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1、Chapter 12: Reactions of Chapter 12: Reactions of AlkenesAlkenesPolyethylenePolyethylene Tear resistantTear resistantAddition Reactions of AlkenesAddition Reactions of Alkenes BondBond is is unsaturatedunsaturated. Reacts . Reacts by by additionaddition. .C CC C+ + A AB BC CC CA AB Bexothermic exoth
2、ermic (usually)(usually)We can calculate We can calculate HH from bond strength from bond strength data, using Ddata, using DHH - -bondbond= 65 kcal/mol. Note, = 65 kcal/mol. Note, however: however: S S is negative ( is negative ( - -30 30 e.ue.u.) .)1. 1. Catalytic HydrogenationCatalytic Hydrogenat
3、ion: H: H2 2+ + catalystcatalystC CC C+ + HH HHC CC CHH HHcat.cat.Heterogeneous Heterogeneous catalystscatalystsWhy catalysts?Why catalysts? Enable a Enable a lower lower E Ea amechanism.mechanism.Alkenes + HAlkenes + H2 2no reaction no reaction without catalystwithout catalystPd/C, PtOPd/C, PtO2 2
4、( ( PtPt) )HH2 2E ECatalyzed pathwayCatalyzed pathwayUncatalyzedUncatalyzed pathwaypathwayMechanismMechanism of hydrogenation: Occurs on of hydrogenation: Occurs on surface of surface of catalystcatalyst. Involves . Involves stereospecificstereospecific HH- - H addition from the H addition from the
5、samesame side (side (synsyn) of double ) of double bond.bond.+ +HH2 2cat.cat.HHHHCHCH3 3CHCH2 2CHCH3 3ciscistranstranscat.cat.D D2 2+ +SSSSD DD DHHHHR RR RLipshutzLipshutzOrbisonOrbisonVegetable Oils, Soft Margarine, and Butter: Vegetable Oils, Soft Margarine, and Butter: Hydrogenation “Hardens”Hydr
6、ogenation “Hardens”“Margarine” comes from “oleomargarine”, in turn derived from : a. Olefins, the original name for alkenes, because addition reactions of many gaseous alkenes give oils as products (from oleum facere, Latin, to make oil); b. margaric acid, one of the constituent fatty acids of marga
7、rine, heptadecanoic acid (forms shiny, “pearly” crystals, Greek, margaron, pearl)The fat molecules in butter and hard (stick) margarines are highly saturated, whereas those in vegetable oils have a high proportion of cis-alkene functions (more than 90%). Partial hydrogenation of these oils yields so
8、ft (tub) margarine. Conditions of hydrogenation also isomerize cis double bonds to trans. For example, hard margarine: 35% saturated fatty acids (SFAs), 12% trans fatty acids (TFAs); natural butter 50% SFAs, 34% TFAs. Soft margarine (less hydrgoenation) 15% SFAs, 5% TFAs.TFAs not metabolized in same
9、 way as cis counterparts; TFAs in foods greatly affect lipid metabolism; TFAs accumulate in cell membranes, increase levels of low- density lipoproteins (“bad cholesterol”) in blood while reducing high-density lipoprotein levels (good cholesterol); Increased risk of breast cancer and heart disease.T
10、FAs are present in fried fast foods (french fries) and many baked goods (cakes, cookies, crackers, and pastries). American Heart Association (2005) recommends: 30% of caloric intake from fat; 10% trans and saturated fats.USDA (1.1.2006) requires all food to list trans fatty acid content.TFAsTFAs and
11、 Healthand Health2. 2. Electrophilic AdditionsElectrophilic Additions Bond isBond is e e- -richrich. Polar r. Polar reagents Aeagents AB B add add to it. to it. + +- -MechanismMechanism for for A = H: A = H: Reverse of E1!Reverse of E1!+ +C CC C+ + HHC CC CHH+ +:B:B- -C CC CHHB B+ + +a. a. Hydrohalo
12、genationHydrohalogenationHHX X+ + HHI I HH+ +I I- - -+ +I IHHRegioelectiveRegioelective: : BrBr HHHHBrBr+ + HClHClClClMarkovnikovsMarkovnikovs RuleRuleHH+ +( (A A + +) adds to ) adds to less substitutedless substituted carboncarbonWhy?Why? Makes the Makes the more substitutedmore substituted cationc
13、ationReverse of Reverse of eliminationelimination18381838- - 190419040C0CPropeneHClPropeneHClLipshutzLipshutzCarusoCarusob. b. MarkovnikovMarkovnikov HydrationHydration+ +HH OHOHOHOHCHCH3 3HHsolventsolventEquilibrium !Equilibrium !Reversible Hydration of 2Reversible Hydration of 2- -Methylpropene:Me
14、thylpropene:HH2 2SOSO4 4HH2 2OO, , HH catalystcatalyst+ +HH+ +consumedconsumedHH+ +regeneratedregeneratedConsequences of Consequences of Reversibility of E1Reversibility of E1Alkene Alkene isomerizationisomerizationHH+ +HH+ +Equilibration = Thermodynamic controlEquilibration = Thermodynamic controlM
15、echanism of Thermodynamic Mechanism of Thermodynamic ControlControlStep 1Step 1Step 2Step 2Reminder: Reminder: C CC C+ +A A B BC C C CA A B Bc. c. HalogenationHalogenation A = B = XA = B = XX XX Xgets polarized during approach gets polarized during approach to alkene.to alkene.+ + BrBr2 2BrBr BrBr S
16、tereospecific : AntiStereospecific : Anti (not syn)(not syn)C CC CC CC CororBrominationsBrominations can be “titrated”can be “titrated”Stereospecific 2-Butene BrominationMechanism:Mechanism:C CC CBrBrBrBr+ +- -C CC CBrBr+ + + +BrBr- -C CC CBrBrBrBrantianti“ “PrecomplexPrecomplex” ”LipshutzLipshutzDjangoDjangoThis Is Hot Recent Stuff!This Is Hot Recent Stuff!BellucciBellucci, Lenoir, , Lenoir, HergesHerges, , J. Am. Chem
