional Group Transformation:
• Two C-H Bonds are formed. (Very Strong)
• Very exothermic: ΔHrxn < 0
• Catalysts: Pd, Pt, Rh, or Ni: all Solid metals (heterogeneous catalysis) • Stereochemistry of addition: Syn Addition: Addition of both H’s takes place on THE SAME FACE of the double bond. (Reaction is Stereospecific)
Reaction happens in one step.
C CC
H H
Catalyst
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Cyclopropanation (AKA: Simmons-Smith Reaction)
Addition Type: Carbene Addition Problems with direct use of carbenes:
1. Diazomethane is VERY EXPLOSIVE 2. Diazomethane is VERY TOXIC 3. UNDESIRED SIDE PRODUCTS usually result
Simmons-Smith Reaction utilizes a CARBENOID to achieve the same transformation. Simmons-Smith reagent: CH2I2 General Equation: Example:
CH2I2
Zn, CuCl
C
H
H
+
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Halogenation
General Equation: General Comments:
• Functional Group Transformation:
• Reaction is stepwise
• Rearrangement:
• Stereochemistry of addition: (Reaction is Stereospecific)
• Intermediate: Mechanistic Example:
X2+XX
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Halogenation of asymmetric alkenes:
Br2
Br2
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Stereochemistry of Halogenation:
Br2
Br2
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Vicinal Halohydrin Formation
General Equation: General Comments:
• Reaction is stepwise
• Rearrangement
• Functional Group Transformation:
• Stereochemistry of addition: (Reaction is Stereospecific)
• Regiochemistry of Addition:
(Reaction is Regioselective)
• Intermediate:
Mechanistic Example:
X2+XHO
H2O+ HX+
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Epoxidation
General Equation: General Comments:
• Functional Group Transformation:
• Mechanism:
• Rearrangement:
• Stereochemistry of addition: (Reaction is Stereospecific)
Mechanistic Example:
MCPBA:
+ +
OO
OR
OH
O
OHR
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Anti Hydroxylation Acid-Catalyzed Epoxide Ring Opening
General Equation: General Comments:
• Functional Group Transformation:
• Mechanism:
• Rearrangement:
• Stereochemistry of addition: (Reaction is Stereospecific)
Mechanistic Example:
In case of asymmetric alkenes:
HO
OH
O
H3O+
MCPBA
CH2Cl2
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Syn Hydroxylation General Equation: General Comments:
• Functional Group Transformation:
• Rearrangement:
• Stereochemistry of addition: (Reaction is Stereospecific)
Mechanistic Example:
1. KMnO4
2. OH–(aq)
OR1. OsO42. H2O2
HO OH
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Oxidative Cleavage by Ozonolysis General Equation: General Comments:
• Reaction is stepwise • Functional Group Transformation:
Oxidative Cleavage by Potassium Permanganate General Equation:
• Functional Group Transformation:
R1
R2
R3
R4
+ +
2. Zn, H2O
2. (CH3)2S
R1
R2
R3
R4
O OOR1. O3, CH2Cl2
R1
R2
R3
H
+ +
R1
R2
R3
OH
O OKMnO4 (Conc.)
Heat