Ch08 Alkenes - Reactions & Synthesis-1

8.1 Preparing Alkenes

• Alkenes are typically prepared from alcohols or alkyl halides using elimination reactions.

• Key elimination reactions: dehydrohalogenation (removal of HX) and dehydration (removal of H2O).

8.2 Halogenation of Alkenes

• Rapid addition of bromine and chlorine to alkenes produces 1,2-dihalides.

• Mechanism involves electrophilic addition forming a carbocation intermediate followed by reaction with Br–.

• Stereochemical outcome is anti-addition with trans stereochemistry in cycloalkenes via bromonium ion formation.

8.3 Halohydrins from Alkenes

• Reaction with halogens and water results in a halohydrin, producing a halo-alcohol (1,2-halo alcohol).

• Intermediate formation via a cyclic bromonium ion, followed by nucleophilic attack by water.

8.4 Hydration via Oxymercuration

• Electrophilic addition of Hg2+ results in Markovnikov addition of water to form alcohols.

• Mechanism: Protonation of alkene forms a carbocation that interacts with water.

8.5 Hydration via Hydroboration

• Hydroboration involves addition of BH3 to yield an organoborane, subsequently oxidized to an alcohol.

• Non-Markovnikov addition occurs with single-step formation and syn stereochemistry is maintained.

8.6 Reduction of Alkenes

• Alkenes can be hydrogenated using H2 in the presence of catalysts like palladium, leading to saturated alkanes.

• Reaction typically occurs with syn stereochemistry due to the reaction mechanism.

8.7 Oxidation: Epoxidation and Hydroxylation

• Oxidation by peroxyacids gives epoxides; subsequent hydrolysis produces diols with syn stereochemistry.

• Alternatively, hydroxylation directly using OsO4 gives diols through a cyclic osmate intermediate.

8.8 Oxidative Cleavage of Alkenes

• Alkenes can be cleaved using ozone or KMnO4, producing carbonyl compounds after ozonolysis.

• Mechanism involves formation of ozonides and subsequent cleavage to yield ketones and/or aldehydes.

8.9 Cyclopropane Synthesis via Carbenes

• Carbenes react with alkenes to form cyclopropane structures directly via a single-step process.

• Simmons-Smith reaction is notable for producing cyclopropane without free carbenes.

8.10 Radical Additions to Alkenes

• Radical reactions can form polymers from alkenes through chain growth mechanisms.

• Industrial significance in synthesizing polymers and understanding biological systems.

8.11 Biological Radical Reactions

• Biological systems conduct radical reactions in a more controlled manner, often avoiding the uncontrolled nature seen in laboratory settings.

8.12 & 8.13 Reaction Stereochemistry

• Examining the stereochemical outcomes of hydration reactions on both armless and chiral alkenes.

• Achiral substrates yield racemic mixtures, while chiral substrates impact the formation of diastereomers.

Conclusion

• Alkenes play a significant role in organic chemistry reactions, requiring a clear understanding of addition, reduction, and oxidation mechanisms for laboratory and biological applications.