wwu -- chemistry alkenes and alkynes i. addition reactions chapter eight
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Alkenes and Alkynes I.Alkenes and Alkynes I.Addition ReactionsAddition Reactions
Chapter Eight
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Sect 8.1: Addition Sect 8.1: Addition ReactionsReactions
C C X Y C C
X
Y
+
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Addition of H-XAddition of H-X
C
H
H
C
H
H
CH
H
C
H
H
H
X
H
H
X
H
H
H
H X
H X
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Addition of WaterAddition of Water
C
H
H
C
H
H
H
O
H
H+ catalyst
CH
H
C
H
H
H
OH
H
H
OH
H
H
H
H
O
H
H+ catalyst
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Sect. 8.2: Introduction Sect. 8.2: Introduction to to MechanismsMechanisms
• mechanism• two step• rate determining step (i.e. slow
step)• energy diagram• transition states• intermediates
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MECHANISMMECHANISMSTEP-BY-STEP ACCOUNT OF WHAT HAPPENS
C C
E+
C C
E+
C C
E
Xstep 2step 1
intermediates areformed during areaction but are not products
:X-
Intermediate
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product
H
intermediate
TS2
TS1
ENERGY PROFILEENERGY PROFILEtwo step reaction
ENERGY
step 1 step 2C C
E+
X -
+C C
C C
E
X
E
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ACTIVATED COMPLEXESACTIVATED COMPLEXEScorrespond to transition states for each
step
ACTIVATED COMPLEXES
C C
E
XX-
+C C
E
E+
C C
C C
E
C C
E
X
intermediate
show bonds in the process of breaking or forming(bonds are half formed or half broken)
+
+
-
+
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Sect 8.3: Electrophilic Sect 8.3: Electrophilic Addition to a Double BondAddition to a Double Bond
CH2 CH2 X Y X CH2 CH2 Y+
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CARBOCATION STABILITYCARBOCATION STABILITYHYPERCONJUGATION
CC
H
H
H
+
R
Relectrons in an adjacent
C-H bond help to stabilizethe 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 Can you recognize the following carbocations? 1following carbocations? 1oo, ,
22oo, 3, 3oo+
+ +
+ +
+
1 o 2 o 2 o
3 o 2 o
2 o
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Sect 8.4: Addition of Sect 8.4: Addition of Hydrogen Halides: Hydrogen Halides: Markovnikov’s ruleMarkovnikov’s rule
H3C
C
CH2
CH3
H3C
C
C
CH3
H3C
C
C
CH3
H
Br
H
H
major product
minor product
H
H
HBr
H-Br
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Markovnikov’s RuleMarkovnikov’s RuleIn the ionic addition of an acid to the carbon-carbon double bond of an alkene, the hydrogen of the acid attaches itself to the carbon atom which already holds the greater number of hydrogens.
–“Them that has, gets!”
–“The richer get richer!”
(V. W. Markovnikov -- 1838 - 1904)
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MARKOVNIKOV RULEMARKOVNIKOV RULE
CH2
+ HCl
CH3
Cl
When adding HX to a double bond thehydrogen of HX goes to the carbonwhich already has the most hydrogens
..... conversely, the anion X adds to the mosthighly substituted carbon ( the carbon withmost alkyl groups attached).
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REGIOSELECTIVEREGIOSELECTIVE REACTION REACTION
CCH3
CH3
CH2 CCH3
CH3
CH3
Cl
+ CHCH3 CH2
Cl
CH3HCl
major minor
one of the possible products is formedin larger amounts than the other one
Compare
REGIOSPECIFICREGIOSPECIFIConly one of the possible products isformed (100%).
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Mechanism (Markovnikov)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
Secondary C+
Major product
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Mechanism (anti-Mechanism (anti-Markovnikov)Markovnikov)
R CH CH2R 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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COMPETING PATHWAYSCOMPETING PATHWAYS
lower energyintermediate
higher energyintermediate
rate-determininng(slow) step
rate-determining step
faster
slower
1 o
2 o
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Markovnikov Addition to Markovnikov Addition to an Alkenean Alkene
C CH2
CH3
CH3
H X CH3 C CH2
CH3
X
H CH3 C CH2
CH3
H
X+ +
major product minor productX = Cl, Br, I
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MechanismMechanism
C CH21)+
+
slow
2)_ fast
+
+
H
Electrophile
Nucleophile
CH3
CH3
CH CH2
CH3
CH3 H
+C CH2
CH3
CH3 H
C CH2CH3
CH3 H
X
X
3 o C+
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SOME ADDITIONAL EXAMPLESSOME 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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Sect 8.5 Addition of Sect 8.5 Addition of Sulfuric Acid to an AlkeneSulfuric Acid to an Alkene
CH CH2R R CH CH2 H
O
S
OH
OO
+ H2SO4
(cold, concentrated)
an alkyl hydrogen sulfate
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H O S O
O
O
H
C C C C
H+ C C
H
OSO3 H
O S O
O
O
H
alkyl hydrogen sulfate
cold
-
ALKYL HYDROGEN SULFATESALKYL HYDROGEN SULFATES
C C
H
OHwaterroom temp
FOLLOWS MARKOVNIKOFFRULE
SLOW
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Addition of Water to an Addition of Water to an AlkeneAlkene
R CH CH2 R CH CH2 H
OH
+ H2OH2SO4
an alcohol
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Mechanism of HydrationMechanism of Hydration
R CH CH2 R CH CH2 H
R CH CH2 H R CH CH2 H
OH2
R CH CH2 H
OH2
R CH CH2 H
OH
O S OH
O
O
H
1) + H2SO4
2)
+
++ H2O
slow
+
3)
+
+
+ HSO4
_
HSO4
_
+
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Sect 8.6 Addition of Sect 8.6 Addition of Bromine to an AlkeneBromine to an Alkene
R CH CH R R CH CH R
Br
Br
+ Br2
CCl4
or CH2Cl2
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ADDITION OF BROMINEADDITION OF BROMINE
C C
Br
Br
+C C
Br
C C
Br Br
BrCCl4
Br Br:+ -
alkene polarizes bromine
SLOW
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H
H
Br
Br
H
H
Br
H
Br
HBr
H
H
Br
Br
Br
cis compound trans compound
NOT OBSERVED
THE REACTION IS STEREOSPECIFICTHE REACTION IS STEREOSPECIFIC
ACTUAL PRODUCT
ANTI ADDITION
anti
syn
antisyn
+
“open“ carbocationwould give bothcis and trans
-
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WHAT WOULD EXPLAIN FORMATIONWHAT WOULD EXPLAIN FORMATION OF ONLY THE OF ONLY THE transtrans PRODUCT ? PRODUCT ?
..... A BRIDGED OR CYCLIC INTERMEDIATE
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+Br
HH
CYCLIC BROMONIUM IONCYCLIC BROMONIUM ION
note size ofbromine
+
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HH
-Br
HH
Br
Br
Br+
BRIDGED BROMONIUM IONBRIDGED BROMONIUM ION
bridgingblocks approachfrom this side
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Br
H
H
Br
Br
H
H
BrH
Br
Br
H
ENANTIOMERS
symmetricintermediate
+
FORMATION OF ENANTIOMERSFORMATION OF ENANTIOMERS
Addition could also startfrom the top with bromideattacking the bottom.
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CH3
H H
CH3CH3
H CH3
H
CH3 CH CH CH3
+
Br2
CH3 CH CH CH3
Br Br
ADDITION OF BROMINE TO 2-BUTENEADDITION 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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CH3
H H
CH3CH3
H CH3
H
BrBr
CH3 CH3
HH
BrBr
H CH3
HCH3
BrBr
CH3 HCH3H+
Br2 / CCl4 Br2 / CCl4
enantiomers meso
NO ! THEY GIVE DIFFERENT PRODUCTSNO ! THEY GIVE DIFFERENT PRODUCTS
These results can only be explained by stereospecific anti addition.
cis trans
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Br
Br
H3CCH3
HH
H3CCH3
HH
Br+
BrBr
H3CCH3
H
H
Br
Br
H3C
CH3
H
H
Br
Br
Br Br
H3C HH
CH3
Br Br
H CH3
H3C H
ROTATEROTATE
ENANTIOMERS
ADD TO RIGHT
ADD TO LEFT
cis-
-
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Br
Br
H3CH
HCH3
H3CH
HCH3
Br
BrBr
H3CH
H
CH3
Br
Br
H3C
H
H
CH3
Br
Br
Br Br
H HH3C
CH3
Br Br
H3CCH3
HH
ROTATEROTATE
ADD TO RIGHT
ADD TO LEFT
MESO
IDENTICAL (also meso)
trans_
_
+
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Bromination of an Bromination of an Unsymmetrical AlkeneUnsymmetrical Alkene
R CH CH2 Br Br R CH CH2
Br
R CH CH2
BrC C
Br
BrH
H
R
H
1)
+slow
+
_
2)+
+ Br: :. _
.
anti-addition(stereospecific)
+ Br
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Stereochemistry of Stereochemistry of Bromination of Alkenes 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 via a bridged ion and some molecules reacting via an open-chain carbocation
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Stereochemistry of Stereochemistry of Bromination of AlkenesBromination of Alkenes
--Part Two--Part Two• In cases where a resonance-stabilized
carbocation is possible, if the solvent is made more polar (acetic acid or nitromethane), the proportion of molecules reacting via an open-chain carbocation increases.
• For simple alkenes, changing solvents has little or no effect on stereochemistry.
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Sect 8.7 Halohydrin Sect 8.7 Halohydrin FormationFormation
Br2
H2O
a bromohydrin
OH
+ HBrR C C
Br
R-CH=CH-R HH R
Br2 + H2O
HO-Br
+ HBr
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MechanismMechanism
R CH CH2 + Br Brslow
R CH CH2
Br
R CH CH2
Br
OH H
R CH CH2
Br
OH H
R CH CH2
Br
OH H
R CH CH2
Br
O
H
+
+ Br
H
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Sect. 8.8: Carbocation Sect. 8.8: Carbocation RearrangementsRearrangements
CHCl
C
major
rearranged!!
+ C
minor
not rearranged!!
CH3
CH3
CH3
CH CH2 CH3
CH3
CH3
CH CH2
H
Cl
CH3
CH3
CH3
CH CH2
Cl
H
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Sect 8.10 Free Radical Sect 8.10 Free Radical Addition of HBr to Alkenes Addition of HBr to Alkenes
((antianti-Markovnikov!)-Markovnikov!)
R CH CH2R CH CH2 Br
H
+ HBrROOR
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ADDITION OF HBrADDITION OF HBr
CH2
conc. HBr
CH3
Br
CH2
conc. HBr
CH2BrH
Markovnikov Addition
Anti-Markovnikov Addition
Oxygen
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R O O R R O O R
R O R O HH Br
CH2 CH R Br CH2 CH R
Br CH2 CH R H Br Br CH2 CH R
H
1)
2)
3)
4)
light or
heat+..
.. ..
... .
..
... + ..
..+ Br:
..
...
Br:..
... + .
. + + Br: ...
.
hydrogen abstraction
hydrogen abstraction
slow
...
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STABILITY OF CARBON RADICAL INTERMEDIATESSTABILITY OF CARBON RADICAL INTERMEDIATES
Radicals are electron-deficient just like carbocationsand have the same stability order.
lowestenergy
highest energy
R C R
R
R C
R
H R C H2 C H3< < <
tertiary secondary primary methyl
and they are stabilized by resonance and / or hyperconjugation.
CH2
( )
( )
( ) etc.CH2
( )
( )( )
( )
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Sect. 8.11 and 8.12: Hydrogenation Sect. 8.11 and 8.12: Hydrogenation of Alkenes and alkynesof Alkenes and alkynes
+ H2
Pt, Pd,
or Ni
syn-addition
(stereospecific)
CCH2
lindlar catalyst
R
H H
R CH CH R
R
R CH CH R
H H
R
R
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Hydrogenation of AlkenesHydrogenation of Alkenes
Also PtO2 -- sometimes Ru, Rh, or Re
R CH CH R R CH CH R
H H
+ H2
Pt, Pd,
or Ni
syn-addition
(stereospecific)
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HYDROGEN ADSORBS ON THE SURFACE OF THE CATALYSTHYDROGEN ADSORBS ON THE SURFACE OF THE CATALYST
n H2 + Pt Pt(H )2n
H H H.Hydrogen Adsorbed onCatalyst Surface
. .finely-dividedparticlesdispersed insolution
Pt Pt PtPt Pt
.H
.H
Pt
.H
Pt Pt Pt Pt Pt
.
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MECHANISM OF HYDROGENATIONMECHANISM OF HYDROGENATION
.H.
H
.H
.H .
H.H.
H
H H
CATALYST
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.H.
H
.H
.H .
H.H.
H
.H
.H
HYDROGENADSORBS
MECHANISM OF HYDROGENATIONMECHANISM OF HYDROGENATION
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MECHANISM OF HYDROGENATIONMECHANISM OF HYDROGENATION
.H.
H
.H
.H .
H.H.
H
.H
RR
R R
. .
.H
ALKENEAPPROACHES
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MECHANISM OF HYDROGENATIONMECHANISM OF HYDROGENATION
.H.
H
.H
.H .
H.H.
H
.H
.H
RR
R R
. .ALKENE PICKS UPTWO HYDROGENS
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MECHANISM OF HYDROGENATIONMECHANISM OF HYDROGENATION
.H.
H
.H
.H .
H.H.
H
RR
R R
. .H
..H
ALKANE ISFORMED
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BOTH HYDROGENS ADD TO THE SAME BOTH HYDROGENS ADD TO THE SAME SIDE OF THE DOUBLE BONDSIDE OF THE DOUBLE BOND
CH3
CH3
CH3
CH3
CH3
CH3
H
H
HH syn
addition
antiaddition
X
notobserved
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EXAMPLESEXAMPLES
CH3 + H2 CH3Pt
CH3 CH CH CH3Pt
H2+ CH3 CH2 CH2 CH3
CH2
PtH2+
CH3
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Sect. 8.14: Addition Sect. 8.14: Addition PolymersPolymers
Ccatalyst
C
ethylene polyethylene
a macromolecule with very high molecular weight
H
H
H
H
C C
H
H H
H
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Polymerization of propenePolymerization of propene
C Ccatalyst three different forms
syndiotactic, isotactic and atactic
CH3CH3 CH3CH3
CH3 CH3CH3
CH3
CH3
H
H
H
isotactic
syndiotactic
Indirect HydrationIndirect Hydration
• Oxymercuration-Demercuration– Markovnikov product formed– Anti addition of H-OH– No rearrangements
• Hydroboration– Anti-Markovnikov product formed– Syn addition of H-OH
HydroborationHydroboration
• Borane, BH3, adds a hydrogen to the most substituted carbon in the double bond.
• The alkylborane is then oxidized to the alcohol which is the anti-Mark product.
C C(1) BH3
C
H
C
BH2
(2) H2O2, OH-
C
H
C
OH
Borane ReagentBorane Reagent
• Borane exists as a dimer, B2H6, in equilibrium with its monomer.
• Borane is a toxic, flammable, explosive gas.• Safe when complexed with tetrahydrofuran.
THF THF . BH3
O B2H6 O+
B-
H
H
H
+2 2
Predict the ProductPredict the Product
Predict the product when the given alkene reacts with borane in THF, followed by oxidation with basic hydrogen peroxide. CH3
D
(1)
(2)
BH3, THF
H2O2, OH-
syn addition
HCH3
DOH
EpoxidationEpoxidation
• Alkene reacts with a peroxyacid to form an epoxide (also called oxirane).
• Usual reagent is peroxybenzoic acid.
CC + R C
O
O O H CC
O
R C
O
O H+
One-Step ReactionOne-Step Reaction
• To synthesize the glycol without isolating the epoxide, use aqueous peroxyacetic acid or peroxyformic acid.
• The reaction is stereospecific.
CH3COOH
O
OH
H
OH
H
Chapter 8Chapter 8 68
Syn Hydroxylation Syn Hydroxylation of Alkenesof Alkenes
• Alkene is converted to a cis-1,2-diol,• Two reagents:
– Osmium tetroxide (expensive!), followed by hydrogen peroxide or
– Cold, dilute aqueous potassium permanganate, followed by hydrolysis with base
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