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ALKENES AND CYCLOALKENES

ALKENES AND CYCLOALKENES. Most of the addition reactions is regio a stereospecific , but some of them are not stereospecific. Addice of H 2 O / H +. nonstereospecific. Markovnikov rule (regioselective reaction). nonstereospecific. Adice H 2 SO 4.

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ALKENES AND CYCLOALKENES

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  1. ALKENES AND CYCLOALKENES Most oftheadditionreactionsisregioa stereospecific, but someofthem are not stereospecific. Addice of H2O / H+ nonstereospecific Markovnikov rule (regioselective reaction) nonstereospecific Adice H2SO4 Markovnikov rule (regioselective reaction) Similarly also addition of hydrohalogenates nonstereospecific

  2. ALKENES AND CYCLOALKENES Ozonization nonstereospecific Reaction examples: The ozonides are decomposed by catalytic hydrogenation or by treatment by dimethylsufide

  3. ALKENES AND CYCLOALKENES Stereospecificreactionscis- (syn)- Hydrogenation Olefin may attack the catalyst also by the other side during reaction enantiomers are formed

  4. ALKENES AND CYCLOALKENES Stereospecificreactionscis- (syn)- Oxidation by KMnO4 or OsO4 enantiomers Similar is reaction with OsO4

  5. ALKENES AND CYCLOALKENES Stereospecificreactionscis- (syn)- Epoxidation Peroxyacid may come also from the other side epoxide Epoxides are not stable = reactivecompounds– in anacidic medium we are able to transfer them to dioles The splitting of the epoxides proceeds under formation of trans- dioles

  6. ALKENES AND CYCLOALKENES Stereospecificreactionscis- (syn)- Markovnikov rule (regioselective reaction) Hydroboration B2H6 2 BH3 + enantiomer The result of the reaction is the same as after addition of water in an acidic medium but the regioselectivity is contrary

  7. ALKENES AND CYCLOALKENES Stereospecificreactionscis- (syn)-

  8. ALKENES AND CYCLOALKENES Stereospecific reaction cis- (syn)- Oxymercuration – demercuration (reaction with mercury acetate, Hg2+ is the electrophile) Demercuration is carried out by NaBH4 Markovnikov rule (regioselective reaction)

  9. ALKENES AND CYCLOALKENES !! Under radical conditions the regioselectivity during addition of hydrohalogens is changing!!! KHARASH phenomenon = antimarkovnikov rule catalysis by UV light ( hn) , radicals, heat initiation propagation more stable than

  10. ALKENES AND CYCLOALKENES At radical conditions you can expect also additions of : chlorine, bromine, sulfane, thioles a polyhalogenated alkanes iniciation

  11. ALKENES AND CYCLOALKENES POLYMERIZATION mechanism: radical polymerization ionic polymerization - kationic - anionic iniciation propagation termination

  12. ALKENES AND CYCLOALKENES POLYMERIZATION Kationic polymerizations, catalysis by BF3, TiCl4, R3Al …… Monomers for the most important polymers

  13. ALKENES AND CYCLOALKENES POLYMERIZATION Anionic polymerizations, catalysis by strong bases: BuLi, NaNH2… This method is suitable for alkenes with electronwithdrawing groups, where that group enables entrance of the base and stabilisation of formed anion. mechanism of the second glue action

  14. ALKENES AND CYCLOALKENES Notice higher reactivity of hydrogenatoms at Csp3 in neighbourghood of double bond Hydrogen atoms at allylic position are reactive in radical reactions (the formed radical is stabilized by the double bond next to it)  In allylic position you can realize halogenation without loss of double bond (without addition) The suitable halogenation agent is NBS (NCS)

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