《有机化学》课程教学课件（McMurry’s Organic Chemistry, 6th edition）Chapter 07 Alkenes - Reactions and Synthesis

Alkenes:Reactions and Synthesis Based on McMurry's Organic Chemistry,6th edition

Alkenes: Reactions and Synthesis Based on McMurry’s Organic Chemistry, 6th edition

Diverse Reactions of alkenes Alkenes react with many electrophiles to give useful products by addition (often through special reagents) OH OH Alcohol Alkane HO OH Halohydrin 1,2-Diol C=0 1,2-Dihalide Carbonyl compound Alkene C. Halide Cyclopropane 2004 Thomson-Brooks/Gol

Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addition (often through special reagents)

Addition Preparation of Alkenes G=0 X-Y Elimination Dehydrohalogenation elimination of HX from alkyl halide-use strong base and heat H H KOH CH3CH2OH KBr H2O H Bromocyclohexane Cyclohexene(81%) Dehydration elimination of H-OH from an alcohol-require strong acids (sulfuric acid,50 C) CH3 CHs OH HS04,H20 THF,50C +H20 1-Methylcyclohexanol 1-Methylcyclohexene (91%)

Preparation of Alkenes Dehydrohalogenation „elimination of HX from alkyl halide – use strong base and heat Dehydration „ elimination of H-OH from an alcohol - require strong acids (sulfuric acid, 50 ºC)

Addition of Halogens to Alkenes ■ Bromine and chlorine add to alkenes to give 1,2-dihalides F2 is too reactive and 12 does not add Cl2 reacts as Cl+Cl,Br2 behaves similarly Br H H Br—Br Br H Br Br Cyclopentene trans-1,2-Dibromo- cis-1,2-Dibromo- cyclopentane cyclopentane (Sole product) (Not formed) 2004 Thomson-Brooks/Cole Reaction occurs with anti stereochemistry What is the mechanism?

Addition of Halogens to Alkenes „ Bromine and chlorine add to alkenes to give 1,2-dihalides „ F 2 is too reactive and I 2 does not add „ Cl 2 reacts as Cl + Cl- , Br 2 behaves similarly „ Reaction occurs with anti stereochemistry What is the mechanism?

H Br-Br Br: Possible B mechanism? H @2004 Thomson-Brooks/Cole + :Br: :Br: C=C An alkene A bromonium ion +:Br: Top side open to attack :Br:- H Br H Br-Br :Br: Br Bottom side shielded from attack Cyclopentene Bromonium ion trans-1,2-Dibromo- intermediate cyclopentane

Halohydrin Formation This is the addition of HO- X to an alkene (with +OH as the electrophile)to give Reaction of the alkene with Br2 yields a bromonium ion intermediate. a 1,2-halo alcohol,called :Br:+ a halohydrin H CH The actual reagent is the .C Water acts as a nucleophile,using a lone pair of electrons to open the :0H2 dihalogen (Br2 or Cl2 in bromonium ion ring and form a bond to carbon.Since oxygen donates its water in an organic electrons in this step,it now has the positive charge. solvent) c0t-H :ǒH X Loss of a proton (H+)from oxygen X then gives HaO+and the neutral H.O HX bromohydrin addition product. Br CH HO +HO H An alkene A halohydrin OH 3-Bromo-2-butanol (A bromohydrin)

Halohydrin Formation „ This is the addition of HO￾X to an alkene (with +OH as the electrophile) to give a 1,2-halo alcohol, called a halohydrin „ The actual reagent is the dihalogen (Br 2 or Cl 2 in water in an organic solvent)

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 OH -Br(NBS) CH-CH2 CH一CH2-Br H2O/CH SOCH3(DMSO) Stvrene 2-Bromo-1-phenvlethanol (76%)

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 C=C H,C H Hydration of an alkene 2-Methylpropene is the addition of H-OH Reaction of an alkene with acid (HA) yields a carbocation intermediate. toto give an alcohol Acid catalysts are used :ǒH in high temperature +C一CH+A industrial processes: Water acts as a nucleophile,using H a lone pair of electrons on oxygen to ethylene is converted to form a bond to carbon.The oxygen atom,having donated electrons,is ethanol now trivalent and has the positive :A charge. H-0:2H HC-C一C-H H H HC H Loss of an H+from oxygen then +H20 H3PO,catalyst CHCH2OH yields the neutral alcohol product 250°C and regenerates the acid catalyst Ethanol HA. HO H Ethylene H:C-C-C-H+HA 24 Thomson-BrooksCole HC H 2-Methyl-2-propanol

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 OAc CH Electrophilie addition of mercurie For laboratory-scale hydration acetate to an alkene produces an intermediate,three-membered of an alkene mercurinium ion. OAc +OAc mercuric(ll)acetate in THF CH followed by sodium borohydride Water as nucleophile then displaces A mercurinium ion ■via mercurinium ion mercury by back-side attack at the more highly substituted carbon, Markovnikov orientation breaking the C-Hg bond. HgOAc 0-H H OAe Loss of Ht yields a neutral organo- mercury addition product. 1.Hg(OAc)2,H2O/THF CH3 HgOAe CH3 2.NaBH OH CH +HOAc OH 1-Methylcyclopentene 1-Methylcyclopentanol Treatment with sodium borohydride (92%) replaces the-Hg by-H and reduces 004Thomson-Brooks.Cole themercuryying product. CH OH +Hg

Oxymercuration Intermediates For laboratory-scale hydration of an alkene „ mercuric (II) acetate in THF followed by sodium borohydride „ via mercurinium ion „ Markovnikov orientation

Addition of Water to Alkenes: Hydroboration --一 Addition of a B-H bond of borane (BH3)to an alkene to give an organoborane Borane An organoborane BHa is highly reactive;is electron deficient;is a H Lewis acid H Reaction occurs with syn H stereochemistry Borane THF BH3-THF complex H OH CH3 BH3 -0H H THF CH3 H202 CHg 1-Methylcyclopentene Alkylborane trans-2-Methylcyclopentanol intermediate (85%) 2004 Thomson-Brooks/Cole

Addition of Water to Alkenes: Hydroboration „ Addition of a B-H bond of borane (BH 3) to an alkene to give an organoborane „ BH3 is highly reactive; is electron deficient; is a Lewis acid „ Reaction occurs with syn stereochemistry