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1 Chapter 7: Alkenes: Reactions and Synthesis Electrophilic Addition

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1 Chapter 7: Alkenes: Reactions and Synthesis Electrophilic Addition Chapter 7: Alkenes: Reactions and Synthesis H OH H H C C C C alcohol alkane X OH HO OH C C C C halohydrin C C 1,2-diol alkene X X C O C C carbonyl 1,2-dihalide H X C C C C C halide halide Addition X Y C C + X Y C C alkene Elimination Electrophilic Addition H H Br ether + HBr H H H Dehydrohalogenation: loss of HX from an alkyl halide to form an alkene H EtOH H Br (ethanol) + KOH + KBr + H2O H H H 1 Hydration: addition of water (H-OH) across a double bond to give an alcohol H H OH H+ + H2O H H H Dehydration: Loss of water (H-OH) from an alcohol to give an alkene H H OH H+ + H2O H H H Addition of Halogens (X2) to Alkenes: 1,2-dihalides X2 X X C C C C alkene 1,2-dihalide Br Br + Br2 + Br Br not observed 1,2-dibromide has the anti stereochemistry Bromonium ion intermediate controls the stereochemistry 2 Halohydrin Formation "X-OH" X OH C C C C alkene halohydrin Br2, H2O X + HBr OH anti stereochemistry Organic molecules are sparingly soluble in water as solvent. The reaction is often done in a mix of organic solvent and water using N-bromosuccinimide (NBS) as the electrophilic bromine source. O OH O Br DMSO, H2O + N Br + N H O O Note that the aryl ring does not react!!! For unsymmterical alkenes, halohydrin formation is Markovnikov-like in that the orientation of the addition of X-OH can be predicted by considering carbocation stability + CH d 3 more d+ charge on the + Br d more substituted carbon d+ H2O adds in the second step and adds to the carbon that has the most d+ charge and ends up on the more substituted end of the double bond HO CH CH 3 Br2, H2O 3 Br H Br adds to the double bond first (formation of bromonium ion) and is on the least substituted end of the double bond 3 Hydration of Alkenes: addition of water (H-OH) across the p-bond of an alkene to give an alcohol. 1. Acid catalyzed hydration- Markovnikov addition of H-OH Not a good method for hydration of an alkene 2. Oxymercuration- Markovnikov addition H-OH CH 1) Hg(OAc)2, H2O HO 3 CH3 2) NaBH4 H H 3. Hydroboration- Anti-Markovnikov addition of H-OH, Syn addition of H-OH 1) B2H6, THF H CH3 CH3 2) H2O2, NaOH, H2O H HO H H H _ O B B 2 H3B O + H H H tetrahydrofuran (THF) B H (diborane) borane-THF 2 6 complex 4 Reaction of Alkenes with Carbenes to give Cyclopropanes Carbene: highly reactive, 6-electron species. (sp2-hybridized) Generation and Reaction of Carbenes: CHCl3 + KOH Cl2C: + H2O + KCl dichlorocarbene H CHCl , KOH 3 Cl Cl H Simmons-Smith Reaction (cyclopropanation) ether CH2I2 + Zn(Cu) I-CH2-Zn-I = “H2C:” methylene carbene H CH2I2, Zn(Cu) ether H 5 The cyclopropanation reaction of an alkene with a carbene takes place in a single step. There is NO intermediate. As such, the geometry of the alkene is preserved in the product. Groups that are trans on the alkene will end up trans on the cyclopropane product. Groups that are cis on the alkene will end up cis on the cyclopropane product. CH I , Zn(Cu) H H 2 2 H H R R ether R R cis-alkene cis-cyclopropane CH I , Zn(Cu) H R 2 2 H R R H ether R H trans-alkene trans-cyclopropane Hydrogenation: Addition of H2 across the p-bond of an alkene to give an alkane. This is a reduction. H2, PtO2 ethanol • The reaction uses H2 and a precious metal catalyst. • The catalysts is not soluble in the reaction media, thus this process is referred to as a heterogenous catalysis. • The catalyst assists in breaking the p-bond of the alkene and the H-H s-bond. • The reaction takes places on the surface of the catalyst. Thus, the rate of the reaction is proportional to the surface area of the catalyst. • To increase the surface area of the catalyst it is finely dispersed on an inert support such as charcoal (carbon, C) • Carbon-carbon p-bond of alkenes and alkynes can be reduced to the corresponding saturated C-C bond. Other p-bond bond such as C=O (carbonyl) and C≡N are not easily reduced by catalytic hydrogenation. The C=C bonds of aryl rings are not easily reduced. 6 Catalysts: Pt2O (Adam’s catalyst) or Pd/C mechanism: The addition of H2 across the p-bond is syn O O H2, PtO2 ethanol O H2, Pd/C C5H11 OH CH3(CH2)16CO2H Linoleic Acid (unsaturated fatty acid) Steric Acid (saturated fatty acid) O O H , Pd/C OCH3 2 OCH3 ethanol H , Pd/C C 2 C N N ethanol H CH3 CH CH H 3 H2, PtO2 3 ethanol CH CH CH3 3 3 H H syn addition Not observed of H2 7 Oxidation of Alkenes to 1,2-Diols and Carbonyl Hydroxylation: formal addition of HO-OH across the p-bond of an alkene to give a 1,2-diol. This is an overall oxidation. 1) OsO4 H OH 2) NaHSO3 syn addition OH H H O O osmate ester intermediate Os O O - not usually isolate H - NaHSO3 breaks down the osmate ester to the product Ozonolysis: oxidative cleavage of an alkene to carbonyl compounds. The p- and s-bonds of the alkene are broken and replaced with C=O doubled bonds. C=C of aryl rings, C≡N and C=O do not react with ozone, C≡C react very slowly with ozone electrical + Ozone (O ): 3 O2 2 O3 O _ 3 discharge O O O3, CH2Cl2 O R R 1 3 -78 °C O O O O Zn R1 R3 R 1 R3 O + O R1 R3 R R2 R4 2 O R4 R2 R4 R2 R4 molozonide ozonide + ZnO 8 1) O3 2) Zn O + O 1) O3 2) Zn H + O= O C H2 1) O3 2) Zn O H O Oxidative Cleavage of 1,2-Diols to Carbonyl Compounds R1 R HO OH NaIO4 3 O + O R1 R3 THF, H2O R2 R4 R2 R4 + ZnO OH O O I O O R1 R3 periodate intermediate R2 R4 OH NaIO4 O H H OH O 9.
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