Chapter 8 Alkenes: Reactions - · PDF fileChapter 8 Alkenes: Reactions Name Type 1. Halo ......

1

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

2

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

3

Example: Mechanism:

Markovnikov’s Rule:

HBr+Acetic Acid

Br

H Br

intermediate

+ Br –

H

Regioselectivity of the first addition

Facial selectivity ofthe second addition

4

Markovnikov Addition vs. Anti-Markovnikov Addition: Example: Write the mechanism & predict the product: (Possibility for rearrangement?)

+ HCl

5

***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

6

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+

7

Hydration by Oxymercuration-Demercuration

General Equation: General Comments:

• Functional Group Transformation:

• Rearrangement?

• Regiochemistry of addition: (Reaction is Regioselective)

Hg(OAc)2(aq)HgOAcHO

NaBH4HHO

8

• Mechanistic Example:

9

Alkoxymercuration-Demercuration

General Equation: General Comments:

• Functional Group Transformation:

• Rearrangement?

• Regiochemistry of addition: (Reaction is Regioselective)

• Mechanistic Example:

Hg(OAc)2HgOAcRO

NaBH4HRO

ROH

10

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

11

Stereochemistry of Hydroboration: (A relevant issue for cyclic alkenes) 2. Oxidation: Exercise: Propose reaction conditions that would achieve the following transformations:

OH

OH

12

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

13

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

+

14

Halogenation

General Equation: General Comments:

• Functional Group Transformation:

• Reaction is stepwise

• Rearrangement:

• Stereochemistry of addition: (Reaction is Stereospecific)

• Intermediate: Mechanistic Example:

X2+XX

15

Halogenation of asymmetric alkenes:

Br2

Br2

16

Stereochemistry of Halogenation:

Br2

Br2

17

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+

18

Epoxidation

General Equation: General Comments:

• Functional Group Transformation:

• Mechanism:

• Rearrangement:

• Stereochemistry of addition: (Reaction is Stereospecific)

Mechanistic Example:

MCPBA:

+ +

OO

OR

OH

O

OHR

19

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

20

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

21

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