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Addition Reactions
(Alkenes)
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MECHANISMSTEP-BY-STEP ACCOUNT OF WHAT HAPPENS
C C
E+
C C
E+
C C
E
Xstep 2step 1
intermediates are
formed during a
reaction but are
not products
:X-
Intermediate
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Electrophilic Addition to a Double
Bond
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CH2
CH2 X Y X CH2CH2 Y+
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CARBOCATION STABILITYHYPERCONJUGATION
CC
H
H
H
+
R
Relectrons in an adjacent
C-H sbond help to stabilize
the positive charge of thecarbocation by proximity
(overlap)
CR
R
R+
R CH R
+
R CH2+
tertiary secondary primary
> >Most
stable
Least
stable
CR
R
R+R CH R
+R CH2
+
..
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Can you recognize the following
carbocations? 1o, 2o, 3o
+
++
+ +
+
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1 o 2o 2 o
3 o 2 o 2o
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Addition of Hydrogen Halides:
Markovnikovsrule
H3C
C
CH2
CH3H3C
C
C
CH3
H3C
C
C
CH3
H
Br
H
H
major
product
minorproduct
H
H
H
Br
H-Br
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Markovnikov
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Markovnikovs Rule
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In the ionic addition of an acid to the
carbon-carbon double bond of an alkene,
the hydrogen of the acid attaches itself tothe carbon atom which
already holds the
greaternumber of hydrogens.
Them that has, gets!The richer get richer!
(V. W. Markovnikov -- 1838 - 1904)
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WWU -- Chemistry
MARKOVNIKOV RULE
CH2
+ HCl
CH3
Cl
When adding HX to a double bond the
hydrogen of HX goes to the carbon
which already has the most hydrogens
..... conversely, the anion X adds to the most
highly substituted carbon ( the carbon with
most alkyl groups attached).
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REGIOSELECTIVE
REACTION
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CC H3
C H3
C H2 CC H3
C H3
C H3
C l
+ C HC H3 C H2
C l
C H3HCl
major minor
one of the possible products is formed
in larger amounts than the other one
Compare
REGIOSPECIFIConly one of the possible products is
formed (100%).
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Mechanism (Markovnikov)
R CH CH2 R CH-CH2
R CH CH2 H
Br
1)+
+
slow
2)_ fast
+
+
H
Br
H
R CH-CH2
+
H
Electrophile
Nucleophile
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Secondary C+
Major product
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Mechanism (anti-Markovnikov)
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R CH CH2
R CH CH2
H
R CH CH2
H
R CH CH2
Br
H
1)+ slow
+2)
_ fast
+
+
H
Br
+
Minor!
Primary carbocation
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WWU -- Chemistry
COMPETING PATHWAYS
lower energy
intermediate
higher energyintermediate
rate-determininng
(slow) step
rate-determining step
faster
slower
1 o
2 o
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Markovnikov Addition to an Alkene
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C CH2
CH3
CH3
H X CH3C CH
2
CH3
X
H CH3C CH
2
CH3
H
X+ +
major product minor product
X = Cl, Br, I
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Mechanism
C CH21)+
+
slow
2) _ fast
+
+
H
Electrophile
Nucleophile
CH3
CH3
CH CH2
CH3
CH3 H
+C CH2
CH3
CH3 H
C CH2CH3
CH3H
X
X
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3 o C+
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SOME ADDITIONAL EXAMPLES
CH3
+ HCl
CH3
Cl
CH2
+ HCl
CH3
Cl
CH CH2 CH CH3
Cl+ HCl
only major product is shown
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Addition of Bromine to an Alkene
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R CH CH R R CH CH R
Br
Br
+ Br2CCl4
or CH2Cl2
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ADDITION OF BROMINE
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C C
B r
B r
+C C
B r
C C
B r B r
B r
C C l4
B r B r:+ -
alkene polarizes bromine
SLOW
Halonium ion
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WWU -- Chemistry
H
H
B r
B r
H
H
B r
H
B r
H
B r
H
H
B r
B r
B r
cis compound transcompound
NOT OBSERVED
THE REACTION IS STEREOSPECIFIC
ACTUAL PRODUCT
ANTI ADDITION
anti
syn
antisyn
+
open carbocationwould give both
cis and trans
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WWU -- Chemistry
WHAT WOULD EXPLAIN FORMATION
OF ONLY THE trans PRODUCT ?
..... A BRIDGED OR CYCLIC INTERMEDIATE
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+
Br
HH
CYCLIC BROMONIUM ION
note size of
bromine
+
Halonium ion
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WWU -- Chemistry
HH
-
Br
HH
Br
Br
Br+
BRIDGED BROMONIUM ION
bridging
blocks approach
from this side
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WWU -- Chemistry
Br
H
H
Br
Br
H
H
BrH
Br
Br
H
ENANTIOMERS
symmetric
intermediate
+
FORMATION OF ENANTIOMERS
Addition could also start
from the top with bromide
attacking the bottom.
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WWU -- Chemistry
C H3
H H
C H3 C H3
H C H3
H
C H3 C H C H C H3+
B r2
C H3 C H C H C H3
B r B r
ADDITION OF BROMINE TO 2-BUTENE
* *
cis-2-butene trans-2-butene
WILL THESE STEREOISOMERS GIVE THE SAME PRODUCTS?
2n possible stereoisomers
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WWU -- Chemistry
CH3
H H
CH3 CH
3
H CH3
H
BrBr
CH3 CH3
HH
BrBr
H CH3
HCH3
BrBr
CH3
HCH3H
+
Br2/ CCl4 Br2/ CCl4
enantiomers meso
NO ! THEY GIVE DIFFERENT PRODUCTS
These results can only be explained by stereospecific anti
addition.
cis trans
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WWU -- Chemistry
B r
B r
H3C
C H3H
H
H3CC H3
H
H
B r+
B rB r
H3C
C H3
H
H
B r
B r
H3C
C H3
H
H
B r
B r
B rB r
H3CH
HC H3
B r B r
H C H3
H3C H
ROTATE
ROTATE
ENANTIOMERS
ADD TO
RIGHT
ADD TO
LEFT
cis-
-
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WWU -- Chemistry
Br
Br
H3CH
H
CH3
H3CH
H
CH3
Br
BrBr
H3C
H
H
CH3
Br
Br
H3C
H
H
CH3
Br
Br
Br Br
H HH3C
CH3
Br Br
H3C CH3
HH
ROTATEROTATE
ADD TO
RIGHT
ADD TO
LEFT
MESO
IDENTICAL (also meso)
trans
__
+
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Bromination of an Unsymmetrical
Alkene
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R CH CH2
Br Br R CH CH2
Br
R CH CH2
Br
C C
Br
BrH
H
R
H
1)
+slow
+
_
2)+
+ Br: :. _
.
anti-addition(stereospecific)
+ Br
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Stereochemistry of Bromination of
Alkenes
Simple alkenes: Addition of bromine or
chlorine goes exclusively anti, with the
formation of a bridged ion If a resonance-stabilized
open-chain
carbocation is possible, there may be a
mixture of mechanisms, with some molecules
reacting viaa bridged ion and some molecules
reacting viaan open-chain carbocation
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Stereochemistry of Bromination of
Alkenes
--Part Two
In cases where a resonance-stabilized
carbocation is possible, if the solvent is mademore polar
(acetic acid or nitromethane), the
proportion of molecules reacting viaan open-
chain carbocation increases.
For simple alkenes, changing solvents has little
or no effect on stereochemistry.
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Addition of Sulfuric Acid
to an AlkeneCH CH
2R R CH CH
2H
O
S
OH
OO
+ H2SO4
(cold, concentrated)
an alkyl hydrogen sulfate
Used in alcohol synthesis
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WWU -- Chemistry
H O S O
O
O
H
C C C C
H+
C C
H
O S O3H
O S O
O
O
H
alkyl hydrogen sulfate
cold
-
ALKYL HYDROGEN SULFATES
C C
H
OH waterroom temp
FOLLOWS
MARKOVNIKOFF
RULE
SLOW
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Mechanism of Hydration
R CH CH2 R CH CH2 H
R CH CH2H R CH CH
2H
OH2
R CH CH2H
OH2
R CH CH2H
OH
O S OH
O
O
H
1) + H2SO4
2)
+
++ H2O
slow
+
3)
+
+
+ HSO4
_
HSO4
_
+
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Halohydrin Formation
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Br2
H2O
a bromohydrin
OH
+ HBrR C C
Br
R-CH=CH-R HH R
Br2 + H2O HO-Br + HBr
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Mechanism
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R CH CH2 + Br Br slow
R CH CH2
Br
R CH CH2
Br
O
H H
R CH CH2
Br
O
H H
R CH CH2
Br
O
H H
R CH CH2
Br
O
H
+
+ Br
H
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Free Radical Addition of HBr to
Alkenes (anti-Markovnikov!)
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R CH CH2 R CH CH2Br
H
+ HBr ROOR
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ADDITION OF HBr
CH2
conc.HBr
CH3
Br
CH2
conc.HBr
CH2Br
H
Markovnikov
Addition
Anti-Markovnikov
Addition
Oxygen
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