6-1 6 organic chemistry william h. brown & christopher s. foote
TRANSCRIPT
6-1
66
Organic Organic Chemistry Chemistry
William H. Brown &William H. Brown &
Christopher S. FooteChristopher S. Foote
6-3
66 Characteristic ReactionsCharacteristic Reactions
H2O+
Hydration
+
Hydrochlorination
CC C C
Cl
H
HCl
CC C C
OH
HH+
Br2+
Bromohydrin formation
+
Bromination
CC C C
BrH
Br
Br2
CC C C
OH
BrH2O
6-4
66 Characteristic ReactionsCharacteristic Reactions
+
Oxymercuration
CC Hg(OAc)2 C C
HgOAcH
OHH2O
OsO4+
Diol formation (oxidation)
+
Hydrogenation (reduction)
CC H2
+
Hydroboration
CC C C
BH2H
BH3
CC C C
OHOH
C C
HH
6-5
66 Reaction MechanismsReaction Mechanisms A reaction mechanism describes details of how a
reaction occurs• which bonds are broken and which new ones are
formed• the order and relative rates of the various bond-
breaking and bond-forming steps• if in solution, the role of the solvent• if there is a catalyst, the role of a catalyst• the position of all atoms and energy of the entire
system during the reaction
6-6
66 Energy DiagramsEnergy Diagrams Energy diagram:Energy diagram: a graph
showing the changes in energy that occur during a chemical reaction
Reaction coordinate:Reaction coordinate: a measure in the change in positions of atoms during a reaction
Reactioncoordinate
Energy
6-7
66 Gibbs Free EnergyGibbs Free Energy Gibbs free energy:Gibbs free energy: a thermodynamic function
relating enthalpy, entropy, and temperature
• exergonic reaction:exergonic reaction: a reaction in which the Gibbs free energy of the products is lower than that of the reactants; an exergonic reaction is spontaneous
• endergonic reaction:endergonic reaction: a reaction in which the Gibbs free energy of the products is higher than that of the reactants; an endergonic reaction is never spontaneous.
ΔG° = ΔH° –TΔS°
6-8
66 Gibbs Free EnergyGibbs Free Energy
ΔH° > 0
ΔH° < 0
Reaction is neverspontaneous
Reaction is spontaneousat higher temperaturewhere TΔS° > ΔH° and, therefore, ΔG° < 0
Reaction is spontaneousat lower temperatureswhere TΔS° < ΔH° and,therefore, ΔG° < 0
Reaction is alwaysspontaneous
ΔS° < 0 ΔS° > 0
ΔG° > 0
ΔG° < 0
6-9
66 Energy DiagramsEnergy Diagrams Heat of reaction:Heat of reaction: the difference in energy
between reactants and products• exothermic reaction:exothermic reaction: a reaction in which the enthalpy
of the products is lower than that of the reactants; a reaction in which heat is released
• endothermic reactionendothermic reaction: a reaction in which the enthalpy of the products is higher than that of the reactants; a reaction in which heat is absorbed
6-10
66 Activation EnergyActivation Energy Transition state:Transition state: • an unstable species of maximum energy formed
during the course of a reaction• a maximum on an energy diagram
Activation Energy, Activation Energy, ΔΔGG‡‡:: the difference in energy between reactants and a transition state• if large, only a few collisions occur with sufficient
energy to reach the transition state; reaction is slow• if small, many collisions occur with sufficient energy
to reach the transition state; reaction is fast
6-11
66 Energy DiagramEnergy Diagram
QuickTime™ and aPhoto - JPEG decompressor
are needed to see this picture.
A one-step reaction with no intermediate
6-12
66 Energy DiagramEnergy Diagram
QuickTime™ and aPhoto - JPEG decompressor
are needed to see this picture.
A two-step reaction with one intermediate
6-13
66 Activation Energy & RateActivation Energy & Rate The relationship between a rate constant, k,
and activation energy is given by the equation
C = a constant (units s-1) that depends on the reaction
R = gas constant, 8.315 x 10-3 kJ (1.987 x 10-3 kcal)•mol-1•K -1
T = temperature in Kelvins
k = C x e-ΔG‡/RT
6-14
66 Activation Energy & RateActivation Energy & RateExample:Example: What is the activation energy for a reaction
whose rate at 35°C is twice that at 25°C?
Solution:Solution:
the ratio of rate constants k2 and k1 for the reaction at temperatures T2 and T1 is
taking the log of both sides and rearranging gives
C x e-ΔG‡/RT 2k2
k1=
C x e-ΔG‡/RT1
log =2.303R
1 - 1k2
k1 T1 T2
ΔG‡
6-15
66 Activation Energy & RateActivation Energy & Rate• substituting values and solving gives
log =2.303 x 8.315 x10-3 kJ•mol-1•K-1
21 298K 308K
1 1-
ΔG‡ = 52.7 (12.6 )/kJ kcal mol
ΔG‡
6-16
66 Developing a MechanismDeveloping a Mechanism How it is done
• design experiments to reveal details of a particular chemical reaction
• propose a set or sets of steps that might account for the overall transformation
• 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
6-17
66 Why Mechanisms?Why Mechanisms?• framework within which to organize descriptive
chemistry• intellectual satisfaction derived from constructing
models that accurately reflect the behavior of chemical systems
• a tool with which to search for new information and new understanding
6-18
66 Electrophilic AdditionsElectrophilic Additions• hydrohalogenation using HCl, HBr, HI
• hydration using H2O, H2SO4
• halogenation using Cl2, Br2
• halohydrination using HOCl, HOBr
• oxymercuration using Hg(OAc)2, H2O
6-19
66 Addition of HXAddition of HX Carried out with pure reagents or in a polar
solvent such as acetic acid Addition is regioselective • regioselective reaction:regioselective reaction: a reaction in which one
direction of bond forming or breaking occurs in preference to all other directions of bond forming or breaking
• regiospecific reaction:regiospecific reaction: a reaction in which one direction of bond forming or breaking occurs to the exclusion of all other directions of bond forming or breaking
6-20
66 Addition of HXAddition of HX H adds to the less substituted carbon
Markovnikov’s rule:Markovnikov’s rule: in the addition of HX, H2O, or ROH to an alkene, H adds to the carbon of the double bond having the greater number of hydrogens
1-Chloropropane (not observed)
2-ChloropropanePropene
++CH3CH=CH2 HCl CH3CH-CH2
H
CH3CH-CH2
H ClCl
6-21
66 HCl + 2-ButeneHCl + 2-Butene A two-step mechanism
Step 1: formation of sec-butyl cation, a 2° carbocation intermediate
Step 2: reaction of the sec-butyl cation (a Lewis acid) with chloride ion (a Lewis base) completes the reaction
-++
δ-δ+H-ClCH3 CH=CHCH3 Cl
sec-Butyl cation(a 2° carbocation
intermediate
slow, ratedetermining +
H
CH3CH-CHCH3
::
:: :::
+
sec-Butyl cation (a Lewis acid)
+
Cl
CH3CHCH2CH3CH3CHCH2CH3
Chloride ion(a Lewis base)
fast
2-Chlorobutane
-Cl:: ::
:::
6-22
66 HCl + 2-ButeneHCl + 2-Butene
QuickTime™ and aPhoto - JPEG decompressor
are needed to see this picture.
6-23
66 CarbocationsCarbocations Carbocation:Carbocation: a species in which a carbon atom has only
six electrons in its valence shell and bears positive charge
Carbocations are• classified as 1°, 2°, or 3° depending on the number of
carbons bonded to the carbon bearing the positive charge
• electrophiles; that is, they are electron-loving • Lewis acids
6-24
66 Carbocation StructureCarbocation Structure• bond angles about a positively
charged carbon are approx. 120°• carbon uses sp2 hybrid orbitals to
form sigma bonds to the three attached groups
• the unhybridized 2p orbital lies perpendicular to the sigma bond framework and contains no electrons
R CR
R+
6-25
66 Carbocation StabilityCarbocation 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, and requires a lower activation energy for its formation
• methyl and primary carbocations are so unstable that they are never observed in solution
6-26
66 Carbocation StabilityCarbocation Stability• relative stability
• methyl and primary carbocations are so unstable that they are never observed in solution
Methyl cation
(methyl)
Ethyl cation(1°)
Isopropyl cation
(2°)
tert-Butyl cation(3°)
Increasing carbocation stability
+ + + +C
H
H
CH3 CCH3
CH3
H
C
CH3
CH3
CH3CH
H
H
6-27
66 Carbocation StabilityCarbocation Stability• we can account for the relative stability of
carbocations if we assume that alkyl groups attached to the positively charged carbon are electron-releasing and thereby help delocalize the positive charge of the cation
• we account for this electron-releasing ability of alkyl groups by (1) the inductive effect, and (2) hyperconjugation
6-28
66 Carbocation StabilityCarbocation Stability The inductive effect• the electron-deficient carbon bearing
the positive charge polarizes electrons of the adjacent sigma bonds toward it
• the positive charge on the cation is not localized on the trivalent carbon, but delocalized over nearby atoms
• the larger the volume over which the positive charge is delocalized, the greater the stability of the cation
H3C
C CH3
H3C
δ+
δ+
δ+δ+
6-29
66 Carbocation StabilityCarbocation Stability Hyperconjugation• partial overlap of the bonding
orbital of an adjacent C-H bond with the vacant 2p orbital of the cationic carbon delocalizes the positive charge and also the electrons of the adjacent bond
• replacing a C-H bond with a C-C bond increases the possibility for hyperconjugation
6-30
66 Addition of HAddition of H22OO• addition of water is called hydration• acid-catalyzed hydration of an alkene is regioselective;
hydrogen adds preferentially to the less substituted carbon of the double bond
CH3CH=CH2 H2 OH2 SO4 CH3CH-CH2
HOH
Propene 2-Propanol
+
2-Methyl-2-propanol2-Methylpropene
+CH3C=CH2 H2 OH2 SO4 CH3C-CH2
CH3
HO
CH3
H
6-31
66 Addition of HAddition of H22OO• Step 1: proton transfer from solvent to the alkene
• Step 2: a Lewis acid/base reaction
• Step 3: proton transfer to solvent
++
+
intermediateA 2o carbocation
+HO
H
HOH
H
CH3CH=CH2 CH3CHCH3
slow, ratedetermining ::
:
::
+
+
+
An oxonium ion
H OHH
CH3CHCH3 O-H CH3CHCH3fast
:
::
++
+OH HO
H
HH
H O HCH3CHCH3 CH3CHCH3fast
OH:
:
::
6-32
66 CC++ Rearrangements Rearrangements In electrophilic addition to alkenes, there is the
possibility for rearrangement Rearrangement:Rearrangement: a change in connectivity of the
atoms in a product compared with the connectivity of the same atoms in the starting material
6-33
66 CC++ Rearrangements Rearrangements• in addition of HCl to an alkene
• in acid-catalyzed hydration of an alkene
+ +
3-Methyl-1-butene
2-Chloro-3-methylbutane
(expected)(40%)
CH3 CH3 CH3
ClCl
CH3CHCH=CH2 HCl CH3CHCHCH3 CH3CCH2 CH3
2-Chloro-2-methylbutane
(rearrangement)(60%)
+
3-Methyl-1-butene 2-Methyl-2-butanol
CH3 CH3
CH3CHCH=CH2 H2 O CH3CCH2 CH3H2SO4
OH
6-34
66 CC++ Rearrangements Rearrangements• driving force is rearrangement of a less stable
carbocation to a more stable one
• the less stable 2° carbocation rearranges to a more stable 3° one by 1,2-shift of a hydride ion
+
A 2° carbocation intermediate
3-Methyl-1-butene
++
CH3
HClH
CH3
CH3CCH=CH2 CH3C-CHCH3
slow, ratedetermining
H::
:-
Cl:: ::
++
A 3° carbocation
CH3
H
CH3
H
CH3C-CHCH3 CH3C-CHCH3fast
6-35
66 CC++ Rearrangements Rearrangements• reaction of the more stable carbocation (a Lewis acid)
with chloride ion (a Lewis base) completes the reaction
-Cl:: ::
2-Chloro-2-methylbutane
++
CH3 CH3
CH3C-CH2CH3 CH3C-CH2CH3fast
Cl: ::
6-36
66 Addition of ClAddition of Cl22 and Br and Br22• carried out with either the pure reagents or in an inert
solvent such as CH2Cl2
• addition is stereoselective
Stereoselective reaction:Stereoselective reaction: a reaction in which one stereoisomer is formed or destroyed in preference to all others
Stereospecific reactionStereospecific reaction: a reaction in which one stereoisomer is formed or destroyed to the exclusion to all others
6-37
66 Addition of ClAddition of Cl22 and Br and Br22
2,3-Dibromobutane2-Butene
+ Br2CH3CH=CHCH3 CH3CH-CHCH3CH2Cl2
trans-1,2-Dibromo-cyclohexane
Cyclohexene
+ Br2
Br
Br
CH2Cl2
Br Br
6-38
66 Addition of ClAddition of Cl22 and Br and Br22• Step 1: formation of a bridged bromonium ion
intermediate
• Step 2: attack of halide ion from the opposite side of the three-membered ring
Anti (coplanar) orientationof added bromine atoms
C C
Br
Br
+ C C
Br ::-
Br:: ::
:::
: ::
C C
Br
C C
Br
Br
+-
Br:: ::
::
:
:
:
:
:
:
6-39
66 Addition of ClAddition of Cl22 and Br and Br22 For a cyclohexene, anti coplanar addition
corresponds to trans diaxial addition
+
trans diaxial(less stable)
trans diequatorial(more stable)
Br
Br
BrBr
Br2
6-40
66 Addition of HOCl and HOBrAddition of HOCl and HOBr Treatment of an alkene with Br2 or Cl2 in water
forms a halohydrin Halohydrin:Halohydrin: a compound containing -OH and -X
on adjacent carbons
1-Chloro-2-propanol (a chlorohydrin)
Propene
+
HO Cl
CH3CH=CH2H2 O
HClCH3CH-CH2Cl2
6-41
66 Addition of HOCl and HOBrAddition of HOCl and HOBr• reaction is both regiospecific (anti addition) and
stereoselective (OH to the more substituted carbon)
+
2-Bromo-1-methylcyclopentanol (anti addition of -OH and -Br)
1-Methylcyclopentene
H3CBr
CH3HOHH
Br2 HBrH2 O
6-42
66 Addition of HOCl and HOBrAddition of HOCl and HOBrStep 1: formation of a bridged halonium ion intermediate
Step 2: attack of H2O on the more substituted carbon opens the three-membered ring
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
::
:
::
:::
6-43
66 Oxymercuration/ReductionOxymercuration/Reduction• oxymercuration followed by reduction results in
hydration of a carbon-carbon double bond
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
6-44
66 Oxymercuration/ReductionOxymercuration/Reduction• addition of Hg(II) and oxygen is anti coplanar
stereoselective
(Anti addition of-OH and -HgOAc)
CyclopenteneH HgOAc
H H
OH HHg(OAc)2
H2 O
6-45
66 Oxymercuration/ReductionOxymercuration/Reduction• Step 1: dissociation of mercury (II) acetate gives
AcOHg+, a Lewis acid
• Step 2: attack of AcOHg+ on the double bond gives a bridged mercurinium ion intermediate in which the two electrons of the pi bond form a two-atom three-center bond
+AcO-Hg-OAc AcO-Hg+
AcO-
6-46
66 Oxymercuration/ReductionOxymercuration/Reduction
HC C
Hg
OAc
H
C C
Hg
OAc
H
HR
HR H
HR
C C
Hg
OAc
HH H
R
CC
Hg
OAc
HH
A bridged mercurinium ion intermediate
(the major contributor)
+
+
An open carbocationintermediate
(a minor contributor)
+
An open carbocationintermediate
(a negligible contributor)
+
6-47
66 Oxymercuration/ReductionOxymercuration/Reduction• Step 3: stereospecific and regioselective attack of H2O
on the bridged intermediate opens the mercurinium ion ring
• Step 4: reduction of the C-HgOAc bond
C C
Hg
OAc
HHR H
OH
H
H
H
OH
C CR
HH
HgOAc
Anti stereospecific additionof HgOAc and HOH
+
+
: ::
C CR
HgOAc
HOH
H
H
NaBH4C C
RH
HO HH
H
+ + Hg0
6-48
66 Oxymercuration/ReductionOxymercuration/Reduction• the fact that oxymercuration occurs without
rearrangement indicates that the intermediate is not a true carbocation, but rather a resonance hybrid closely resembling a bridged mercurinium ion intermediate
• regioselectivity is accounted for by at least some carbocation character in the bridged intermediate
• stereospecificity is accounted for by anti attack on the bridged intermediate
6-49
66 Hydroboration/OxidationHydroboration/Oxidation Hydroboration: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 Diborane
2BH3 B2H6
6-50
66 Hydroboration/OxidationHydroboration/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 BH3B2H6
BH3•THF
:::
6-51
66 Hydroboration/OxidationHydroboration/Oxidation Hydroboration is both regioselective (boron to
the less hindered carbon) and stereospecific (syn addition)
CH3H
BH3
BR2
H H3C
H
+
1-Methylcyclopentene (Syn addition of BH3)(R = 2-methylcyclopentyl)
6-52
66 Hydroboration/OxidationHydroboration/Oxidation• mechanism involves concerted regioselective and
stereospecific addition of B and H to the carbon-carbon double bond
δ− δ+H B
CH3CH2CH2CH=CH2 CH3CH2CH2CH-CH2
H B
6-53
66 Hydroboration/OxidationHydroboration/Oxidation• trialkylboranes are rarely isolated• oxidation with alkaline hydrogen peroxide gives an
alcohol and sodium borate
The result of hydroboration/oxidation is regioselective and stereospecific hydration of a carbon-carbon double bond
(RO)3B 3ROH + Na3BO3
A trialkyl-borate
+ 3NaOH
An alcohol
6-54
66 Hydroboration/OxidationHydroboration/Oxidation
A trialkylborane(R = 2-methylcyclopentyl)
1-Methylcyclopentene
trans-2-Methylcyclopentanol
H3CH
H
CH3H
BR R
OH H
H H3CH2 O2NaOH
BH3
6-55
66 Oxidation/ReductionOxidation/Reduction Oxidation:Oxidation: the loss of electrons• or the loss of H, the gain of O, or both
Reduction:Reduction: the gain of electrons• or 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 solution
3. complete a charge balance using electrons, e-
6-56
66 Oxidation/ReductionOxidation/Reduction• three balanced half-reactions
CH3CH=CH2 CH3CHCH3+ H2O
Propene 2-Propanol
OH
CH3CH=CH2 CH3CHCH2+ 2H2O + 2H+ + 2e-
Propene 1,2-Propanediol
HO OH
CH3CH2CH3+ 2H+ + 2e-
Propene
CH3CH=CH2
Propane
6-57
66 Oxidation with OsOOxidation with OsO44 Oxidation by OsO4 converts an alkene to a glycol,
a compound with -OH groups on adjacent carbons• oxidation is syn stereospecific
OsO4 NaHSO3
H2OH
O
H
O
H
OH
H
OHOs
O Ocis-1,2-Cyclopentanediol (a cis glycol)
A cyclic osmic ester
6-58
66 Oxidation with OsOOxidation with OsO44• OsO4 is both expensive and highly toxic
• it is used in catalytic amounts with another oxidizing agent to reoxidize its reduced forms and, thus, recycle OsO4
Hydrogenperoxide
tert-Butyl hydroperoxide (t-BuOOH)
CH3
CH3
HOOH CH3COOH
6-59
66 Oxidation with OOxidation with O33 Treatment of an alkene with ozone followed by a
weak reducing agent cleaves the C=C and forms two carbonyl groups in its place
Propanal(an aldehyde)
Propanone(a ketone)
2-Methyl-2-pentene
CH3 O O
CH3C=CHCH2 CH31. O32. (CH3)2S
CH3CCH3 + HCCH2CH3
6-60
66 Oxidation with OOxidation with O33 • the initial product is a molozinide which rearranges to
an isomeric ozonide
Acetaldehyde
2-Butene
O
CH3CH=CHCH3O3
(CH3 )2S CH3CH
CH3CH-CHCH3
O OO
O OC
OC
H
CH3
H
H3C
A molozonide
An ozonide
6-61
66 Reduction of AlkenesReduction 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
+ H2Pd
Cyclohexene Cyclohexane
25°C, 3 atm
6-62
66 Reduction of AlkenesReduction of Alkenes Most common pattern is syn stereoselectivity
30% to15%70% to 85%cis-1,2-Dimethyl-
cyclohexane
1,2-Dimethyl-cyclohexene
++
CH3
CH3
CH3
CH3
CH3
CH3
H2Pt
trans-1,2-Dimethyl-cyclohexane
6-63
66 Reduction of AlkenesReduction of Alkenes Mechanism of catalytic hydrogenation• H2 is absorbed on the metal surface with formation of
metal-hydrogen bonds• the alkene is also absorbed with formation of metal-
carbon bonds• a hydrogen atom is transferred to the alkene forming
one new C-H bond• a second hydrogen atom is transferred forming the
second C-H bond
6-64
66 ΔΔH° of HydrogenationH° of Hydrogenation Reduction of an alkene to an alkane is
exothermic• there is net conversion of one pi bond to one sigma
bond
ΔH° depends on the degree of substitution• the greater the substitution, the lower the value of ΔH°
ΔH° for a trans alkene is lower than that of an isomeric cis alkene
6-65
66 ΔΔH° of HydrogenationH° of Hydrogenation
CH2=CH2
CH3CH=CH2
CH3CH2 CH=CH2
NameStructural Formula
Δ °H ( )/kJ kcal mol
Ethylene
Propene
1-Butene
cis-2-Butene
trans-2-Butene
2- -2-Methyl butene
2,3- -2-Dimethyl butene
-137 (-32.8 )
-126 (-30.1 )
-127 (-30.3 )
-120 (-28.6
-115 (-27.6 )
-113 (-26.9 )
-111 (-26.6 )
CH3 CH=CHCH3
CH3 CH=CHCH3
(CH3 ) 2 C=CHCH3
(CH3 ) 2 C=C( CH3 ) 2
6-66
66 ΔΔH° of Hydrogenation H° of Hydrogenation The greater the degree of substitution of a
double bond, the lower its heat of hydrogenation• the greater the degree of substitution, the more stable
the double bond
The heat of hydrogenation of a trans alkene is lower than that of the isomeric cis alkene• a trans alkene is more stable than its isomeric cis
alkene• the difference is due to nonbonded interaction strain
in the cis alkene
6-67
66 ΔΔH° of HydrogenationH° of Hydrogenation
cis-2-Butene(less stable)
trans-2-Butene(more stable)
6-68
66 Reaction StereochemistryReaction Stereochemistry In several of the reactions presented in this
chapter, stereocenters are created Where one or more stereocenters are created, is
the product• one enantiomer and, if so, which one?• a pair of enantiomers?• a meso compound?• a mixture of stereoisomers?• or what?
6-69
66 Reaction StereochemistryReaction Stereochemistry Which of the three possible stereoisomers of 2,3-
dibromobutane are formed in the addition of bromine to trans-2-butene?
• the three possible stereoisomers for this product are a pair of enantiomers and a meso compound
CH3-CH-CH-CH3Br2
Br Br
CCl4
H
C C
CH3
H3C H
6-70
66 Reaction StereochemistryReaction Stereochemistry Reaction of bromine with the alkene forms a
cyclic bromonium ion intermediate
• which is then opened by attack of bromide ion from the side opposite the bromine bridge
trans-2-Butene (achiral)
CH
CH3C
H3C
H Br2C
HCH3
C
H3CH
Br+
6-71
66 Reaction StereochemistryReaction Stereochemistry
C
HCH3
C
H3CH
Br+
Br-
2 3
Br-
identical;a meso compound
(2S,3R)-2,3-Dibromo-butane
(2R,3S)-2,3-Dibromo-butane
2
2
3
3C
H3C
H
CH3C
C
Br
CH3H
Br
C
Br
H
Br
HCH3
6-72
66 Reaction StereochemistryReaction Stereochemistry How many and what kind of stereoisomers are
formed in the oxidation of cis-2-butene by OsO4?
cis-2-Butene (achiral)
C CH
H3C
H
CH3
OsO4
ROOHCH3-CH-CH-CH3
three stereoisomers arepossible for 2,3-butanediol;
a meso compound and apair of enantiomers
OH OH
6-73
66 Reaction StereochemistryReaction Stereochemistry
cis-2-Butene (achiral)
C CH
H3C
H
CH3
OsO4
ROOH
identical;a meso compound
(2S,3R)-2,3-Butanediol
(2R,3S)-2,3-Butanediol
2
2
3
3C
HO
HO
CH
C
OH
HCH3
H3C
C
OH
H3CH
CH3
H
6-74
66 Reaction StereochemistryReaction Stereochemistry How many and what kind of stereoisomers are
formed in the oxidation of trans-2-butene by OsO4?
(2R,3R)-2,3-Butanediol
(2S,3S)-2,3-Butanediola pair ofenantiomers;a racemicmixture
2
2
3
3
C CCH3
H
C
HH3C
OH
H3C
CH
CH3
OH
H
HO
HO
trans-2-Butene (achiral)
32
CH
CH3C
H3C
H OsO4
ROOH
6-75
66 Reaction StereochemistryReaction Stereochemistry Enantiomerically pure products can never be
formed from achiral starting materials and reagents
An enantiomerically pure product can be generated in a reaction if at least one of the reactants is enantiomerically pure, or if the reaction is carried out in an achiral environment
6-76
66 Prob 6.15Prob 6.15 Draw the isomeric carbocations formed on
treatment of each alkene with HCl. Which is the more stable?
(d)(c)
(b)(a)
CH3CH2
CH3CH2 CH=CHCH3CH3CH2 C=CHCH3
CH3
6-77
66 Prob 6.16Prob 6.16 Arrange the alkenes in each set in order of increasing
rate of reaction with HI.
(a)
(b)
CH3CH=CHCH3 CH3C=CHCH3and
and
CH3
6-78
66 Prob 6.17Prob 6.17 Write the major product formed on treatment of 2-butene
with each reagent.
(a) (b) (c)
(d) (e) (f)
(g) (h)
H2O (H2SO4) Br2 Cl2
Br2 in H2O HI Cl2 in H2O
prdt (g) + NaBH4Hg(OAc)2, H2O
6-79
66 Prob 6.18Prob 6.18 What alkene undergoes acid-catalyzed hydration to give
each alcohol as the major product?
(a) (b)
(c) (d)
3-hexanol 2-methylcyclohexanol
2-methylbutan ol 2-propanol
6-80
66 Prob 6.19Prob 6.19 Reaction of 2-methyl-2-pentene with each reagent is
regiospecific. What is the regiospecificity and how it is accounted for?(a) (b)
(c) (d)
(e)
HI HBr
H2O, H2SO4 Br2 in H2O
Hg(OAc)2 in H2O
6-81
66 Prob 6.21Prob 6.21 Draw the alkene of indicated molecular formula that gives
the compound shown as the major product.
+
+H2 SO4
C5H1 0 Br2
C5H1 0 H2 OOH
BrBr
(a)
(b)
(c) +CH3
ClC7H1 2 HCl
6-82
66 Prob 6.22Prob 6.22 Account for the fact that addition of HCl to 1-
bromopropene gives 1-bromo-1-chloropropane.
+CH3CH=CHBr HCl CH3CH2CHBrCl
1-Bromopropene 1-Bromo-1-chloropropane
6-83
66 Prob 6.23Prob 6.23 Propenoic acid reacts with HCl to give 3-chloropropanoic
acid. Account for this result.
+ HClCH2=CHCOH ClCH2CH2COH
CH3CHCOH
Propenoic acid)(Acrylic acid)
3-Chloropropanoic acid
2-Chloropropanoic acid(this product is not formed)
O O
OCl
6-84
66 Prob 6.24Prob 6.24 Draw a structural formula for the alkene of molecular
formula C5H10 that reacts with Br2 to give each product.
(b)(a) (c)BrBr
Br
Br
BrBr
6-85
66 Prob 6.26Prob 6.26 Draw a structural formula of the cycloalkene of molecular
formula C6H10 that reacts with Cl2 to give each compound.
(b)(a)
(c)
Cl
Cl
Cl
ClCH3
H3C ClCl
CH2Cl
Cl(d)
(b)(a)
(c)
Cl
Cl
Cl
ClCH3
H3C ClCl
CH2Cl
Cl(d)
6-86
66 Prob 6.27Prob 6.27 Treatment of this bicycloalkene with Br2 gives a trans
dibromide. Of the two possible trans dibromides, only one is formed. Which is formed? Account for its formation to the exclusion of its isomer.
+ or
(a) (b)H
CH3CH3
H
Br
Br Br
BrCH3
H
Br2CCl4
6-87
66 Prob 6.28Prob 6.28 Propose a structural formula for terpin. How many
cis,trans isomers are possible for the structural formula you have proposed?
Terpin
Limonene
H2 SO42H2 O C10H20O2+
6-88
66 Prob 6.29Prob 6.29 Propose a mechanism for this reaction.
CH3-C=CH2 ICl+ CH3C-CH2I
CH3
Cl
CH3
6-89
66 Prob 6.30Prob 6.30 Propose a mechanism for this reaction.
CH3C=CH2
CH3
CH3OH+H2SO4
CH3C-OCH3
CH3
CH3
6-90
66 Prob 6.31Prob 6.31 Propose a mechanism for the formation of each product.
CH3OHCH3CH=CHCH2CH3
Cl2
CH3CHCHCH2CH3
Cl OCH3
CH3CHCHCH2CH3
H3CO Cl
CH3CHCHCH2CH3
Cl Cl
50% 35% 15%
++
6-91
66 Prob 6.32Prob 6.32 Propose a mechanism for the formation of each product.
Cyclohexyl acetate (15%)
Bromocyclohexane (85%)
Cyclohexene
+
+
OCCH3Br
HBrCH3COH
O
O
6-92
66 Prob 6.33Prob 6.33 Propose a mechanism for this reaction.
+ +
+
1-Pentene
1-Bromo-2-pentanol
Br2 H2 O
HBrBrOH
6-93
66 Prob 6.34Prob 6.34 Propose a mechanism for this reaction.
4-Penten-1-ol+ +
O CH2BrBr2 HBr
OH
6-94
66 Prob 6.35Prob 6.35 Propose a mechanism for each reaction.
(a) OH
O CH3
H2 SO4
H2 O
(b) H
COOH O C
O
Br
H HH
HH
Br2
NaOH+ NaBr + H2O
6-95
66 Prob 6.36Prob 6.36 Propose a mechanism for this reaction.
1-Chloro-1,2-dimethyl- cyclohexane
+ HCl
Cl
1-Methyl-1-vinyl- cyclopentane
6-96
66 Prob 6.37Prob 6.37 Draw a structural formula for the alcohol formed by
treatment of each alkene with B2H6 in THF followed by treatment with alkaline H2O2.
(c) (d)
(e)
(a) (b)CH2 CH3
CH3C=CHCH2 CH3 CH2=CH(CH2)5CH3
(CH3 )3CCH=CH2
CH3
6-97
66 Prob 6.38Prob 6.38 Of the four possible cis,trans isomers possible for this
compound, one is formed in 85% yield. Propose a structure for this isomer.
α-Pinene
OH1. BH3
2. H2O2, NaOH
6-98
66 Prob 6.41Prob 6.41 Draw a structural formula of the alkene that gives each
set of products.
(a)C7H1 21. O3
2. (CH3)2S
O O
(b)C10H181. O3
2. (CH3)2S H H
O O O O+ +
(c)C10H181. O3
2. (CH3)2S
OH
O
6-99
66 Prob 6.47Prob 6.47 State the number and kind of stereoisomers formed when
(R)-3-methyl-1-pentene is treated with each reagent.
(R)-3-Methyl-1-pentene
CH3H
(d)
(b) H2 / Pt(a) Hg(OAc)2, H2O followed by NaBH4
Br2 in CCl4(c) BH3 followed by H2O2 in NaOH
6-100
66 Prob 6.49Prob 6.49 For each reaction determine (1) how many stereoisomers
are possible for the product, (2) which of the possible ones are formed, and (3) whether the product is optically active or inactive.
(a)2. NaBH4
1. Hg(OAc)2, H2 OOH
+(b) Br2 CCl4 Br
Br
+(c) Br2 CCl4 Br
Br
6-101
66 Prob 6.49 (cont’d)Prob 6.49 (cont’d)+(d) HCl
Cl
+(e)OH
Cl
Cl2 in H2O
(f)
OH
OH
OsO4
ROOH
(g)
CH3
OH
CH3 2. H2O2, NaOH
1. BH3
+(h)CH3
CH3
BrHBr