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University of Minnesota Twin Cities
CHEM 2331H
T.Andrew Taton

Chapter 8 Book Notes Reactions of Alkenes • All alkenes have C=C double bond, which = reactive, has characteristic reactions 8.1) Reactivity of the Carbon-Carbon Double Bond • Single bonds = more stable than pi bonds  most common rxns of double bonds make pi bond  sigma bond • 3 major types of rxns o Addition: 2 molecules combine 1 product molecule  2 groups add to C’s of double bond  C’s become saturated o Elimination: 1 molecule splits  2 fragment molecules o Substitution: 1 fragment replaces another fragment in molecule • Addition = most common alkene rxn o can add suitable reagents to alkene double bonds  variety of functional groups 8.2) ElectrophilicAddition to Alkenes • Hypothetically, many energetically favorable rxns exist, but not all = @ convenient rates  catalyst (ex: platinum, palladium, nickel) helps ↑rate, although some reagents can react w/o catalyst • Sigma bond electrons tightly held, but pi bond delocalized above & below sigma bond, electrons farther from nuclei  more loosely held o strong electrophile wants these loosely held electrons  pulls them away  form new bond  one of C has only 3 bonds and + charge (carbocation) o Double bond = nucleondile, gives electron pair to electrophile • Most addition rxns have 2 step where nucleophile attacks carbocation  stable product o in product, both electrophile and nucleophile bonded to C’s that were double bonded o  Electrophilic addition • Mechanism: ElectrophilicAddition toAlkenes o Step 1:Attack of pi bond on electrophile forms carbocation  Strong electrophile attacks loosely held electrons from pi bond of alkene o Step 2:Attack by nucleophile gives the addition product  Electrophile forms sigma bond to one of C’s of former double bond, other C becomes carbocation  Carbocation (strong electrophile) reacts w/ nucleophile (often weak) to form another sigma bond st o Basically, 2 different sets of electrophiles/nucleophiles react for step 1 and 2. In 1 step, alkene = nucleophile and other thing = electrophile; In step 2, former alkene = electrophile and something else new = nucleophile • Reminders/cautions o Regiochemistry: orientation of addition; which part of reagent adds to which end of double bond o Pay attention to stereochemistry if rxn = stereospecific 8.3) Addition of Hydrogen Halides to Alkenes • 8.3A) Orientation of Addition: Markovnikov’s Rule st o Addition of HBr to but-2-ene; 1 step: protonate double bond  if protonates secondary C  tertiary carbocation (more stable, favored)  if protonates tertiary C  secondary carbocation o 2 half of mechanism: add HBr to 2-methylbut-2-ene  Note: protonating one of the C’s in double bond  other C that wasn’t protonated becomes carbocation   proton adds to less substituted end of double bond  more substituted carbocation (more stable) o Mechanism: IonicAddition of HX to anAlkene  Step 1: Protonation of the pi bond forms a carbocation  Step 2:Attack by the halide ion gives the addition product o Regioselective: 1 of 2 possible orientations of addition results preferentially over the other (ex: protonating less substituted C  more substituted carbocation) o Markovnikov’s Rule: addition of proton acid to double bond of alkene  product w/ acid proton bonded to C that already holds greater # H’s  rxns follow Markovnikov orientation  Markonikov product  If want to extend to electrophiles other than proton acids  Markonikov Rule (extended): In electrophilic addition to alkene, electrophile adds in such a way as to generate most stable intermediate o HI and HCl also follow same rule • 8.3B) Free-RadicalAddition of HBr:Anti-MarkovnikovAddition o anti-Markovnikov reactions: additions of HBr (but not HI or HCl)  products = opposite of Markonikov’s rule  likely when reagents/solvents came from old supplies that accumulated peroxides from air exposure  O-O bond in peroxide = weak  splits into alkoxy radicals  causes addition (anti-Markovnikov) by radical mechanism o Mechanism: Free-RadicalAddition of HBr toAlkenes  Initiation: Formation of Radicals • peroxide generates free radicals, which react w/ HBr  Br radicals  Propagation: radical reacts to generate another radical • Step 1: Br radical adds to double bond  (new) alkyl radical on more substituted C o Br radical lacks octet, electron deficient, electrophilic • Step 2:Alkyl radical abstracts H from HBr (forms C-H bond)  product + Br radical (which reacts w/ another alkene  chain rxn) • Both steps = moderately exothermic  faster than termination steps • Each step starts w/ 1 free radical, ends w/ 1 free radical (# free radicals constant) until reactants consumed, radicals come together, terminate chain rxn o RadicalAddition of HBr to UnsymmetricalAlkenes  Add Br to less substituted end of double bond  unpaired electron’s on most substituted C  This new radical now reacts w/ HBr  anti-Markovnikov product: H added to more substituted end of double bond (the end that started w/ fewer H) o  But both HBr addition mechanisms (w/ and w/o peroxides) follow extended version of Markovnikov’s rule  Electrophile adds to less substituted end of double bond  most stable intermediate (carbocation/free radical)  for ionic rxn, H = electrophile; peroxide-catalyzed free-radical rxn: Br radical = electrophile o Markovnikov orientation does take place in presence of peroxides, together w/ free radical rxn but peroxide-catalyzed rxn = faster  If just little bit peroxide present  Markovnikov + anti-Markovnikov product mixture  If lot of peroxide, radical chain rxn much faster than uncatalyzed ionic rxn  only anti-Markovnikov product observed o Peroxide effect: reversal of orientation in presence of peroxides  Happens only w/ HBr addition to alkenes  Doesn’t happen w/ HCl b/c alkyl radical + HCl rxn = strongly endothermic  HI + alkene rxn also = strongly endothermic 8.4) Addition of Water: Hydration ofAlkenes • Hydration: addition of water, H adds to 1 C, hydroxyl group adds to other C; alkene + water (in presence of strongly acidic catalyst) alcohol; reverse of dehydration of alcohol • Dehydrating alcohols: concentrated dehydrating acid (like H S2 or4H PO 3 d4ives equilibrium to favor alkene; hydration of alkene: add excess water, drives equilibrium toward alcohol • 8.4A) Mechanism of Hydration o Principle of microscopic reversibility: forward and reverse rxn happening under same conditions (like in equilibrium) must follow same rxn pathway in microscopic detail  ex: hydration and dehydration = 2 complementary rxns in an equilibrium  must follow same rxn pathway  Lowest-energy transition states and intermediates for reverse and forward rxn = same, except in reverse order o  Can write hydration mechanism by reversing dehydration mechanism steps  Double bond protonated  carbocation  Water (nucleophile) attacks  proton lost  alcohol o Mechanism:Acid-Catalyzed Hydration of anAlkene  Step 1: Protonation of double bond  carbocation  Step 2: Nucleophillic attack by water  alcohol  Step 3: Deprotonation alcohol • 8.4B) Orientation of Hydration o Hydration follows Markovnikov’s rule (like HBr addition): in product, new H added to less substituted end of double bond  proton adds to less substituted end of double bond  more substituted carbocation  water attacks carbocation  (after proton loss) alcohol w/ -OH on most substituted C   hydration = regioselective o Hydration may have rearrangement of carbocation intermediate 8.5) Hydration by Oxymercuration-Demercuration • Many alkenes don’t easily undergo hydration in aqueous acid o some alkenes = nearly insoluble in aqueous acid o others undergo side rxn (ex: rearrangement, polymerization, charring) in strongly acidic conditions m o sometimes overall eqb favors alkene, not alcohol o  can’t cause rxn to occur w/ catalysis if energetics aren’t favorable • Oxymercuriation-demercuriation: another way of converting alkenes  alcohols w/ Markovnikov orientation o works w/ many alkenes that don’t easily go direct hydration o milder conditions o no free carbocation formed no rearrangement/polymerization • MercuricAcetate (Hg(OCOCH ) or H3 2Ac) ) = reag2nt for mercuration o = electrophile (many theories of how it does that) o si+plest theory: mercuric acetate dissociates slightly  +charged mercury species Hg(OAc) • Oxymercuriation: electrophilic attack on double bond by + mercury species  mercurinium ion (organometallic cation w/ 3 membered ring) nd o 2 step: water from solvent attacks mercurinium ion  (after protonation) organomercurial alcohol o  demercuriation: remove mercurys o Sodium borohydride (NaBH , red4cing agent) replaces mercuric acetate fragment w/ H • Mechanism: Oxymercuriation of anAlkene o Step 1: Electrophilic attack forms a mercurinium ion o Step 2: Water opens the ring  organomercurial alcohol  Demercuriation replaces mercuric fragment w/ H  alcohol • Oxymercuriation-demercuriation of unsymmetrical alkene  Markovnikov orientation of addition (ex: propene in ↑ example) o Mercurinium ion has good amt + charge, on both of its C’s but more +charge on more substituted C  more stable o water attacks electrophilic C  Markovnikov orientation o electrophile Hg(OAc) remains on less substituted end of double bond o reduction of organomercurial alcohol  Markovnikov alcohol • Oxymercuriation-demercuriation accomplishes pretty much same as acid-catalyzed hydration, but no rearrangement o most commonly used for alkenes in laboratory o better yields, no rearrangement, no harsh conditions o BUT, organomercurial compounds = highly toxic safety in using/disposing 8.6) Alkoxymercuriation-Demercuriation • Alkoxymercuriation-demercuriation: alkenes  ethers by adding alcohol across double bond of alkene o mercuriation in alcohol solvent: alcohol = nucleophile, attacks mercurinium ion  product has alkoxy (-O-R) group o alkene reacts  mercurinium ion, which = attacked by nucleophilic solvent (alcohol )organomercurial ether, which can be reduced to the ether • Solvent attacks mercurinium ion @ more substituted end of double bond (where there’s more δ charge)  Markovnikov orientation of addition o Hg(OAc) group’s on less substituted end of double bond o reduction  Markovnikov product w/ H @ less substituted end of double bond 8.7) Hydroboration ofAlkenes • What if we want alkene  anti-Markovnikov alcohol? o diborane (B H 2 a6ds to alkenes w/ anti-Markovnikov orientation  alkylboranes o  oxidized  anti-Markovnikov alcohols • Diborane = dimer composed of 2 borane (BH ) molecules 3 o unconventional bonding: 3-centered (banana-shaped) bonds w/ protons in middle of them o diborane = in equilibrium w/ small amt borane (strong Lewis acid w/ only 6 valence electrons) o o But diborane = toxic, flammable, explosive  inconvenient o  more easily used as complex w/ tetrahydrofuran (THF): a cyclic ether   acts like diborane, but sol’n easily manufactured and transferred • BH 3THF = form of borane commonly used in organic rxns o BH add3 to double bond of alkene  alkylborane o Basic hydrogen peroxide oxidizes alkylborane  alcohol o Hydroboration-oxidation: alkenes  alcohols by adding water across double bond w/ anti-Markovnikov orientation • 8.7A) Mechanism of Hydroboration o Borane = electron-deficient; 6 valence electrons, no octet  wants octet  unusual bonding structure (ex: banana bonds)  strong electrophile, adds to double bond  hydroboration: 1 step, B adds to less substituted end of double bond o Transition state: B withdraws electrons from pi bond, C @ other end gets partial + charge  + charge more stable on more substituted C  B bonded to less substituted end of double bond, H on more substituted end  steric hindrance  B adding to less substituted end = more favorable o Mechanism: Hydroboration of anAlkene  Single Step: borane adds to double bond by adding to less hindered, less substituted C, H adds to more substituted C o B removed by oxidation, using aqueous sodium hydroxide and hydrogen peroxide  replaces B w/ hydroxyl (-OH) group  oxidation doesn’t affect prstuct orientation b/c anti-Markovnikov orientation established in 1 step when adding BH 3 o Hydration of alkene by hydroboration-oxidation doesn’t follow original Markovnikov’s rule, but follows reasoning behind it (extended version)  electrophilic B adds to less substituted end of double bond, + charge (and H) on more substituted end • 8.7B) Stoichiometry of Hydrocarbon o 3 moles of alkene can react w/ each mole of BH ; 13addition  alkylborane, 2 nd rd addition  dialkylborane, 3 addition  trialkylborane o Trialkylboranes react same way, oxidize to give anti-Markovnikov alcohols  = bulky  B prefers to add to less hindered C of double bond  Boranes often drawn as 1:1 monoalkylboranes to simplify structure & emphasize organic part of molecule • 8.7C) Stereochemistry of Hydroboration o syn addition: B and H add across double bond to same side of molecule
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