Alkene%20and%20Arene

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Organic Chemistry
Alkene and Arene
Aromatic Alkene
C. Y. Yeung, 06/07
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C. Y. Yeung p. 02
Preparations of Alkenes
Elimination of ROH and RX
Endothermic DH gt 0
1. Dehydration (- H2O)
2. Dehydrohalogentation (- HX)
conc. H2SO4
alc. KOH
Saytzeff rule is used to predict the major
product of elimination.
(Higher substituted alkene is more stable!)
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C. Y. Yeung p. 03
Reactions of Alkenes (I)
Addition reaction
Exothermic DH lt 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
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C. Y. Yeung p. 04
Reactions of Alkenes (II)
3. Ozonolysis
(i) O3, CH3CCl3, lt200C
(ii) Zn / H2O
alkene ? smaller aldehyde / ketone
a good method to deduce the position of CC in
the alkene !
4. Polymerization
? mechanism Free Radical Addition
initiation ? propagation ? termination
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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!
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C. Y. Yeung p. 06
Free Radical Addition Polymerization (cont)
(ii). Chain propagation
reaction between radical and molecule
react with another alkene molecule
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C. Y. Yeung p. 07
Free Radical Addition Polymerization (cont)
(iii). Chain termination
reaction between radicals
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C. Y. Yeung p. 08
Note
Haloalkanes (mixture)
Alkane
Alkene
Polymer (mixture)
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C. Y. Yeung p. 09
Arenes
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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
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C. Y. Yeung p. 11
DHhydrogenation
Delocalization stabilization energy -152 kJ
mol-1
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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).
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C. Y. Yeung p. 13
Prove delocalization of p e- in benzene
? by Ozonolysis of 1,2-dimethylbenzene !