Organic Chemistry AL Chemistry Alkene and Arene Aromatic

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

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

C. Y. Yeung p. 02 Preparations of Alkenes * Elimination of ROH and RX:

C. Y. Yeung p. 02 Preparations of Alkenes * Elimination of ROH and RX: Endothermic [DH > 0] 1. Dehydration (- H 2 O) # 2. Dehydrohalogentation (- HX) heat under reflux conc. H 2 SO 4 alc. KOH Saytzeff rule is used to predict the major product of elimination. (Higher substituted alkene is more stable!)

C. Y. Yeung p. 03 Reactions of Alkenes (I) * Addition reaction: Exothermic [DH

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) + Br 2 in CCl 4 # Markownikoff rule is used to predict the major product of addition. (for unsymmetrical alkenes) (b) + dry H-Br (c) + HO-Br (bromine water) (d) + H-OSO 3 H [cold]

C. Y. Yeung p. 04 Reactions of Alkenes (II) 3. Ozonolysis: (i) O 3,

C. Y. Yeung p. 04 Reactions of Alkenes (II) 3. Ozonolysis: (i) O 3, CH 3 CCl 3, <200 C (ii) Zn / H 2 O * alkene smaller aldehyde / ketone “break down” ^ a good method to deduce the position of C=C in the alkene ! 4. Polymerization: mechanism: Free Radical Addition * initiation propagation termination “build up”

C. Y. Yeung p. 05 Free Radical Addition Polymerization (i). Chain initiation: * the

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

C. Y. Yeung p. 06 Free Radical Addition Polymerization (con’t) (ii). Chain propagation: *

C. Y. Yeung p. 06 Free Radical Addition Polymerization (con’t) (ii). Chain propagation: * reaction between radical and molecule (alkene) (R ) R R + H H H CH 3 C C H H C C + H H H CH 3 R H C C CH 3 react with another alkene molecule C H H C CH 3 H R H CH 3 H C C H H C CH 3 C H H

C. Y. Yeung p. 07 Free Radical Addition Polymerization (con’t) (iii). Chain termination: *

C. Y. Yeung p. 07 Free Radical Addition Polymerization (con’t) (iii). Chain termination: * reaction between radicals R H CH 3 H C C H H C CH 3 H C + C H H x C H H R CH 3 H C C H H R y H CH 3 H C C C H H x+1 C H H R y+1

C. Y. Yeung p. 08 Note: Alkane Alkene free radical substitution free radical addition

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

C. Y. Yeung p. 09 Arenes NO 2 p. 152 - 159 p. 160

C. Y. Yeung p. 09 Arenes NO 2 p. 152 - 159 p. 160 X R p. 161 SO 3 H p. 160 COOH p. 162 -164

C. Y. Yeung p. 10 Stability of Aromatic Compounds Much more stable than Non-aromatic

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

C. Y. Yeung p. 11 DHhydrogenation Enthalpy + 3 H 2 - 360 k.

C. Y. Yeung p. 11 DHhydrogenation Enthalpy + 3 H 2 - 360 k. J mol-1 (estimated) - 208 k. J mol-1 (expt. ) Delocalization stabilization energy [-152 k. J mol-1] + H 2 - 120 k. J mol-1 (expt. )

C. Y. Yeung p. 12 Q. Explain why the DHhydrogenation of cyclohexa-1, 3 -diene

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

C. Y. Yeung p. 13 Prove “delocalization of p e-” in benzene … by

C. Y. Yeung p. 13 Prove “delocalization of p e-” in benzene … by Ozonolysis of 1, 2 -dimethylbenzene ! CH 3 (1) O 3, CH 3 CCl 3, <200 C (2) Zn / H 2 O CH 3 O O O CH 3 OO O O CH 3 O O Evidence !