Chem241a_Lecture_18 - Organic Chemistry 241a Prof. Bogdan...

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Unformatted text preview: Organic Chemistry 241a Prof. Bogdan Olenyuk Contact Information Office: Old Chemistry 309 Phone: (520) 626-0754 Course web site: Chapters 6 & 7: Alkenes Today we will cover: Structure and stability of cabocations Reactions of alkenes Reading Assignment: McMurry (the BOOK), Ch. 6.10-6.11, 7.1-7.6 Competing Reactions and the Hammond Postulate Normal Expectation: Faster reaction gives more stable intermediate Intermediate resembles transition state Mechanism of Electrophilic Addition: Rearrangements of Carbocations Carbocations undergo structural rearrangements following set patterns 1,2-H and 1,2-alkyl shifts occur Goes to give more stable carbocation Can go through less stable ions as intermediates Chapter 7: Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addition (often through special reagents) Preparation of Alkenes: A Preview of Elimination Reactions Alkenes are commonly made by elimination of HX from alkyl halide (dehydrohalogenation) Uses heat and KOH elimination of H-OH from an alcohol (dehydration) require strong acids (sulfuric acid, 50 C) Addition of Halogens to Alkenes Bromine and chlorine add to alkenes to give 1,2dihaldes, an industrially important process F2 is too reactive and I2 does not add Cl2 reacts as Cl+ Cl Br2 is similar Addition of Br2 to Cyclopentene Addition is exclusively trans Mechanism of Bromine Addition Br+ adds to an alkene producing a cyclic ion Bromonium ion, bromine shares charge with carbon Gives trans addition Bromonium Ion Mechanism Electrophilic addition of bromine to give a cation is followed by cyclization to give a bromonium ion This bromoniun ion is a reactive electrophile and bromide ion is a good nucleophile The Reality of Bromonium Ions Bromonium ions were postulated more than 60 years ago to explain the stereochemical course of the addition (to give the trans-dibromide from a cyclic alkene Olah showed that bromonium ions are stable in liquid SO2 with SbF5 and can be studied directly Addition of Hypohalous Acids to Alkenes: Halohydrin Formation This is formally the addition of HO-X to an alkene to give a 1,2-halo alcohol, called a halohydrin The actual reagent is the dihalogen (Br2 or Cl2 in water in an organic solvent) Mechanism of Formation of a Bromohydrin Br2 forms bromonium ion, followed by the addition of water Orientation toward stable C+ species Aromatic rings do not react An Alternative to Bromine Bromine is a difficult reagent to use for this reaction N-Bromosuccinimide (NBS) produces bromine in organic solvents and is a safer source Addition of Water to Alkenes: Oxymercuration Hydration of an alkene is the addition of H-OH to to give an alcohol Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol Oxymercuration Intermediates For laboratory-scale hydration of an alkene Use mercuric acetate in THF followed by sodium borohydride Markovnikov orientation via mercurinium ion Addition of Water to Alkenes: Hydroboration Herbert Brown (HB) invented hydroboration (HB) Borane (BH3) is electron deficient is a Lewis acid Borane adds to an alkene to give an organoborane Hydroboration-Oxidation Forms an Alcohol from an Alkene Addition of H-BH2 (from BH3-THF complex) to three alkenes gives a trialkylborane Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene Orientation in Hydration via Hydroboration Regiochemistry is opposite to Markovnikov orientation OH is added to carbon with most H's H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition) Mechanism of Hydroboration Borane is a Lewis acid Alkene is Lewis base Transition state involves anionic development on B The components of BH3 are added across C=C More stable carbocation is also consistent with steric preferences Addition of Carbenes to Alkenes The carbene functional group is "half of an alkene" Carbenes are electrically neutral with six electrons in the outer shell They add symmetrically across double bonds to form cyclopropanes Formation of Dichlorocarbene Base removes proton from chloroform Stabilized carbanion remains Unimolecular elimination of Cl- gives electron deficient species, dichlorocarbene Reaction of Dichlorocarbene Addition of dichlorocarbene is stereospecific cis Simmons-Smith Reaction Equivalent of addition of CH2: Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species Forms cyclopropanes by cycloaddition ...
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