carbo cation
TRANSCRIPT
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6-1
Organic
Chemistry
William H. Brown
Christopher S. Foote
Brent L. Iverson
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6-2
Reactions
of Alkenes
Chapter 6
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6-3
Characteristic Reactions
CC
C C
C C
Br2
( HX)HCl
H2O
( X2 )
C C Br2( X2 )
H2 O
(X)
C C
H
OH
C C
Br
Br (X)
C C
HO
Br (X)
C C
H
Cl (X)
Descriptive N ame(s )React ion
+
+
+
Bromination
(halogenation)
Hydrochlorination
(hydrohalogenation)
Hydration
+ Bromo(halo)hydrin
formation
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6-4
Characteristic Reactions
CC
C C
CC
BH3
OsO4
H2
C C Hg(OAc) 2H2 O
C C
BH2H
C C
HO OH
C CHH
C C
HO
HgOAc
+
+
+
Hydroboration
Diol formation
(oxidation)
Hydrogenation(reduction)
+ Oxymercuration
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6-6
Gibbs Free Energy
Gibbs free energy change, G0: a thermodynamic
function relating enthalpy, entropy, andtemperature
exergonic reaction: a reaction in which the Gibbs freeenergy of the products is lower than that of thereactants; the position of equilibrium for an exergonicreaction favors products
endergonic reaction: a reaction in which the Gibbs freeenergy of the products is higher than that of thereactants; the position of equilibrium for anendergonic reaction favors starting materials
G0 = H0T S0
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Gibbs Free Energy
a change in Gibbs free energy is directly related to
chemical equilibrium
summary of the relationships between G0, H0, S0,and the position of chemical equilibrium
G0
= -RT ln Keq
At higher temperatures
when T S0 > H0 and
G0 < 0, the position of
equ ilibrium favors
products
G0 > 0; the
position of equilib rium
favors reactants
G0 < 0; the
position of equilib rium
favors products
At lower temperatures
whenT S0 < H0 and
G0 < 0, the po sition of
equ ilibrium favors
products
H0 > 0
H0 < 0
S0 < 0 S0 > 0
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6-8
Energy Diagrams
Enthalpy change, : the difference in total
bond energy between reactants and products a measure of bond making (exothermic) and bond
breaking (endothermic)
Heat of reaction, :the difference in enthalpybetween reactants and products
exothermic reaction:a reaction in which the enthalpyof the products is lower than that of the reactants; areaction in which heat is released
endothermic reaction:a reaction in which the enthalpyof the products is higher than that of the reactants; areaction in which heat is absorbed
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Energy Diagrams
Energy diagram: a graph
showing the changes inenergy that occur during achemical reaction
Reaction coordinate:ameasure in the change inpositions of atoms duringa reaction
Reactioncoordinate
Energy
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Energy Diagram
a one-step reaction with no intermediate
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6-12
Energy Diagram
A two-step reaction with one intermediate
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Developing a Reaction Mechanism
How it is done
design experiments to reveal details of a particular chemicalreaction
propose a set or sets of steps that might account for the overalltransformation
a mechanism becomes established when it is shown to be
consistent with every test that can be devised this does mean that the mechanism is correct, only that it is the
best explanation we are able to devise
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6-14
Why Mechanisms?
they are the framework within which to organize
descriptive chemistry they provide an intellectual satisfaction derived from
constructing models that accurately reflect thebehavior of chemical systems
they are tools with which to search for new informationand new understanding
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Electrophilic Additions
hydrohalogenation using HCl, HBr, HI
hydration using H2O in the presence of H2SO4 halogenation using Cl2, Br2
halohydrination using HOCl, HOBr
oxymercuration using Hg(OAc)2, H2O followed by
reduction
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6-16
Addition of HX
Carried out with pure reagents or in a polar
solvent such as acetic acid
Addition is regioselective regioselective reaction:an addition or substitution
reaction in which one of two or more possibleproducts is formed in preference to all others that
might be formed Markovnikovs rule:in the addition of HX, H2O, or ROH
to an alkene, H adds to the carbon of the double bondhaving the greater number of hydrogens
CH3 CH=CH2 HBr CH3 CH-CH2
Br H
CH3 CH-CH2
H Br
1-Bromopropane(not observed)
2-BromopropanePropene
++
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HBr + 2-Butene
A two-step mechanism
Step 1: proton transfer from HBr to the alkene gives a carbocationintermediate
Step 2: reaction of the sec-butyl cation (an electrophile) withbromide ion (a nucleophile) completes the reaction
CH3 CH=CHCH3 H Br CH3 CH-CHCH3
H
Br++
s ec-Butyl cation(a 2 carbocation
intermediate)
slo w, ratedetermining
Br CH3CHCH2 CH3 CH3CHCH2 CH3
Br
sec-Butyl cation(an electrophile)
+
Bromide ion(a nucleophile)
fast
2-Bromobutane
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HBr + 2-Butene
An energy diagram for the two-step addition of
HBr to 2-butene the reaction is exergonic
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Carbocations
Carbocation:a species in which a carbon atom has only
six electrons in its valence shell and bears positivecharge
Carbocations are
classified as 1, 2, or 3 depending on the number of
carbons bonded to the carbon bearing the positivecharge
electrophiles; that is, they are electron-loving
Lewis acids
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Carbocations
bond angles about a positively charged carbon are
approximately 120 carbon uses sp2 hybrid orbitals to form sigma bonds
to the three attached groups
the unhybridized 2porbital lies perpendicular to the
sigma bond framework and contains no electrons
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Carbocation Stability
a 3 carbocation is more stable than a 2 carbocation,
and requires a lower activation energy for its formation a 2 carbocation is, in turn, more stable than a 1
carbocation,
methyl and 1 carbocations are so unstable that they
are never observed in solution
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Carbocation Stability
relative stability
methyl and primary carbocations are so unstable thatthey are never observed in solution
Methylcation
(methyl)
Ethylcation
(1)
Isopropylcation
(2)
t ert-Butylcation
(3)Increasin g carbocation s tability
+ + + +C
H
HCH3 CCH3
CH3
H
C
CH3
CH3
CH3CHH
H
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Carbocation Stability
we can account for the relative stability of
carbocations if we assume that alkyl groups bonded tothe positively charged carbon are electron releasingand thereby delocalize the positive charge of thecation
we account for this electron-releasing ability of alkylgroups by (1) the inductive effect, and (2)hyperconjugation
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The Inductive Effect the positively charged carbon polarizes electrons of
adjacent sigma bonds toward it
the positive charge on the cation is thus localized overnearby atoms
the larger the volume over which the positive charge isdelocalized, the greater the stability of the cation
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Hyperconjugation
involves partial overlap of the -bonding orbital of an
adjacent C-H or C-C bond with the vacant 2porbital ofthe cationic carbon
the result is delocalization of the positive charge
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Addition of H2O
addition of water is called hydration
acid-catalyzed hydration of an alkene is regioselective;hydrogen adds preferentially to the less substitutedcarbon of the double bond
HOH adds in accordance with Markovnikovs rule
CH3CH=CH2 H2OH2SO4
CH3CH-CH2HOH
Propene 2-Propanol
+
CH3C=CH2
CH3
H2O
H2SO4
HO
CH3
HCH3C-CH2
2-Methyl-2-propanol2-Methylpropene
+
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Addition of H2O
Step 1: proton transfer from H3O+ to the alkene
Step 2: reaction of the carbocation (an electrophile)
with water (a nucleophile) gives an oxonium ion
Step 3: proton transfer to water gives the alcohol
++
+
intermediateA 2o carbocation
+HO
H
HOH
H
CH3CH=CH2 CH3CHCH3slow, ratedetermining
::
:
:
:+
+
+
An oxonium ion
H OHH
CH3CHCH3 O-H CH3CHCH3fast
:
:
:
++
+OH H
OH
H
H
H O HCH3 CHCH3 CH3 CHCH3fast
OH:
:
:
:
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Carbocation Rearrangements
In electrophilic addition to alkenes, there is the
possibility for rearrangement Rearrangement:a change in connectivity of the
atoms in a product compared with theconnectivity of the same atoms in the startingmaterial
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Carbocation Rearrangements
in addition of HCl to an alkene
in acid-catalyzed hydration of an alkene
HCl+ +
2-Chloro-3,3-dimethylbutane(the expected p roduct; 17%)
2-Chloro-2,3-dimethylbutane(the major product; 83%)
3,3-Dimethyl-1-butene
Cl Cl
H2 SO4H2 O
OH
3-Methyl-1-butene 2-Methyl-2-butanol
+
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Carbocation Rearrangements
the driving force is rearrangement of a less stable
carbocation to a more stable one
the less stable 2 carbocation rearranges to a morestable 3 one by 1,2-shift of a hydride ion
+
A 2 carbocation
intermediate
3-Methyl-1-butene
++
CH3
H
ClH
CH3
CH3 CCH=CH2 CH3 C-CHCH3
slow, ratedetermining
H
:
:
:-
Cl
:
:::
++
A 3 carbocation
CH3
H
CH3
H
CH3C-CHCH3 CH3C-CHCH3fast
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Carbocation Rearrangements
reaction of the more stable carbocation (an
electrophile) with chloride ion (a nucleophile)completes the reaction
-Cl:
:
::
2-Chloro-2-methylbutane
++
CH3 CH3
CH3C-CH2CH3 CH3C-CH2CH3fast
Cl: ::
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Addition of Cl2 and Br2
carried out with either the pure reagents or in an inert
solvent such as CH2Cl2
addition of bromine or chlorine to a cycloalkene gives
a trans-dihalocycloalkane
addition occurs with anti stereoselectivity; halogenatoms add from the opposite face of the double bond
we will discuss this selectivity in detail in Section 6.7
Br2CH2 Cl2
Br
Br
Br
Br
+
trans-1,2-Dibromocyclohexane(a racemic mixture)
Cyclohexene
+
CH3 CH=CHCH3 Br2 CH2Cl2CH3CH-CHCH3
Br Br
2,3-Dibromobutane2-Butene
+
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Addition of Cl2 and Br2
Step 1: formation of a bridged bromonium ion
intermediate
C C
Br
Br
C C
Br
C C
Br
C C
Br
Br-
The bridged bromoniumion retains the geometry
These carbocations are majorcontribu ting s tructures
+
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Addition of Cl2 and Br2
Step 2: attack of halide ion (a nucleophile) from the
opposite side of the bromonium ion (an electrophile)opens the three-membered ring to give the product
Anti (coplanar) orie ntation
of added bromine atoms
C C
Br
Br
Br
Br
N ewman projection
of the produ ct
C C
Br
Br-
Anti (coplanar) orie ntation
of added bromine atoms
C CBr
Br-
CC
Br
BrBr
Br
New man projection
of the prod uct
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Addition of Cl2 and Br2
for a cyclohexene, anti coplanar addition corresponds
to transdiaxial addition the initial transdiaxial conformation is in equilibrium
with the more stable transdiequatorial conformation
because the bromonium ion can form on either face of
the alkene with equal probability, both transenantiomers are formed as a racemic mixture
Br2
Br
Br
Br
Br
BrBr
BrBr
+
(1R,2R)-1,2-Dibromo-
cyclohexane
(1S,2S)-1,2-Dibromo-
cyclohexane
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Addition of HOCl and HOBr
Treatment of an alkene with Br2 or Cl2 in water
forms a halohydrin Halohydrin:a compound containing -OH and -X
on adjacent carbons
CH3CH=CH2 Cl2 H2O CH3CH-CH2
ClHO
HCl
1-Chloro-2-propanol(a chlorohydrin )
Propene
+++
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Addition of HOCl and HOBr
reaction is both regiospecific (OH adds to the more
substituted carbon) and anti stereoselective both selectivities are illustrated by the addition of
HOBr to 1-methylcyclopentene
to account for the regioselectivity and the antistereoselectivity, chemists propose the three-stepmechanism in the next screen
2-Bromo-1-methylcyclopentanol( a racemic mixture )
Br2 / H2OOH
1-Methylcyclopentene
+ HBr+
HBr
OH
HBr
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Addition of HOCl and HOBr
Step 1: formation of a bridged halonium ion intermediate
Step 2: attack of H2O on the more substituted carbonopens the three-membered ring
C C
Br
OH
H
HR
OH
H H H
+
C C
Br
R HH H
::
:
:
:
:::
C C
Br
R H
H HC C
Br
R H
H HC C
R H
H H - Br-
bridged bromoniumion
minor contributingstructure
BrBr :
:
:
:
:
:
:: :::
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Addition of HOCl and HOBr
Step 3: proton transfer to H2O completes the reaction
As the elpot map on the next screen shows
the C-X bond to the more substituted carbon is longer
than the one to the less substituted carbon because of this difference in bond lengths, the
transition state for ring opening can be reached moreeasily by attack of the nucleophile at the moresubstituted carbon
H3 O+
+C C
Br
OH
H
H
R
H H
+
O H
H
C C
Br
OH
H
H
R
H
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Addition of HOCl and HOBr
bridged bromonium ion from propene
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O i /R d i
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Oxymercuration/Reduction
an important feature of oxymercuration/reduction is
that it occurs without rearrangement
oxymercuration occurs with anti stereoselectivity
3,3-Dimethyl-2-butanol3,3-Dimethyl-1-butene
1. Hg(OAc)2 , H2 O
2. NaBH4OH
H H
Hg(OAc) 2
H2O
H HgOAc
OH HNaBH4
OH H
HH
(Anti addition ofOH and HgOAc)
CyclopentanolCyclopentene
O i /R d i
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Oxymercuration/Reduction
Step 1: dissociation of mercury(II) acetate
Step 2: formation of a bridged mercurinium ionintermediate; a two-atom three-center bond
O ti /R d ti
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Oxymercuration/Reduction
Step 3: regioselective attack of H2O (a nucleophile) on
the bridged intermediate opens the three-memberedring
Step 4: reduction of the C-HgOAc bond
O ti /R d ti
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Oxymercuration/Reduction
Anti stereoselective
we account for the stereoselectivity by formation ofthe bridged bromonium ion and anti attack of thenucleophile which opens the three-membered ring
Regioselective of the two carbons of the mercurinium ion
intermediate, the more substituted carbon has thegreater degree of partial positive character
alternatively, computer modeling indicates that the C-Hg bond to the more substituted carbon of the bridged
intermediate is longer than the one to the lesssubstituted carbon
therefore, the ring-opening transition state is reachedmore easily by attack at the more substituted carbon
H d b ti /O id ti
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Hydroboration/Oxidation
Hydroboration:the addition of borane, BH3, to an
alkene to form a trialkylborane
Borane dimerizes to diborane, B2H6
Borane
H B
H
H
3CH2=CH2 CH3CH2 B
CH2CH3
CH2CH3
Triethylborane(a trialkylborane)
+
Borane D iborane
2BH3 B2H6
H d b ti /O id ti
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Hydroboration/Oxidation
borane forms a stable complex with ethers such as
THF the reagent is used most often as a commercially
available solution of BH3 in THF
22
Tetrahydrofuran(THF)
-++O O BH
3
B2 H6
BH3 TH F
:::
H d b ti /O id ti
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Hydroboration/Oxidation
Hydroboration is both
regioselective (boron to the less hindered carbon) and syn stereoselective
CH3H
BH3
BR2
H H3C
H
+
1-Methylcyclopentene (Syn addition of BH3)(R = 2-methylcyclopentyl)
H d b ti /O id ti
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Hydroboration/Oxidation
concerted regioselective and syn stereoselective
addition of B and H to the carbon-carbon double bond
trialkylboranes are rarely isolated
oxidation with alkaline hydrogen peroxide gives analcohol and sodium borate
H B
CH3 CH2 CH2 CH=CH2 CH3 CH2 CH2 CH-CH2
H B
R3 B H2 O2 NaOH 3ROH Na3 BO3A trialkyl-
borane
+
An alcohol
++
H d b ti /O id ti
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Hydroboration/Oxidation
Hydrogen peroxide oxidation of a trialkylborane
step 1: hydroperoxide ion (a nucleophile) donates apair of electrons to boron (an electrophile)
step 2: rearrangement of an R group with its pair ofbonding electrons to an adjacent oxygen atom
B
R
R
R O O H B
R
R O
R
O-H-
+
B
R
R
R B
R
R
R O-O-H B
R
R
R O O HB
R
R
R O O H+
A trialkylborane
(an electroph ile)
Hydroperoxide ion(a nucleophile)
H droboration/O idation
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Hydroboration/Oxidation
step 3: reaction of the trialkylborane with aqueous
NaOH gives the alcohol and sodium borate( RO)3 B 3NaOH 3ROH + Na3 BO3
A trialky lbo rate Sodiu m borate
+
Oxidation/Reduction
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Oxidation/Reduction
Oxidation:the loss of electrons
alternatively, the loss of H, the gain of O, or both
Reduction:the gain of electrons
alternatively, the gain of H, the loss of O, or both
Recognize using a balanced half-reaction1. write a half-reaction showing one reactant and its
product(s)
2. complete a material balance; use H2O and H+ in acid
solution, use H2O and OH-
in basic solution3. complete a charge balance using electrons, e-
Oxidation/Reduction
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Oxidation/Reduction
three balanced half-reactions
CH3 CH=CH2 CH3 CHCH3+ H2 O
Propene 2-Propanol
OH
CH3 CH=CH2 CH3 CHCH2+ 2 H2 O + 2 H+ + 2 e -
Propene 1,2-Propanediol
HO OH
CH3 CH2 CH3+ 2 H+ + 2 e -
Propene
CH3 CH=CH2
Propane
Oxidation with OsO
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Oxidation with OsO4
OsO4 oxidizes an alkene to a glycol, a compound
with OH groups on adjacent carbons oxidation is syn stereoselective
OsO4
OOs
O O
O
NaHSO3
H2O
cis-1,2-Cyclopentanediol(a cis glycol)
A cyclic osmate
OH
OH
Oxidation with OsO
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Oxidation with OsO4
OsO4 is both expensive and highly toxic
it is used in catalytic amounts with another oxidizingagent to reoxidize its reduced forms and, thus, recycleOsO4
HOOH CH3 COOH
CH3
CH3
Hydrogenperoxide
t ert -Butyl hydroperoxide(t -BuOOH)
Oxidation with O
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Oxidation with O3
Treatment of an alkene with ozone followed by a
weak reducing agent cleaves the C=C and formstwo carbonyl groups in its place
Propanal(an aldehyde)
Propanone(a ketone)
2-Methyl-2-pentene
CH3 O O
CH3C=CHCH2CH31. O32. (CH
3)2S
CH3CCH3 + HCCH2CH3
Oxidation with O
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Oxidation with O3
the initial product is a molozonide which rearranges to
an isomeric ozonide
Acetaldehyde
2-Butene
O
CH3CH=CHCH3O3
(CH3)2S CH3CH
CH3CH-CHCH3
O OO
O O
CO
CH
CH3
H
H3C
A molozonide
An ozonide
Reduction of Alkenes
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Reduction of Alkenes
Most alkenes react with H2 in the presence of a
transition metal catalyst to give alkanes
commonly used catalysts are Pt, Pd, Ru, and Ni
the process is called catalytic reduction or,alternatively, catalytic hydrogenation
addition occurs with syn stereoselectivity
+ H2Pd
Cyclohexene Cyclohexane
25C, 3 atm
Reduction of Alkenes
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Reduction of Alkenes
Mechanism of catalytic hydrogenation
Reduction of Alkenes
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Reduction of Alkenes
even though addition syn stereoselectivity, some
product may appear to result from transaddition
reversal of the reaction after the addition of the firsthydrogen gives an isomeric alkene, etc.
H2/ Pt
CH3
CH3
CH3
CH3
H H
H
PtCH3
1,2-Dimethyl-cyclohexene
1,6-Dimethyl-cyclohexene
CH3
CH3
CH3
CH3
CH3
CH3
CH330% to15%70% to 85%
cis-1,2-Dimethyl-cyclohexane1,2-Dimethyl-cyclohexene
+
t rans-1,2-Dimethyl-cyclohexane(racemic)
H2 / Pt
H0 of Hydrogenation
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H0 of Hydrogenation
Reduction of an alkene to an alkane is
exothermic there is net conversion of one pi bond to one sigma
bond
H0 depends on the degree of substitution
the greater the substitution, the lower the value of H
H0 for a transalkene is lower than that of anisomeric cisalkene
a transalkene is more stable than a cisalkene
H0 of Hydrogenation
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H0 of Hydrogenation
CH2=CH2
CH3CH=CH2
Name
Structural
Formula
H
[kJ (kcal)/m ol]Ethylene
Propene
1-Butene
cis -2-Butene
t rans -2-Butene
2-Methyl-2-butene
2,3-D ime thyl-2-bute ne
-137 (-32.8)
-126 (-30.1)
-127 (-30.3)
-120 (-28.6)
-115 (-27.6)
-113 (-26.9)
-111 (-26.6)
Reaction Stereochemistry
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Reaction Stereochemistry
In several of the reactions presented in this
chapter, chiral centers are createdWhere one or more chiral centers are created, is
the product
one enantiomer and, if so, which one?
a pair of enantiomers as a racemic mixture?
a meso compound?
a mixture of stereoisomers?
As we will see, the stereochemistry of theproduct for some reactions depends on thestereochemistry of the starting material; that is,some reactions are stereospecific
Reaction Stereochemistry
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Reaction Stereochemistry
We saw in Section 6.3D that bromine adds to 2-
butene to give 2,3-dibromobutane
two stereoisomers are possible for 2-butene; a pair ofcis,transisomers
three stereoisomers are possible for the product; a
pair of enantiomers and a meso compound if we start with the cisisomer, what is the
stereochemistry of the product?
if we start with the transisomer, what is the
stereochemistry of the product?
CH3 CH=CHCH3 Br2 CH2Cl2CH3CH-CHCH3
Br Br
2,3-Dibromobutane2-Butene
+
Bromination of cis-2-Butene
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Bromination of cis-2-Butene
reaction of cis-2-butene with bromine forms bridged
bromonium ions which are meso and identical
Bromination of cis-2-Butene
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Bromination of cis-2-Butene
attack of bromide ion at carbons 2 and 3 occurs with
equal probability to give enantiomeric products as aracemic mixture
Bromination of trans-2-Butene
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Bromination of trans-2-Butene
reaction with bromine forms bridged bromonium ion
intermediates which are enantiomers
Bromination of trans-2-Butene
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Bromination of trans-2-Butene
attack of bromide ion in either carbon of either
enantiomer gives meso-2,3-dibromobutane
Bromination of 2-Butene
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Bromination of 2 Butene
Given these results, we say that addition of Br2
or Cl2 to an alkene is stereospecific bromination of cis-2-butene gives the enantiomers of
2,3-dibromobutane as a racemic mixture
bromination of trans-2-butene gives meso-2,3-
dibromobutane Stereospecific reaction: a reaction in which the
stereochemistry of the product depends on thestereochemistry of the starting material
Oxidation of 2-Butene
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Oxidation of 2 Butene
OsO4 oxidation of cis-2-butene gives meso-2,3-
butanediol
C CH
H3 C
H
CH3
OsO4
ROOH
C
HO
C
OH
HCH3
H3CH
HO
C
HH3 C
C
OH
CH3
H
cis-2-Butene(achiral)
identical;a meso
compound
(2S,3R)-2,3-Butanediol
(2R,3S)-2,3-Butanediol
2
2
2
3
3
3
Oxidation of 2-Butene
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Oxidation of 2 Butene
OsO4 oxidation of an alkene is stereospecific
oxidation of trans-2-butene gives the enantiomers of2,3-butanediol as a racemic mixture (optically inactive)
and oxidation of cis-2-butene gives meso 2,3-butanediol (also optically inactive)
C
H
CH3C
H3 C
H OsO4
ROOH
C CCH3
H
OH
H3 CH
HO
C
HH3 C
CH
CH3
OHHO
(2R,3R)-2,3-Butanediol
(2S,3S)-2,3-Butanediola pair ofenantiomers;a racemicmixture
trans-2-Butene
(achiral)
32
2
2
3
3
Reaction Stereochemistry
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Reaction Stereochemistry
We have seen two examples in which reaction of
achiral starting materials gives chiral products in each case, the product is formed as a racemic
mixture (which is optically inactive) or as a mesocompound (which is also optically inactive)
These examples illustrate a very important pointabout the creation of chiral molecules
optically active (enantiomerically pure) products cannever be produced from achiral starting materials and
achiral reagents under achiral conditions although the molecules of product may be chiral, the
product is always optically inactive (either meso or apair of enantiomers)
Reaction Stereochemistry
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Reaction Stereochemistry
Next let us consider the reaction of a chiral
starting material in an achiral environment the bromination of (R)-4-tert-butylcyclohexene
only a single diastereomer is formed
the presence of the bulky tert-butyl group controls theorientation of the two bromine atoms added to the ring
Br2
Br
Br
(1S,2S,4R)-1,2-Dibromo-4-t ert -butylcyclohexane(R)-4-t ert -Butyl-
cyclohexene
Br
Br
redraw asa chair
conformation
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Reaction Stereochemistry
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Reaction Stereochemistry
treating (R)-BINAP with ruthenium(III) chloride forms a
complex in which ruthenium is bound in the chiralenvironment of the larger BINAP molecule
this complex is soluble in CH2Cl2 and can be used as ahomogeneous hydrogenation catalyst
using (R)-BINAP-Ru as a hydrogenation catalyst, (S)-naproxen is formed in greater than 98% ee
H3 CO
COOH
CH2
H2(R)-BINAP-Ru
H3 CO
COOH+
pressure
(S)-Naproxen(ee > 98%)
CH3
(R)-BINAP RuCl3 (R)-BINAP-Ru+
Reaction Stereochemistry
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Reaction Stereochemistry
BINAP-Ru complexes are somewhat specific for the
types of C=C they reduce to be reduced, the double bond must have some kind
of a neighboring group that serves a directing group
(S)-BINAP-Ru
OHH2
(R)-BINAP-Ru
OH
OH(E)-3,7-Dimethyl-2,6-octadien-1-ol
(Geraniol)
(R)-3,7-Dimethyl-6-octen-1-ol
(S)-3,7-Dimethyl-6-octen-1-ol
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