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Alkene and Arene

AL Chemistry. Organic Chemistry. Alkene and Arene. Ar omatic Alk ene. C. Y. Yeung, 06/07. heat under reflux. heat under reflux. C. Y. Yeung p. 02. Preparations of Alkenes. * Elimination of ROH and RX:. Endothermic [ D H > 0]. 2. Dehydrohalogentation (- HX). 1. Dehydration (- H 2 O).

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Alkene and Arene

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  1. AL Chemistry Organic Chemistry Alkene and Arene Aromatic Alkene C. Y. Yeung, 06/07

  2. heat under reflux heat under reflux C. Y. Yeung p. 02 Preparations of Alkenes * Elimination of ROH and RX: Endothermic [DH > 0] 2. Dehydrohalogentation (- HX) 1. Dehydration (- H2O) conc. H2SO4 alc. KOH # Saytzeff rule is used to predict the major product of elimination. (Higher substituted alkene is more stable!)

  3. C. Y. Yeung p. 03 Reactions of Alkenes (I) * Addition reaction: Exothermic [DH < 0] 1. Catalytic Hydrogenation Application: “Hardening of Vegetable oil  Margarine 2. Electrophilic Addition: (a) + Br2 in CCl4 (b) + dry H-Br # Markownikoff rule is used to predict the major product of addition. (for unsymmetrical alkenes) (c) + HO-Br (bromine water) (d) + H-OSO3H [cold]

  4. “break down” “build up” C. Y. Yeung p. 04 Reactions of Alkenes (II) (i) O3, CH3CCl3, <200C 3. Ozonolysis: (ii) Zn / H2O * alkene  smaller aldehyde / ketone ^ a good method to deduce the position of C=C in the alkene ! 4. Polymerization:  mechanism: Free Radical Addition * initiation  propagation  termination

  5. O O UV O R C O R C O 2 O R C + CO2 2 R C. Y. Yeung p. 05 Free Radical Addition Polymerization (i). Chain initiation: * the generation of “free radical” (R ) from “diacyl peroxide” (unstable) (homolysis) very reactive  reacts with alkene (monomer) … chain propagation!

  6. (alkene) (R) + R CH3 CH3 H H C C R R C C H H CH3 CH3 CH3 H CH3 H H H H H C C C C H H H H + C R C C C H H H H C. Y. Yeung p. 06 Free Radical Addition Polymerization (con’t) (ii). Chain propagation: * reaction between radical and molecule react with another alkene molecule

  7. CH3 CH3 H H C C C R C + H H H H y CH3 CH3 H H C C C R C CH3 CH3 H H H H H H C R C C R C x H H H H y+1 x+1 C. Y. Yeung p. 07 Free Radical Addition Polymerization (con’t) (iii). Chain termination: * reaction between radicals

  8. free radical substitution free radical addition C. Y. Yeung p. 08 Note: Haloalkanes (mixture) Alkane Alkene Polymer (mixture)

  9. SO3H NO2 p. 160 p. 160 p. 152 - 159 COOH R X p. 162-164 p. 161 p. 161 C. Y. Yeung p. 09 Arenes

  10. destroy ! C. Y. Yeung p. 10 Stability of Aromatic Compounds Much more stable than Non-aromatic Alkenes  more inert towards Electrophilic Addition !  Extra stabilization of delocalization of e- ref. p.159 Table 31-2 ----- Compare the reactivity of cyclohexane, cyclohexene and methylbenzene Estimate the “extent” of stabilization …. ? based on DHhydrogenation

  11. Enthalpy + 3 H2 + 3 H2 - 208 kJ mol-1 (expt.) - 360 kJ mol-1 (estimated) + H2 - 120 kJ mol-1 (expt.) C. Y. Yeung p. 11 DHhydrogenation Delocalization stabilization energy [-152 kJ mol-1]

  12. It has an extra stability by delocalization of p-electrons. [conjugated system] + 2 H2 + 2 H2 - 240 kJ mol-1 + H2 - 231 kJ mol-1 - 120 kJ mol-1 (expt.) C. Y. Yeung p. 12 Q. Explain why the DHhydrogenation of cyclohexa-1,3-diene (-231 kJ mol-1) is less exothermic than the DHhydrogenation of cyclohexa-1,4-diene (-240 kJ mol-1).

  13. CH3 CH3 CH3 CH3 CH3 CH3 (1) O3, CH3CCl3, <200C (2) Zn / H2O (1) O3, CH3CCl3, <200C (2) Zn / H2O CH3 CH3 CH3 O O CH3 O O O O Evidence ! O O O O O O C. Y. Yeung p. 13 Prove “delocalization of p e-” in benzene …  by Ozonolysis of 1,2-dimethylbenzene !

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