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KIMIA ORGANIK FISIK
STEREOKIMIA
HALOGENASI RADIKAL BEBAS DARI SENYAWA
KIRAL
CAMPURAN RASEMIK
3
The Stereochemistry of SN1 Reactions
When the leaving group leaves from a stereogenic center of an
optically active compound in an SN1 reaction, racemization will
occur
This is because an achiral carbocation intermediate is formed
Racemization: transformation of an optically active compound to a
racemic mixture
4
5
Carbocations
A carbocation has only 6 electrons, is sp2 hybridized and has an
empty p orbital
6
The Stereochemistry of SN2 Reactions
Backside attack of nucleophile results in an inversion of
configuration
In cyclic systems a cis compound can react and become trans
product
Chapter 6 8
The E2 Reaction
E2 reaction involves concerted removal of the proton,
formation of the double bond, and departure of the leaving
group
Both alkyl halide and base concentrations affect rate and
therefore the reaction is 2nd order
10
The E1 Reaction
The E1 reaction goes through a carbocation
intermediate
12
Stereochemistry of Alkene Adition
C C
Y
Z
C C C C
Y ZYZ YZ
anti additionsyn addition
Catalytic hydrogenation proceeds by syn addition:
CH3
CH3
H
H
H2
Pt
Stereochemistry of hydrogenation
Chapter 8 13
Stereochemistry of the Ionic Addition to an
Alkene
Addition of HBr to butene yields a chiral molecule
A racemic mixture is produced because the
intermediate carbocation is achiral
14
Hydroboration-oxidation
Mechanism:
CH3 CH CH2
B2H6 2 BH3 H BH2
+-
diborane boranemore
electropositivethan H
H BH2
CH3 CH CH2
BH2H-
+CH3 CH CH2
H BH2
- +
OH
more substituted C
bears larger +
syn addition
antiMarkovnikov
Chapter 8 15
Chapter 8 16
Oxidations of Alkenes: Syn 1,2-Dihydroxylation
Either OsO4 or KMnO4 will give 1,2 diols (glycols)
Mechanism for Syn Hydroxylation of Alkenes
Cyclic intermediates result from reaction of the
oxidized metals
The initial syn addition of the oxygens is preserved
when the oxygen-metal bonds are cleaved and the
products are syn diols
17
Epoxidation
C C + R C
O
O OH C C
O
+ R C
O
OH
peroxy acidepoxide
CH3 C
O
O OH
peroxyacetic acid m-chloroperoxybenzoic acid (mCPBA)
O
O
OH
ClO
O
O
epoxyethane (ethylene oxide)
1,2-epoxypropane (propylene oxide)
1,2-epoxycyclohexane (cyclohexene oxide)
18
Epoxidation
Stereospecific syn addition:
CH3CO3H
"
O
H3C CH3
HH
O
H3C H
CH3H
cis-2,3-epoxybutane
trans-2,3-epoxybutane
Chapter 8 19
Stereochemistry of the addition of Halogens to Alkenes
The net result is anti addition because of SN2 attack on the
bromonium ion intermediate
When cyclopentene reacts the product is a racemic
mixture of trans-1,2-dibromocyclopentane enantiomers
Chapter 8 20
Stereospecific Reactions
A reaction is stereospecific if a particular stereoisomeric form of
the starting material reacts in such a way that it gives a specific
stereoisomeric form of the product
Example: cis- and trans-2-butene give stereoisomeric products
when halogenated
Halogenation of double bonds is stereospecific
Chapter 8 21
Stereospecific Reactions
A reaction is stereospecific if a particular stereoisomeric form of
the starting material reacts in such a way that it gives a specific
stereoisomeric form of the product
Example: cis- and trans-2-butene give stereoisomeric products
when halogenated
Halogenation of double bonds is stereospecific
