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University of Waterloo
CHEM 266
Steve Forsey

1 R R C C H H R 2 3 R H 3H H 3 + 82% )2 R (Racemic) 2H 3 R H C CH H H CH C cis-1-ethyl-2-methylcyclohexane R H R s for hydrogenation reactions are: 2 Sec. 7 - alkenes/alkynes ,2EtOH, Pd / H syn additione) ,2PtO R H R syn addition) PalldlumialClatsemd ndcarbonc(td-s), PtO Hydration of Alkenes (stereospecific) CH C 2 3 CH CH R R Inrate,eveoscerequeceayystaturstayethydrogenations proceed at a steady In heontsencueof ocatrlev,hyathighnteipnsrofresenes, although exothermic, do not Vollhardt: 12-1 to 12-13 2 hydrogenation reactions is to provide Sec. 7 - alkenes/alkynes Comcetinacol,ants uhedlfcr The erilaorufdnctorogfeaaatmlsstinthe catalyst surface. H H H 3 CC X H H Why? X Fast (base) Nucleophile 2 H H H Reactivity parallels acid strength. Theon. alkene is a very weak base so you need H-CR + R CC (acid) H Electrophile H Sec. 7 - alkenes/alkynes Slow HX + 2 + Addition Reactions to Alkenes (acid) Addi2ion of HydrogElectrophileo Alkenes. R H H CC Nuwleakbase)Unsymmetrical alkene H General Reaction Order of reactivity = HI > HBr > HCl > HF 4 an intermediate (You already knew this l rself to aorarbon atom to the double bondith the Sec. 7 - alkenes/alkynes Markovnikov’s Rule greater number of hydrogens N reactions. The most stable carbocation intermediate forms) Hydrogen becomes attached to the car In the ionic addition of an unsymmetricaitn as 5 Examples Sec. 7 - alkenes/alkynes Br H Br H top bottom 3 + 3 H CH CH C 2 sp 3 C C C CH 3 3 H H 6 O S O H O O 2 3 OH CH H 3 + O2- weak base +O C CH 3 H 4 also inHgreater concentration CH C3 poor nucleophileHbut better + C C C than HSO 2 3 3 H H H 4 3 Sec. 7 - alkenes/alkynes S2 OH CH H 3 S OH C CH O O hydradilute I – Cl water present 3 H H 2 2 or H CH CH + + O + O 3 H H H C C C3 3 3 C H H H H Examples r 7 H B OH HO B A) B) C) D) E) Examples Sec. 7 - alkenes/alkynes 4 4 SO 4 diHuteat SO S2 H2 NaBr H concentrated concentrated H H HO What is the product of the following reactions 8 ese conditions, a reaction is said to be under Sec. 7 - alkenes/alkynes Alkene hydration and alcohol dehydration are equilibrium processes The reversibility of alkene protonation leads to alkene equilibration. Allltties.rTreversidliitrePrequthermodynamic control.macti.Usnderyinterconvert related alkenes and produce an 9 Halareehaiuasmaoabanos)nnttm(i.e. Sec. 7 - alkenes/alkynes ThistoetcehrnismrecataclAddition of Bromine and Chlorine to Alkenes Haglgingmvclnaldehaasoeact as electrophiles with alkenes 10 is racemic since the initial attack of No cis-1,2-dibromocyclohexane is formed. Sec. 7 - alkenes/alkynes Bromination takes place through anti addition. Consider the bromination of cyclohexene.omine can occur with equal probability at either face of the cyclohexene. R Br 11 R CC r B R R anti addition forms trans products Br (base) nucleophile + Br R R R R + + r Br B (acid) CC CC intermediate Mechanism bromonium ionhile R R R Sec. 7 - alkenes/alkynes R R R R but okay for our needs + R δ− δ not true diagrams of orbitals δ− δ Br Br Br Br CC (base) CC nucleophile R R R R (acid) electrophile polarized 12 r B Br OH Br B) D) OH OH Br OH w it relates to the cyclic bromonium ion A) C) Sec. 7 - alkenes/alkynes solvent and is in large excess. 2 Br – Br (solvent) Unsymmetrical Alkene and other nucleophiles WhaoAlso think about carbocation stability and ho 13 Br r B + Br Br O 2 H Na OH O Sec. 7 - alkenes/alkynes H + lots of water -H O 2 Br Br – Br Halohydrin Formation (halogen + alcohol2 O Br H HO 14 e not sufficiently add in a Markovnikov-like fashion: The the two carbons ar Bromonium Ion Sec. 7 - alkenes/alkynes Elelectrxphres of troduoctssretfmetric reagents 15 +, Sec. 7 - alkenes/alkynes -, can undergo stereo- and regiospecific Reagents of the type A-B, in which A acts as the electrophile, A 16 rearrangements Hyattached to the carbonens Sec. 7 - alkenes/alkynes OAc The reaction shows Markovnikov selectivity The procedure gives high yields of alcohols and avoids AlMoarolvnromv aldiies through Oxymercuration-Demercuration 17 + Hg 2 2 OH H 3 nuanti additionCH CC HO C H c A 4 g 2 H H + HgOAc + OAc NaBNaOH complex mechanism HC + C C 3 3 3 2 H H A + H 2 Sodium borohydride replaces the mercury witha Mechanism HgO Sec. 7 - alkenes/alkynes Hg(OAc) 3 attack CH HC HO electrophilic3°cecaunhaaibeCtrmooectinnipnotpnvoevodriata H + HgOAc 2 Electrophile OH +2 HgOAc H CHC HC O+ 2 3 C H H3C H 3H NucleophileH3 Dissociation Mercuric acetate 3 Br 3 OCH 18 OCH Br OH OH B) D) 3 B) D) OCH 3 OH Br Br OH OH 3 OCH Br OCH Br A) C) E) A) C) E) Sec. 7 - alkenes/alkynes O3 /CH 2 O 4 2 O3 Br – Br(solvent) KOH, H 1. Hg(OAc)BH What is the major product formed in the following reaction? 19 the double bond Syadded to the same side of e alkene is electron rich, an initial lvent tetrahydrofuran (THF) is often used Sec. 7 - alkenes/alkynes Synthesis of Alkylboranes AlAont-larromnikovenes tddouigh Hydroboration-Oxidation: HydBeLewis acid-base complex similar to the bromonium ion can form 20 ). Here, bstituted carbon syn addition side at the most su ed carbon more easily electronic factors. The boron binds to ansition state to re to three molecules of alkene east steSec. 7 - alkenes/alkynes Mechanism of Hydroboration δ charge in the tr anti-Markovnikov product Boranboncoftes aoabhlebtndthe least substituted Boron hydride adds successHydstehie asctorinderemdoseumstitrted)tharbon.reospecific ( ThThTihirieiatiaionleaalotos aoach the l 21 Sec. 7 - alkenes/alkynes Mechanism of Hydroboration electrophile 22 place with retention of stereochemistry Sec. 7 - alkenes/alkynes at the carbon bonded to boron Oxidation and Hydrolysis of Alkylboranes Oxidation and hydrolysis to the alcohol takes 3 23 3 + B(OH) CH 2 OH CH 2 2 + Hg CH 90% CH 2 2 2 CH H 96% 2 O 2) H CHCH 2H-CH C2 2 CH 2H 2) C2 2 1-hexanol 2H 2-hexanol 2 C3 B 2 3H HgOAc H 2H-CH 3 CH CH CH-CH CH HO 2 2 H 2H-CH CH CH 2 2 2H 2 CH CH 2 3 CH 2 H , NaOH 2 ,4NaOH 3H 3THF CH CH H H 2 3 CH Sec. 7 - alkenes/alkynes CH 1) B) H O + 1) Hg(OAc) H -H H 2 δ 2 2 2 H B δ+HgOAc Hydroboration Versus Oxymercuration 2H=CH H C2δ+H CH=CH 2H-CH 2 δ+ 2H 1) C2 1) 2H C2 2H C2 2H 1-hexene transitio2 state CH C3 CH 3 C3 3 CH CH CH CH 24 ? OH E) Br 2 CH 3 1) NaH) CH ? D) :THF2NaOH 3 O2 H C) 1) B) H ? Sec. 7 - alkenes/alkynes O2 N2i )2 O CH) C3 )3 (LDA) B) (CH ((CH r B O A) What is the final product formed in this series of reactions? 25 ion of diazomethane C: (the simplest carbon is surrounded by t initiated decomposit e decomposition of diazomethane 2:, in which the central Sec. 7 - alkenes/alkynes Divalent Carbon Compounds: Carbenes Theyceoptonpanefiihnthcgrbyneerctdvlysdecieomaethlee
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