Alkenes: Reactions, Synthesis and Mechanisms - Prof. Yu-Lin Jiang, Study notes of Organic Chemistry

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Alkenes: Reactions and Synthesis Based on McMurry’s Organic Chemistry, 6th edition 2 Diverse Reactions of Alkenes  Alkenes react with many electrophiles to give useful products by addition (often through special reagents)  alcohols (add H-OH)  alkanes (add H-H)  halohydrins (add HO-X)  dihalides (add X-X)  halides (add H-X)  diols (add HO-OH)  cyclopropanes (add :CH2) 5 A Preview of Elimination Reactions  Alkenes are commonly made by elimination of HX from alkyl halide: (dehydrohalogenation); using heat and KOH 6 A Preview of Elimination Reactions  elimination of H-OH from an alcohol (dehydration); requires strong acids (sulfuric acid, 50 ºC) alkene products from dehydration? 7 CH3C H,C CH,CH,CH3 3-Methy1-3-hexanol | CH, ©2004 Thomson - Brooks/Cole 10 Addition of Br2 to Cyclopentene  Addition is exclusively trans (stereospecific) 11 Mechanism of Bromine Addition  Br+ adds to an alkene producing a cyclic cation: a bromonium ion, in which bromine shares charge with carbon 12 Mechanism of Bromine Addition  Since the Br blocks one face, one must get anti (trans) addition 15 The Reality of Bromonium Ions  Bromonium were postulated more than 60 years ago to expain the stereochemical course of the addition (to give the trans-dibromide from a cyclic alkene  George Olah (Nobel Prize 1994) showed that bromonium ions are stable in liquid SO2 with SbF5 and can be studied directly by nuclear magnetic resonance spectroscopy 16 7.3 Halohydrin Formation  This is formally 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 (Br2 or Cl2 with water in an organic solvent) Mechanism of Formation of a Bromohydrin H CH, es Br--Br a N H,C H Reaction of the alkene with Br, | yields a bromonium ion intermediate. oO ae 17 Reaction of the alkene with Br yields a bromonium ion intermediate. Water acts as a nucleophile, using a lone pair of electrons to open the bromonium ion ring and form a bond to carbon, Since oxygen donates its electrons in this step, it now has the positive charge. Loss of a proton (H*) from oxygen then gives H,0* and the neutral bromohydrin addition product. ©2004 Thomson - Brooks/Cole H CH, ‘ hk ee Br 13 gs \ H,C A ?Br:* cee _C—C. + Br HY WCH, H,C H :OH, Br CH; ~e—c" si a M H,C c:0*-H a ft be = Pe H —C~” +H,0* Hf i H,C OH 3-Bromo-2-butanol (A bromohydrin) 20 21 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 22 7.4 Addition of Water to Alkenes: Hydration  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 25 Oxymercuration/ Demercuration  For laboratory-scale hydration of an alkene under mild conditions (room temperature, neutral pH)  Use mercuric acetate in THF followed by sodium borohydride reduction  Markovnikov orientation  via mercurinium ion Electrophilic addition of mercuric acetate to an alkene produces an intermediate, three-membered | mercurinium ion, eS Ou, Water as nucleophile then displaces A mereurinium ion mercury by back-side attack at the | more highly substituted carbon, breaking the C—Hg bond. re CH, + at Zo CH; ot i OAc Loss of H* yields a neutral organo- | mercury addition product. HgOAc C Ne He + HOAc OH Treatment with sodium borohydride replaces the —-Hg by —H and reduces | aBH, the mercury, yielding an alcohol Boat CH, + Hg OH 27 Problem 7.8: Which alkene? (a) oH CH,CCH,CH,CH,CH, CHs (b) OH 31 7.5 Addition of Water to Alkenes: Hydroboration  Herbert Brown (HB) invented hydroboration (HB)  Borane (BH3) is electron deficient: a Lewis acid  Borane adds to an alkene to give an organoborane 32 BH3 is a Lewis Acid  Six electrons in outer shell  Coordinates to oxygen electron pairs in ethers 35 Mechanism of Hydroboration  Borane is a Lewis acid  Alkene is Lewis base  Transition state involves anionic development on B  The components of BH3 are across C=C Hydroboration: Orientation in Addition Step C7 Partial 3° cation H cH, —BEs (more stable transition state) 3 1-Methylcyclopentene H [| —~._CHs us —z a B --H H. os H BH, Partial 2° cation NOT formed (less stable transition state) © 2004 Thomson/Brooks Cole 36 37 Hydroboration, Electronic Effects Give Non-Markovnikov  More stable carbocation is also consistent with steric preferences Two Possible Orientations: Practice Problem 7.1 CH,CH; CH,CH, 1, BH;, THF CT 1, Hg(OAc),, Hy CH,CH, on coal ‘i “ _-OH 9. HQ», OH™ 2, NaBHy “OH ~H H H (a) Syn, non-Markovnikov (b) Markovnikov addition of H20 addition of H,0 ©2004 Thomson - Brooks/Cole 40 Practice Problem 7.2: Which alkene? 1 = ——= CHsCH,CHCHCH.CHs | OH ©2004 Thomson - Brooks/Cole Which alkene would work best? CH Add —OH here CH Add —OH here [8 / 8 CH;CH,CHCH—CHCHs3 CH;CH,.C = CHCH,CH3 4-Methyl-2-hexene 3-Methyl-3-hexene ©2004 Thomson - Brooks/Cole 42 Problem 7.10: Which Alkenes? (a) (CH3),>CHCH,CH,OH (b) eae emis (c) ( )- CH,OH OH 46 7.6 Addition of Carbenes to Alkenes  Carbenes are electrically neutral with six electrons in the outer shell  They add symmetrically across double bonds to form cyclopropanes 47 Formation of Dichlorocarben e  Base removes proton from chloroform  Carbanion is stabilized by chlorines  Unimolecular Elimination of Cl- gives the neutral, electron deficient species: dichlorocarbene 50 Reaction of Dichlorocarbene  Addition of dichlorocarbene is stereospecific cis 51 Simmons-Smith Reaction  Equivalent to addition of CH2:  Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species  Forms cyclopropanes by cycloaddition Problem 7.12: Major products? (a) (ron, +cHcl, “2% 2 (b) (CH,),>CHCH,CH=CHCH, + CHpl, ©2004 Thomson - Brooks/Cole Zn(Cu) 9 syn Addition CH, CH, | Ho, PtO. H CH3CO2H H CH | . CH; 1,2-Dimethylcyclohexene cis-1,2-Dimethylcyclohexane (82%) ©2004 Thomson - Brooks/Cole 55 Mechanism of Catalytic Hydrogenation O Heterogeneous - reaction between phases \ 4 \ / f H H pr %. H H C=C WITTITIT Paes : fis Catalyst Hydrogen adsorbed Complex of alkene on catalyst surface to catalyst Na Cc. H” No we Soe ce * ~~ loam fp * IIITTTTIT H H Regenerated Insertion of hydrogen Alkane product catalyst into carbon—carbon double bond 56 © Thomson - Brooks Cole Steric effect: Top side of double bond blocked by methyl group HC i H3C CHg i CH; a-Pinene ©2004 Thomson - Brooks/Cole H3C._ CH; H CH3 (NOT formed) 57 Problem 7.13: Major Products? (a) y _ CH3C = CHCH,CHs (b) CH; Ken, Solid Fats from Liquid Oils NNN Ester of linoleic acid (a constituent of vegetable oil) [2 Hg, Pd/C O ON NNN Ester of stearic acid ©2004 Thomson - Brooks/Cole 61 62 7.8 Oxidation of Alkenes: Hydroxylation  Hydroxylation adds OH to each end of C=C  Oxidizing agent is osmium tetroxide  Stereochemistry of addition is syn  Product is a 1,2-dialcohol or diol (also called a glycol) 65 7.9 Biological Alkene Addition Reactions  Living organisms convert organic molecules using enzymes as catalysts  Many reactions are similar to organic chemistry conversions, except they occur in neutral aqueous solution at 37oC  Usually very specific for reactant and stereochemistry 66 Biological Hydration Example  The conversion of fumaric acid to malic acid is catalyzed by fumarase, which is specific for the trans stereoisomer (maleic acid is cis)  This is one step in the Citric Acid cycle, the final phase in the catabolism of fats and carbohydrates to carbon dioxide STEP 1 Acetyl CoA adds to L oxaloacetate to yield citrate. HjC” ~SCaA HSCoA Tain Oxaloacetate a CH, ea H ‘ co," STEPS The cycle is completed 1% . | = by oxidation of malate c=0 CH, s fi ae é TEP 2 Citrate zed back to oxaloacetate. | - | - _ Ai cof the OH co, CO, group to yield isocitrate, el NaD* COs cH, NADH/I* Malate vi oe CH—co.- 2 = | COs a 7 CH—OH _Isocitrate Sar RSS Citric Acid | Fe is H,0 to fuma- a Cycle apne — rate gives + C0, aeaie G0r ADEE STEP 3 lsocitrate oi coo —e + 004 CO, loses COs to yield ll HSCoA | a-ketoglutarate, Fumarate H—C Ely . +NAD* We ee ee 1,0 7 CH, CH, i le a-Ketoglutarate STEP 6 Succinate is ] CH, ee dehydrogenated by CH, | co,.- the coenzyme FAD i i cH, Oy to give fumarate. a | Ster 4 a-Ketoglutarate Succinate cpp C=0 loses CO, and reacts +HPo- | with HSCoA to yield GTP succinyl CoA. Step 5 Suecinyl CoA is Succinyl CoA hydrolyzed to give succinate plus HSCoA, and a GDP molecule is phosphorylated to give GTP. ©2004 Thomson - Brooks/Cole 67 70 Free Radical Polymerization: Initiation  Initiation - a few radicals are generated by the homolysis of the O-O bond in benzoyl peroxide  The benzoyloxy radical adds to ethylene to form a carbon radical 71 Polymerization: Propagation  Radical from initiation adds to alkene to generate alkene derived radical  This radical adds to another alkene, and so on many times 72 Polymerization: Termination  Chain propagation ends when two radical chains combine, or react with some radical scavenger  Not controlled specifically but affected by reactivity and concentration 75 Chain Branching During Polymerization  During radical propagation chain can develop forks leading to branching  One mechanism of branching is short chain branching in which an internal hydrogen is abstracted 76 Long Chain Branching  In long chains, a hydrogen from another chain is abstracted 77 Cationic Polymerization  Vinyl monomers react with Brønsted or Lewis acid to produce a reactive carbocation that adds to alkenes and propagates via lengthening carbocations