Lecture and Chapter 8 Notes

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University of Toronto St. George
Thavarajah/ Woolley/ Nitz

CHM138H1 Jasmyn Lee Chapter 8: Alkenes: Reactions and Synthesis (Not Simmons-Smith reactions or epoxidations, not 8.10 or 8.11) Topics:  Synthesis via elimination reactions  Halogenation and halohydrin formation  Addition of water to alkenes (oxymercuration, hydroboration-oxidation)  Carbene reactions (:2CL )  Hydrogenation (H /cat.) 2 -  Dihydroxylation (O4O ), diol cleavage4with IO  Ozonolysis  Stereochemistry of alkene addition 8.1 Preparing Alkenes: A Preview of Elimination Reactions Synthesis of Alkenes via Elimination Reactions:  Base Elimination  Acid-Catalyzed Elimination  Dehydrohalogenation – the loss of HX from an alkyl halide o Usually occurs by reaction of an alkyl halide with strong base (eg/ KOH)  Dehydration – the loss of water from an alcohol o Often carried out by treatment of an alcohol with a strong acid Quick Review: Addition of HX (X=CL, Br, I) to Alkenes  The reaction goes through the more stable carbocation  When both carbocations have similar stability: 1 CHM138H1 Jasmyn Lee 8.2 Halogenation of Alkenes: Addition of X 2 From H-X Addition  By this mechanism, should see 50:50 mixture of cis and trans  In reality, only the trans isomer is observed  Reaction occurs with anti stereochemistry – atoms add from opposite faces of the double bond o Bromonium ion (R B2 ) is formed as an intermediate  Bromonium ion “shields” one face of ring – only the opposite face is available  ANTI addition 8.3 Halohydrins from Alkenes: Addition of HOX Halohydrin Formation  Reaction of alkenes with the hypohalous acids HO-Cl or HO-Br to yield 1,2-halo alcohols (halohydrins)  Halohydrin formation does not take place by direct reaction of an alkene with HOBr or HO Cl o Addition is done indirectly by reaction of the alkene with eithe2 Br o2 Cl in the presence of water  Recall: When Br2reacts with an alkene, the cyclic bromonium ion intermediate reacts with the only nucleophile - present, Br ion  If the reaction is carried out in the presence of an additional nucleophile, the intermediate bromonium ion can be intercepted by the added nucleophile and diverted to a different product - o In the presence of high concentration of 2 O – water competes with Br ion as nucleophile and reacts with the bromonium ion intermediate to yield a bromohydrin 2 CHM138H1 Jasmyn Lee o Net Effect – addition of HO-Br to the alkene Halohydrin Formation (2) Bromohydrin Formation with NBS  Few alkenes are soluble in water – bromoydrin formation is often carried out in solvent CH 3OCH (3MSO)  NBS – source of B2 , stable, easily handled compound that slowly decomposes in water to yield B2 at a controlled rate  NBS reacts slowly with water to produce a low constant level of 2r  This is a safer way of running this reaction; note, ring doesn’t react 3 CHM138H1 Jasmyn Lee Hydration of Alkenes: 8.4 Addition of H O2by Oxymercuration/ 8.5 Addition of H O by 2ydroboration  Hydration – H 2 adds to alkenes to yield alcohols o Strong acid catalyst (eg/ 2 SO4)  In Industry:  In Living Cells Electrophilic Addition Reactions Review Hydration Reactions in the Lab Oxymercuration  Involves electrophilic addition of Hg to the alkene on reaction with mercury (II) acetate, (3H 2 2) Hg (abbrv. Hg(OAC) 2 in aqueous THF solvent 4 CHM138H1 Jasmyn Lee 2+  Initiated by electrophilic addition of Hg (mercuric) ion to the alkene  Gives a mercurinium ion  Nucleophilic addition of water and loss of a proton  Yields a stable organomercury product  Demercuration of the organomercury compound by reaction with sodium borohydride  Note – Markovnikov addition of water Hydroboration  Addition of B-H bond of borane, B3 , to an alkene to yield an organoborane intermediate, RBH 2  Oxidation of the organoborane by reaction with basic hydrogen peroxide2 2 O , gives an alcohol  Borane is a good electrophile with an empty p-orbital o Borane is very reactive as a Lewis Acid – boron atom has only 6 valence electron3; BH accepts an electron pair form a solvent molecule to complete its octet  Syn Stereochemistry o Boron and hydrogen add to the alkene from the same face of the double bond – boron attaches to the less highly substituted carbon o Curing the oxidation step, the boron is replaced by an –OH with the same stereochemistry – resulting in an overall syn non-Markovnikov addition of water o Occurs in a single step without a carbocation intermediate – because both C-H and C-B bonds form at the same time and from the same face of the alkene, syn stereochemistry results o Non Markovnikov regiochemistry occurs – because attachment of boron is favoured at the less sterically crowded carbon atom of the alkene 5 CHM138H1 Jasmyn Lee 8.6 Reduction of Alkenes: Hydrogenation  Hydrogenation – alkenes react with 2 in the presence of a metal catalyst (palladium or platinum) to yield a saturated addition product  Reduction o General Chemistry – the gain of one or more electrons by an atom o Organic Chemistry – a reaction that results in a gain of electron density by carbon, caused by either a bond formation between C and a less EN atom eg/ H), or by a bond breaking between C and a more EN atom (eg/ O, N, X)  Addition of 2 gas to alkenes gives alkanes (or cycloalkanes)  Homogeneous Process – hydrogenation reaction occurs o the surface of solid catalyst particles  SYN addition – both H atoms add to the same face of alkenes  Catalyst: often Pd/C or 2tO (Adam’s Catalyst)  Adsorption of H2 ontotalyst surface  Complexation between catalyst and alkene – a vacant orbital on the metal interacts with the filled alkene  orbital  Hydrogen is inserted into the double bond  The saturated product diffuses away from the catalyst  The stereochemistry of hydrogenation is syn because both hydrogen’s add to the double bond from the same catalyst surface  The double bond is hydrogenated or reduced by the addition o2 H  Other multiple bonds are usually unaffected by these conditions o Alkenes are more reactive than other unsaturated functional groups o Aldehydes, ketones, esters and nitriles often are unaffected by hydrogenation 8.7 Oxidation of Alkenes: Epoxidation and Hydroxylation  Oxidation o General Chemistry – the loss of one or more electrons b tab atom o Organic Chemistry – a reaction that results in a loss of electron density by C, caused by either a bond formation between C and a more EN atom (O, N, X), or by a bond breaking between C and a less EN atom (H) o Often adds oxygen (vs. reduction – adds hydrogen) 6 CHM138H1 Jasmyn Lee Epoxidation  Alkenes are oxidized to give epoxides on treatment with perox
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