Download Chapter 7: Alkenes - Reactions, Synthesis and Mechanisms and more Study notes Organic Chemistry in PDF only on Docsity! 1 Chapter 7: Alkenes: Reactions and Synthesis C C C C OHH C C HH C C OHX C C XX alcohol alkane halohydrin 1,2-dihalide C C XH halide alkene C C OHHO 1,2-diol C C halide C C O carbonyl C C alkene X Y+ C C YX Elimination Addition Electrophilic Addition Dehydrohalogenation: loss of HX from an alkyl halide to form an alkene + HBr BrH H H H H ether Br H H H + KOH EtOH (ethanol) H H + KBr + H2O 2 Hydration: addition of water (H-OH) across a double bond to give an alcohol Dehydration: Loss of water (H-OH) from an alcohol to give an alkene + H2O OHH H H H H H+ OH H H H H H + H2O H+ Addition of Halogens (X2) to Alkenes: 1,2-dihalides C C C C XX 1,2-dihalidealkene X2 1,2-dibromide has the anti stereochemistry Bromonium ion intermediate controls the stereochemistry + Br2 Br Br + Br Br not observed 5 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 CH2I2 + Zn(Cu) ether I-CH2-Zn-I = “H2C:” Simmons-Smith Reaction (cyclopropanation) methylene carbene CHCl3, KOH Cl Cl H H CH2I2, Zn(Cu) H H ether 6 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. H H R R cis-alkene CH2I2, Zn(Cu) ether H H R R cis-cyclopropane H R R H trans-alkene CH2I2, Zn(Cu) ether H R R H trans-cyclopropane Hydrogenation: Addition of H2 across the p-bond of an alkene to give an alkane. This is a reduction. • 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. H2, PtO2 ethanol 7 Catalysts: Pt2O (Adam’s catalyst) or Pd/C mechanism: The addition of H2 across the p-bond is syn H2, PtO2 ethanol O O OCH3 O H2, Pd/C ethanol OCH3 O C N C N H2, Pd/C ethanol H2, PtO2 ethanol CH3 CH3 CH3 CH3 H H syn addition of H2 CH3 H H CH3 Not observed C5H11 OH O Linoleic Acid (unsaturated fatty acid) H2, Pd/C CH3(CH2)16CO2H Steric Acid (saturated fatty acid)
