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8. Alkena: Pembuatan dan Reaksi
Based on McMurry’s Organic Chemistry
6th edition
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McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003
2
Pembuatan Alkena: Reaksi Eliminasi
Alkena biasanya dibuat dengan cara:
•Dehydration Alkohol Tersier
CH3
OHH2SO4, H2O CH3
+ H2O
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McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003
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Pembuatan Alkena: Reaksi Eliminasi
•Dehydrohalogenation
Br
H
H
H
KOHH
H
+ KBr + H2O
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McMurry Organic Chemistry 6th edition Chapter 6 (c) 2003
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8.1 Preparation of Alkenes: A Preview of Elimination Reactions
Dehidrohalogenasi
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McMurry Organic Chemistry 6th edition Chapter 6 (c) 2003
5
Reaksi-reaksi alkena
Alkena bereaksi dengan banyak elektrofil menghasilkan produk melalui reaksi adisi • Menghasilkan alkohol (bila mengadisi H-OH)
• Menghasilkan alkana (bila mengadisi H-H)
• Menghasilkan halohidrin (bila mengadisi HO-X)
• Menghasilkan dihalida (bila mengadisi X-X)
• Menghasilkan halida (bila mengadisi H-X)
• Menghasilkan diol (bila mengadisi HO-OH)
• Menghasilkan siklopropana (bila mengadisi :CH2)
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McMurry Organic Chemistry 6th edition Chapter 6 (c) 2003
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8.2 Adisi Halogen pada Alkena
2,3-Dibromobutane2-Butene
+ Br2CH3CH=CHCH3 CH3CH-CHCH3CH2Cl2
trans-1,2-Dibromo-cyclohexane
Cyclohexene
+ Br2
Br
Br
CH2Cl2
Br Br
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• Step 1: Pembentukan bridged bromonium ion intermediate
• Step 2: Penyerangan ion halida dari arah yang berlawanan (opposite side) dari cincin anggota tiga
Anti (coplanar) orientationof added bromine atoms
C C
Br
Br
+ C C
Br ::-
Br
:: ::
::
:
: ::C C
Br
C C
Br
Br
+-
Br
:: ::
::
:
:
:
:
:
:
Mechanism
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McMurry Organic Chemistry 6th edition Chapter 6 (c) 2003
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8.3 Pembentukan Halohidrin
Reagennya Br2 atau Cl2 )
1-Chloro-2-propanol (a chlorohydrin)
Propene
+
HO Cl
CH3CH=CH2H2 O
HClCH3CH-CH2Cl2
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McMurry Organic Chemistry 6th edition Chapter 6 (c) 2003
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Mekanisme pembentukan Bromohidrin
C C
Br
R HH H
C C
Br
R HH H
C CR H
H H -Br -
bridged bromoniumion
minor contributingstructure
Br
Br:
:
:
:
:
:
:: ::
:
C C
Br
OH
H
HR
OH
H H H
+C C
Br
R HH H
::
:
:
:
::
:Step 1
Step 2
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McMurry Organic Chemistry 6th edition Chapter 6 (c) 2003
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8.4 Adisi air pada alkena: Oksimerkurasi/Reduksi
2-Butanol
+
Acetic acid
HgOAc
OH OH
CH3COOHCH3CHCHCH3NaBH4 CH3CHCHCH3 + Hg
H
Acetic acidAn organomercury compound
Mercury(II) acetate
2-Butene++
OH
HgOAc
Hg(OAc)2CH3CH=CHCH3H2 O
CH3CHCHCH3 CH3COOH
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McMurry Organic Chemistry 6th edition Chapter 6 (c) 2003
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Addition of Hg(II) and oxygen is anti & stereoselective
(Anti addition of-OH and -HgOAc)
CyclopenteneH HgOAc
H H
OH HHg(OAc)2
H2 O
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McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003
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Mechanism of Oxymercuration 1. Electrophilic addition of Hg(OAc)2 produces
mercurinium ion. 2. Water acts as a nucleophile, attacks the more
substituted carbon atom to form a C-O bond. Acetate ion removes hydrogen, forming a neutral
product, but the HgOAc is still attached. NaBH4 reduces the HgOAc, replacing it with a
hydrogen atom to form the alcohol product.
H3CHg OAc
OAc
H3CHg
OAcH3C Hg
OAc
H2O
H3C
O
H H
HgOAc
OAc OH
HgOAcH3C
NaBH4- H+
H3C
HO
H
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McMurry Organic Chemistry 6th edition Chapter 6 (c) 2003
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8.5 Adisi air pada alkena: Hidroborasi
Boran adalah asam Lewis yang mampu menerima elektron
Boran diadisi alkena menghasilkan organoboran
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Orientasi dalam hidrasi melalui Hidroborasi
Regiokimianya berlawanan dengan orientasi Markovnikov• OH terikat pada karbon dengan atom H paling banyak
H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)
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Hidroborasi, Efek Elektronik memberikan Non-Markovnikov
More stable carbocation is consistent with steric preferences
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Mekanisme Hidroborasi
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8.7 Reduksi Alkena: Hidrogenasi Alkena bereaksi dengan H2dengan adanya katalis
logam transisi menghasilkan alkana
• commonly used catalysts are Pt, Pd, Ru, and Ni
Proses ini disebut reduksi katalitik (catalytic reduction) atau hidrogenasi katalitik (catalytic hydrogenation)
+ H2Pd
Cyclohexene Cyclohexane
25°C, 3 atm
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Mechanism of Hydrogenation
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8.8 Oksidasi alkena: Hidroksilasi dan Pemutusan
Hidroksilasi mengadisi OH pada kedua karbon C=C Dikatalisis oleh osmium tetroksida Stereokimia adisi adalah syn Produknya adalah 1,2-dialkohol or diol (disebut juga
glikol = glycol)
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Ozonolisis alkena
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Structure Elucidation With Ozone
Cleavage products reveal an alkene’s structure