Chapter 7. Alkenes: Reactions and Synthesis

Transcription

1 Chapter 7. Alkenes: Reactions and Synthesis 1 Synthesis of Alkenes: Elimination Reactions 1. Dehydrohalogenation of alkyl halides. loss of requires CH 2 CH 2 Cl Zaitsev s Rule: CH 2 C 2. Dehydration of an Alcohol loss of requires CH

2 2 CH 2 C Addition Reactions of Alkenes 1. Electrophilic Addition of HX to Alkenes CH=CH 2 + HX X = to add HI, use: Regiochemistry: Carbocation intermediate, therefore Stereochemistry: In words: Reactions: RCH CH 2 HX ether

3 HCl CH CH ether 3 KI H 3 PO 4 Question: What is the product of the reaction: CH + H? a) CH 2 c) CHCH 2 b) CH d) CH 2. Addition of Halogens to Alkenes C C X 2 CH 2 Cl 2 Stereochemistry: Intermediate: In the second step, the nucleophile attacks from 2 CH 2 Cl 2

4 4 Cl 2 CH 2 Cl 2 2 CH 2 Cl 2 3. Halohydrin formation X 2 H 2 O Stereochemistry: Regiochemistry: In words: 1) 2) 3) Question: Why does the go to the more highly substituted carbon? For bromohydrin formation, the reagents that are usually used are:

5 5 Cl 2 H 2 O NBS H 2 O, DMSO 4. Acid-Catalyzed Hydration or CH CH 2 H 3 O + CH CH 2 dil. H 2 SO 4 Regiochemistry: Question: What cation is formed when 1-methylcyclohexene is treated with dilute sulfuric acid? a) c) b) CH 2 d) 5. Oxymercuration CH CH 2 1. H 2 O, Hg(OAc) 2 2. NaBH 4 Regiochemistry: A net hydration: A two-step reaction: This is the method of choice for the lab synthesis of Markovnikov alcohols. This is used in the lab rather than acidcatalyzed hydration.

6 6. Hydroboration 6 CH CH 2 2. H 2 O 2, 1. BH 3, THF Regiochemistry: Stereochemistry: Draw the mechanism of hydroboration, including the structure of the transition state: Question: Starting with an appropriate alkene, show how you would use either hydroboration/oxidation or oxymercuration to make the two alcohols below. For each product shown, show the starting alkene and the appropriate reagents. a) CH C CH 2 b) CH CH CH 2 CH 2

7 Question: What are products A and B (in that order) in the reaction below? 7 1. H 2 O, Hg(OAc) 2 2. NaBH 4 A CH 2 1. BH 3 -THF 2. H 2 O 2, B a) and CH 2 H c) Both give b) CH 2 and H d) Both give CH 2 H 7. Addition of Carbenes General Six Points about Carbenes: A. Dichlorocarbene addition: Stereochemistry: Mechanism for the formation of dichlorocarbene:

8 B. Simmons-Smith reaction: 8 Starting with the appropriate alkene, show how you could synthesize cis-12-dimethylcyclopropane. Starting with the appropriate alkene, show how you could synthesize trans-12-dimethylcyclopropane. 8. Hydrogenation of Alkenes Oxidation Reduction General Chemistry Organic Chemistry H 2 catalys t Catalysts: Stereochemistry: H 3 C H 2 catalys t

9 Under normal conditions, only the alkene double bond is reduced: 9 H 2 O Pd/C, Et O C O H 2 Pd/C, Et Isotopic Labeling: D D 2 Pd/C, Et H 3 C D 2 Pd/C, Et 9. Epoxidation with a peroxyacid. MCPBA 10. Epoxidation of halohydrins

10 11. Acid-catalyzed epoxide hydroxylation 10 Stereochemistry: 12. Syn hydroxylation with OsO 4 Intermediate: 13. Cleavage of 1,2 diols Example: 14: Oxidative Cleavage: Ozonolysis

11 Summary of ozonolysis: Oxidative Cleavage with KMnO 4 From a previous exam: Compound A has the formula C 6 H 8. Compound A reacts with two molar equivalents of hydrogen gas to form Compound B, C 6 H 12. When Compound A is treated with ozone gas, followed by reduction with zinc in acetic acid, only one product is formed: Compound C, shown below. Determine the structures of Compound A and Compound B. O O H C CH 2 C Compound C H

12 16. Free Radical Addition of H to Alkenes 12 This reaction requires that H be added in the presence of. CH CH 2 H peroxide Regiochemistry: Stability Order of Free Radicals: H peroxide