3/27/2011. Chapter 8 Reactions of Alkenes and Alkynes. Alkene Addition Reactions. 8.1 Preparing Alkenes: A Preview of Elimination Reactions

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1 John E. McMurry Chapter 8 Reactions of Alkenes and Alkynes Richard Morrison University of Georgia, Athens Alkene Addition Reactions Alkene addition reactions Addition of a halogen to give 1,2-dihalide Addition of a hypohalous acid to give halohydrin Addition of water to give alcohol Addition of hydrogen to give alkane Addition of single oxygen to give three-membered cyclic ether: epoxide Addition of two hydroxyl groups to give 1,2-diol 8.1 Preparing Alkenes: A Preview of Elimination Reactions Preparation of alkenes: elimination reactions Precursors to alkenes Biological systems usually alcohols Laboratory either alcohols or alkyl halides Alkenes and alcohols are chemically related through addition and elimination reactions Alkenes add H 2 O to form alcohols Alcohols eliminate water to form alkenes 1

2 Preparing Alkenes: A Preview of Elimination Reactions Dehydrohalogenation Loss of HX from alkyl halide Usually occurs by reaction of an alkyl halide with a strong base Preparing Alkenes: A Preview of Elimination Reactions Dehydration Loss of water from an alcohol Usually occurs by treatment of an alcohol with a strong acid Preparing Alkenes: A Preview of Elimination Reactions In biological pathways dehydrations normally take place on substrates in which OH is positioned two carbons away from a carbonyl group 2

3 8.2 Halogenation Addition reaction of alkenes Addition of Br 2 and Cl 2 to alkenes to yield 1,2-dihalides Halogenation of cycloalkenes Only trans-stereoisomer of dihalide product is formed Reaction occurs with anti stereochemistry the two halogen atoms come from opposite faces of doublebond, one from top face and one form bottom face Reaction occurs through an intermediate bromonium ion (R 2 Br + ), formed by interaction of the alkene with Br 2 and simultaneous loss of Br - 3

4 Bromonium ion shields one side of molecule so that reaction with Br - ion occurs only from opposite side Alkene halogenation reaction Common laboratory reaction Limited primarily to marine organisms in nature Carried out by enzymes called haloperoxidases that oxidize Br - or Cl - ions to a biological equivalent of Br + or Cl Halohydrins from Alkenes Halohydrin Formation (electrophilic addition) Reaction of alkenes with hypohalous acids HO-Cl or HO-Br to yield 1,2-halo alcohols called halohydrins In marine organisms halohydrin formation is carried out by haloperoxidases that oxidize Br - or Cl - ions to corresponding HOBr or HOCl bonded to a metal atom in the enzyme for subsequent addition to the double bond of substrate 4

5 Halohydrins from Alkenes X 2 reacts with alkene to give cyclic halonium ion intermediate Intermediate halonium ion is intercepted by water nucleophile Oxygen loses proton to give the neutral halohydrin product 8.4 Alkenes undergo an acid catalyzed addition reaction with water to yield alcohols Hydration of ethylene is not of much use in the laboratory because of the high temperatures often required Uncommon in biological pathways Acid-catalyzed hydration of double bond adjacent to carbonyl group More common in biological pathways Adjacent carbonyl group required for elimination of water Not an electrophilic addition mechanism 5

6 Laboratory hydrations of alkenes Oxymercuration Electrophilic addition of Hg 2+ to alkene on treatment with mercury(ii) acetate [(CH 3 CO 2 ) 2 Hg, or Hg(OAc) 2 ] in aqueous tetrahydrofuran (THF) solvent Reaction yields an alcohol Product corresponds to Markovnikov regiochemistry (more highly substituted alcohol formed) Hydroboration/oxidation Addition of a B-H bond of borane, BH 3, to an alkene Occurs in single step No carbocation intermediate Reaction yields an alcohol Syn stereochemistry Both C-H and C-B bonds form at the same time and from the same face of the double-bond Product has non-markovnikov regiochemistry Alkene Hydroboration 6

7 Worked Example 8.1 Predicting the Products of a Hydration Reaction What products would you obtain from reaction of 2- methylpent-2-ene with: (a) BH 3, followed by H 2 O 2,OH - (b) Hg(OAc) 2, followed by NaBH 4 Worked Example 8.2 Synthesizing an Alcohol How might you prepare the following alcohol? 7