6
Organic
Chemistry
William H. Brown &
Christopher S. Foote
6-1
6
Alkenes II
Chapter 6
6-2
6 Characteristic Reactions
Hydrochlorination
C
C
H
+ HCl
C
C
Cl
Hydration
C
C
+
H2 O
H
+
H
C
C
OH
Bromination
C
C
Br
+ Br2
C
C
H
Br
Bromohydrin formation
Br
C
C
+
Br2
H2 O
C
C
OH
6-3
6 Characteristic Reactions
Oxymercuration
C
C
+ Hg ( OAc ) 2
OH
H2 O
C
C
H
Hg OA c
Hydroboration
C
C
+ BH3
C
C
H
BH2
C
C
Diol formation (oxidation)
C
C
+
OsO 4
OH OH
Hydrogenation (reduction)
C
C
+ H2
C
C
H
H
6-4
6 Reaction 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 bondbreaking 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
6-5
6 Energy Diagrams
diagram: a graph
showing the changes in energy
that occur during a chemical
reaction
 Reaction coordinate: a
measure in the change in
positions of atoms during a
reaction
Energy
 Energy
Reaction
coordinate
6-6
6 Gibbs Free Energy
 Gibbs
free energy: a thermodynamic function
relating enthalpy, entropy, and temperature
G° = H° –TS°
• 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: a reaction in which the Gibbs free
energy of the products is higher than that of the
reactants; an endergonic reaction is never
spontaneous.
6-7
6 Gibbs Free Energy
S° < 0
H° > 0
G° > 0
Reaction is never
spontaneous
S° > 0
Reaction is spontaneous
at higher temperature
where TS° > H° and,
therefore,G° < 0
Reaction is spontaneous G° < 0
at lower temperatures
H° < 0 where TS° < H° and,
Reaction is always
spontaneous
therefore,G° < 0
6-8
6 Energy Diagrams
 Heat
of reaction: the difference in energy
between reactants and products
• 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 reaction: a reaction in which the enthalpy
of the products is higher than that of the reactants; a
reaction in which heat is absorbed
6-9
6 Activation Energy
 Transition
state:
• an unstable species of maximum energy formed
during the course of a reaction
• a maximum on an energy diagram
Energy, G‡: the difference in energy
between reactants and a transition state
 Activation
• 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
6-10
6 Energy Diagram
A one-step reaction with no intermediate
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6-11
6 Energy Diagram
A two-step reaction with one intermediate
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6-12
6 Activation Energy & Rate
 The
relationship between a rate constant, k,
and activation energy is given by the equation
‡
-G
/RT
k= Cxe
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
6-13
6 Activation Energy & Rate
Example: What is the activation energy for a reaction
whose rate at 35°C is twice that at 25°C?
Solution:
the ratio of rate constants k2 and k1 for the reaction at
temperatures T2 and T1 is
k2
k1
=
‡
-G
/RT2
Cxe
‡
-G
/RT1
Cxe
taking the log of both sides and rearranging gives
log
k2
k1
‡
G
=
2.303R
1 - 1
T1 T2
6-14
6 Activation Energy & Rate
• substituting values and solving gives
‡
2
G
log
=
1
2.303 x 8.315 x10-3 kJ•mol-1•K -1
1
1
298K 308K
G‡ = 52.7 kJ (12.6 kcal)/mol
6-15
6 Developing 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
6-16
6 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
6-17
6 Electrophilic Additions
•
•
•
•
•
hydrohalogenation using HCl, HBr, HI
hydration using H2O, H2SO4
halogenation using Cl2, Br2
halohydrination using HOCl, HOBr
oxymercuration using Hg(OAc)2, H2O
6-18
6 Addition of HX
 Carried
out with pure reagents or in a polar
solvent such as acetic acid
 Addition is regioselective
• 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: a reaction in which one
direction of bond forming or breaking occurs to the
exclusion of all other directions of bond forming or
breaking
6-19
6 Addition of HX
H
adds to the less substituted carbon
Cl H
CH3 CH= CH2 + HCl
Propene
H
Cl
CH3 CH- CH 2 + CH3 CH- CH 2
2-Chloropropane 1-Chloropropane
(not observed)
 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
6-20
6 HCl + 2-Butene
 A two-step mechanism
Step 1: formation of sec-butyl cation, a 2° carbocation intermediate
:
:
H
+
:
CH3 CH- CHCH3 + Cl :
:
:
slow, rate
+ CH3 CH= CH CH 3 + H- Cl : determining
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
: Cl :
+
: Cl :
+ CH 3 CHCH 2 CH 3 fast
CH3 CHCH 2 CH3
:
:
Chloride ion
(a Lewis base)
sec-Butyl cation
(a Lewis acid)
2-Chlorobutane
6-21
6 HCl + 2-Butene
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6-22
6 Carbocations


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
6-23
6 Carbocation 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
+
C
R
R
6-24
6 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, and requires a lower activation energy for
its formation
• methyl and primary carbocations are so unstable that
they are never observed in solution
6-25
6 Carbocation Stability
• relative stability
H
H
C+
H
Methyl
cation
(methyl)
CH3
H
C+
H
Ethyl
cation
(1°)
CH3
CH3
C+
H
Isopropyl
cation
(2°)
CH3
CH3
C+
CH3
tert-Butyl
cation
(3°)
Increasing carbocation stability
• methyl and primary carbocations are so unstable that
they are never observed in solution
6-26
6 Carbocation 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
6-27
6 Carbocation Stability
 The
inductive effect
• the electron-deficient carbon bearing
the positive charge polarizes electrons +
of the adjacent sigma bonds toward it H3 C
+
+
• the positive charge on the cation is not
C
CH3
localized on the trivalent carbon, but
delocalized over nearby atoms
H3 C
• the larger the volume over which the
+
positive charge is delocalized, the
greater the stability of the cation
6-28
6 Carbocation 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
6-29
6 Addition of H2O
• 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
CH3 CH= CH 2 +
H2 O
H2 SO 4
Propene
2-Methylpropene
CH3 CH- CH2
2-Propanol
CH3
CH3 C= CH2 +
OH H
H2 O
H2 SO 4
CH3
CH3 C- CH2
HO H
2-Methyl-2-propanol
6-30
6 Addition of H2O
• Step 1: proton transfer from solvent to the alkene
CH3 CH= CH 2 + H O H
+
CH3 CHCH 3
A 2o carbocation
intermediate
H
:
:
+
slow, rate
determining
+
:O H
H
• Step 2: a Lewis acid/base reaction
:
+
CH 3 CHCH 3 +
: O-H
fast
CH 3 CHCH 3
O+
H
:
H
H
An oxonium ion
• Step 3: proton transfer to solvent
H
H
+
CH3 CHCH 3 + H O H
: OH
H
:
O:
O+
:
H
:
H
:
CH3 CHCH 3
fast
6-31
6 C+ 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 the
connectivity of the same atoms in the starting
material
6-32
6 C+ Rearrangements
• in addition of HCl to an alkene
CH 3
CH3
CH 3 CHCH = CH 2 + HCl
3-Methyl-1-butene
CH 3
CH3 CHCH CH 3 + CH3 CCH2 CH 3
Cl
2-Chloro-3methylbutane
(expected)
(40%)
Cl
2-Chloro-2methylbutane
(rearrangement)
(60%)
• in acid-catalyzed hydration of an alkene
CH3
CH3 CHCH = CH 2 +
3-Methyl-1-butene
H2 O
H2 SO 4
CH3
CH3 CCH2 CH 3
OH
2-Methyl-2-butanol
6-33
6 C+ Rearrangements
• driving force is rearrangement of a less stable
carbocation to a more stable one
CH3
+
+
: Cl :
:
CH3 C- CHCH3
:
Cl :
CH3
:
:
CH3 CCH= CH2 + H
H
3-Methyl-1-butene
slow, rate
determining
H
A 2° carbocation
intermediate
• the less stable 2° carbocation rearranges to a more
stable 3° one by 1,2-shift of a hydride ion
CH3
CH3 C- CHCH3
+
H
fast
CH3
CH3 C- CHCH3
+
H
A 3° carbocation
6-34
6 C+ Rearrangements
• reaction of the more stable carbocation (a Lewis acid)
with chloride ion (a Lewis base) completes the reaction
CH3
:
CH3 C- CH2 CH3 + : Cl :
+
fast
CH3
:
:
CH3 C- CH2 CH3
: Cl :
2-Chloro-2-methylbutane
6-35
6 Addition of Cl2 and Br2
• carried out with either the pure reagents or in an inert
solvent such as CH2Cl2
• addition is stereoselective
 Stereoselective
reaction: a reaction in which one
stereoisomer is formed or destroyed in
preference to all others
 Stereospecific reaction: a reaction in which one
stereoisomer is formed or destroyed to the
exclusion to all others
6-36
6 Addition of Cl2 and Br2
CH3 CH= CH CH 3
+ Br2
2-Butene
CH2 Cl 2
Br Br
CH3 CH- CHCH3
2,3-Dibromobutane
Br
+ Br2
Cyclohexene
CH2 Cl 2
Br
trans-1,2-Dibromocyclohexane
6-37
6 Addition of Cl2 and Br2
• Step 1: formation of a bridged bromonium ion
intermediate
:
: Br :
: Br :
:
: Br :
:
+ : Br :
C
C
C
C
:
• Step 2: attack of halide ion from the opposite side of
the three-membered ring
:
:
: Br : +
: Br :
:Br :
:
C
C
C
C
:
: Br :
Anti (coplanar) orientation
of added bromine atoms
6-38
6 Addition of Cl2 and Br2
 For
a cyclohexene, anti coplanar addition
corresponds to trans diaxial addition
Br
+ Br2
Br
Br
trans diaxial
(less stable)
Br
trans diequatorial
(more stable)
6-39
6 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
HO
CH3 CH= CH 2
Propene
Cl 2
H2 O
Cl
CH3 CH- CH2
+
HCl
1-Chloro-2-propanol
(a chlorohydrin)
6-40
6 Addition of HOCl and HOBr
• reaction is both regiospecific (anti addition) and
stereoselective (OH to the more substituted carbon)
Br2
H
CH3
1-Methylcyclopentene
Br
H3 C
+ HBr
H2 O
H
OH
2-Bromo-1-methylcyclopentanol
(anti addition of -OH and -Br)
6-41
6 Addition of HOCl and HOBr
Step 1: formation of a bridged halonium ion intermediate
:
: Br :
: Br :
:
H
R
C
C
H - Br
H
:
: Br :
-
: Br :
C
C
H
H
R
H
bridged bromonium
ion
C
C
H
H
R
H
minor contributing
structure
Step 2: attack of H2O on the more substituted carbon
opens the three-membered ring
:
H O:
C
H
R
C
: Br :
C
H
C
+
H
H
O
:
H
H
R
:
: Br :
H
H
H
6-42
6 Oxymercuration/Reduction
• oxymercuration followed by reduction results in
hydration of a carbon-carbon double bond
CH3 CH= CH CH 3 + Hg ( OAc ) 2
2-Butene
Mercury(II)
acetate
OH
OH
H2 O
CH3 CHCH CH 3
Hg OA c
An organomercury
compound
+ CH3 COOH
Acetic acid
OH
CH3 CHCH CH 3
Hg OA c
N aBH4
CH3 CHCH CH 3 + CH3 COOH + Hg
H
2-Butanol
Acetic acid
6-43
6 Oxymercuration/Reduction
• addition of Hg(II) and oxygen is anti coplanar
stereoselective
Hg ( OAc ) 2
H2 O
H
H
Cyclopentene
OH
H
H
Hg OA c
(Anti addition of
-OH and -HgOAc)
6-44
6 Oxymercuration/Reduction
• Step 1: dissociation of mercury (II) acetate gives
AcOHg+, a Lewis acid
AcO-Hg-OAc
AcO-Hg
+
+ AcO -
• 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
6-45
6 Oxymercuration/Reduction
OA c
+
Hg
H
R
C
C
H
H
+
H
C
OA c
OA c
OA c
Hg
Hg
Hg
+
C
C
H
R
H
An open carbocation
intermediate
(a minor contributor)
H
C
H
H
C
+
C
H
R
H
H
R
A bridged mercurinium An open carbocation
ion intermediate
intermediate
(the major contributor) (a negligible contributor)
6-46
6 Oxymercuration/Reduction
• Step 3: stereospecific and regioselective attack of H2O
on the bridged intermediate opens the mercurinium
ion ring OA c
Hg
+
C
H
R
C
C
H
:
H
H
H
C
+
H
O
H
H
H
Anti stereospecific addition
of HgOAc and HOH
:
R
H O:
Hg OA c
• Step 4: reduction of the C-HgOAc bond
H
R
HO
Hg OA c
C
+ N aBH4
C
H
H
H
R
C
HO
H
C
H
H
+ Hg0
6-47
6 Oxymercuration/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
6-48
6 Hydroboration/Oxidation
 Hydroboration:
the addition of borane, BH3, to an
alkene to form a trialkylborane
H
H B
CH2 CH3
+ 3 CH2 = CH2
H
Borane
 Borane
CH3 CH2 B
CH2 CH3
Triethylborane
(a trialkylborane)
dimerizes to diborane, B2H6
2 BH3
Borane
B2 H6
Diborane
6-49
6 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
2
: O:
+ B2 H6
Tetrahydrofuran
(THF)
2
+ :O BH3
BH3 • THF
6-50
6 Hydroboration/Oxidation
 Hydroboration
is both regioselective (boron to
the less hindered carbon) and stereospecific
(syn addition)
+
H
CH3
1-Methylcyclopentene
BH3
H
H3 C
BR2
H
(Syn addition of BH3 )
(R = 2-methylcyclopentyl)
6-51
6 Hydroboration/Oxidation
• mechanism involves concerted regioselective and
stereospecific addition of B and H to the carboncarbon double bond
 
H B
H
CH3 CH2 CH2 CH= CH2
B
CH3 CH2 CH2 CH-CH 2
6-52
6 Hydroboration/Oxidation
• trialkylboranes are rarely isolated
• oxidation with alkaline hydrogen peroxide gives an
alcohol and sodium borate
(RO) 3 B
A trialkylborate
+
3 NaOH
3ROH +
An alcohol
Na 3 BO 3
 The
result of hydroboration/oxidation is
regioselective and stereospecific hydration of a
carbon-carbon double bond
6-53
6 Hydroboration/Oxidation
BH3
H
CH3
1-Methylcyclopentene
H
H3 C
B
H
R
R
A trialkylborane
(R = 2-methylcyclopentyl)
H2 O2
N aOH
H
OH
H3 C
H
trans-2-Methylcyclopentanol
6-54
6 Oxidation/Reduction
 Oxidation:
the loss of electrons
• or the loss of H, the gain of O, or both
 Reduction:
the gain of electrons
• or the gain of H, the loss of O, or both
 Recognize
using a balanced half-reaction
1. 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-
6-55
6 Oxidation/Reduction
• three balanced half-reactions
OH
CH 3 CH= CH2 + H2 O
Propene
CH3 CHCH3
2-Propanol
HO OH
CH 3 CH= CH2 + 2 H 2 O
Propene
CH 3 CH= CH2 + 2 H+ + 2 e Propene
CH3 CHCH 2 + 2 H+ + 2 e 1,2-Propanediol
CH3 CH 2 CH 3
Propane
6-56
6 Oxidation with OsO4
 Oxidation
by OsO4 converts an alkene to a
glycol, a compound with -OH groups on adjacent
carbons
• oxidation is syn stereospecific
OsO 4
H
H
O
O
N aHSO 3
Os
O
O
A cyclic osmic ester
H2 O
H
H
OH
OH
cis-1,2-Cyclopentanediol
(a cis glycol)
6-57
6 Oxidation with OsO4
• 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
CH3
HOOH
CH3 COOH
CH3
Hydrogen
peroxide
tert-Butyl hydroperoxide
(t-BuOOH)
6-58
6 Oxidation with O3
 Treatment
of an alkene with ozone followed by a
weak reducing agent cleaves the C=C and forms
two carbonyl groups in its place
CH3
CH3 C= CHCH2 CH 3
2-Methyl-2-pentene
O
1 . O3
CH3 CCH3
2 . ( CH3 ) 2 S
Propanone
(a ketone)
O
+ HCCH 2 CH3
Propanal
(an aldehyde)
6-59
6 Oxidation with O3
• the initial product is a molozinide which rearranges to
an isomeric ozonide
CH3 CH= CH CH 3
2-Butene
O3
O OO
CH3 CH- CHCH3
A molozonide
H
O
H
O
C
C
( CH3 ) 2 S
H3 C
CH3 CH
CH3
O O
Acetaldehyde
An ozonide
6-60
6 Reduction of Alkenes
 Most
alkenes react with H2 in the presence of a
transition metal catalyst to give alkanes
+ H2
Cyclohexene
Pd
25°C, 3 atm
Cyclohexane
• commonly used catalysts are Pt, Pd, Ru, and Ni
 The
process is called catalytic reduction or,
alternatively, catalytic hydrogenation
6-61
6 Reduction of Alkenes
 Most
common pattern is syn stereoselectivity
CH3
+ H2
CH3
1,2-Dimethylcyclohexene
Pt
CH3
CH3
+
CH3
70% to 85%
cis-1,2-Dimethylcyclohexane
CH3
30% to15%
trans-1,2-Dimethylcyclohexane
6-62
6 Reduction 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 metalcarbon 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
6-63
6 H° of Hydrogenation
 Reduction
of an alkene to an alkane is
exothermic
• there is net conversion of one pi bond to one sigma
bond
 H°
depends on the degree of substitution
• the greater the substitution, the lower the value of H°

H° for a trans alkene is lower than that of an
isomeric cis alkene
6-64
6 H° of Hydrogenation
H°
kJ (kcal)/mol
Name
Structural
Formula
Ethylene
CH2 = CH2
-137 (-32.8 )
Propene
CH3 CH= CH 2
-126 (-30.1 )
1-Butene
CH3 CH2 CH= CH2
-127 (-30.3 )
cis-2-Butene
CH3 CH= CHCH3
-120 (-28.6
trans-2-Butene
CH3 CH= CHCH3
-115 (-27.6 )
2-Methyl-2-butene
( CH3 ) 2 C= CHCH 3
-113 (-26.9 )
2,3-Dimethyl-2-butene ( CH3 ) 2 C= C( CH 3 ) 2 -111 (-26.6 )
6-65
6 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
6-66
6 H° of Hydrogenation
cis-2-Butene
(less stable)
trans-2-Butene
(more stable)
6-67
6 Reaction 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?
6-68
6 Reaction Stereochemistry
 Which
of the three possible stereoisomers of 2,3dibromobutane are formed in the addition of
bromine to trans-2-butene?
H
CH3
C
H3 C
C
H
Br2
CCl4
Br Br
CH3 -CH -CH- CH3
• the three possible stereoisomers for this product are a
pair of enantiomers and a meso compound
6-69
6 Reaction Stereochemistry
 Reaction
of bromine with the alkene forms a
cyclic bromonium ion intermediate
+
Br
H
C
C
CH3
H3 C
H
trans-2-Butene
(achiral)
Br2
H
H3 C
C
C
CH3
H
• which is then opened by attack of bromide ion from
the side opposite the bromine bridge
6-70
6 Reaction Stereochemistry
Br
+
Br
H
H3C
2
C
-
Br
C
3
C
CH3
H
-
Br
2
3
C
CH3
H
H
Br
H3C
(2R,3S)-2,3-Dibromobutane
H3C
H
2
C
3
C
Br
identical;
a meso
compound
CH3
Br
H
(2S,3R)-2,3-Dibromobutane
6-71
6 Reaction Stereochemistry
 How
many and what kind of stereoisomers are
formed in the oxidation of cis-2-butene by OsO4?
H
H3 C
C
C
H
CH3
cis-2-Butene
(achiral)
OsO 4
ROOH
CH3 -CH -CH- CH3
OH OH
three stereoisomers are
possible for 2,3-butanediol;
a meso compound and a
pair of enantiomers
6-72
6 Reaction Stereochemistry
H3 C
H
H3 C
C
C
H
CH3
cis-2-Butene
(achiral)
OsO 4
H
H
2
C
3
C
CH3
HO
OH
(2S,3R)-2,3-Butanediol
ROOH
HO
2
C
identical;
a meso
compound
OH
3
C
H
H
CH3
H3 C
(2R,3S)-2,3-Butanediol
6-73
6 Reaction Stereochemistry
 How
many and what kind of stereoisomers are
formed in the oxidation of trans-2-butene by
OsO4?
H3 C
H
2
C
H
2
3
C
C
CH3
H3 C
H
trans-2-Butene
(achiral)
OsO 4
ROOH
3
C
CH 3
H
HO
OH
(2S,3S)-2,3-Butanediol
OH
HO
2
C
a pair of
enantiomers;
a racemic
mixture
3
C
CH3
H
H
H3 C
(2R,3R)-2,3-Butanediol
6-74
6 Reaction 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
6-75
6 Prob 6.15
Draw the isomeric carbocations formed on
treatment of each alkene with HCl. Which is the
more stable?
CH3
(a) CH3 CH2 C= CH CH 3
(c)
CH3
(b) CH3 CH2 CH= CHCH3
(d)
CH2
6-76
6 Prob 6.16
Arrange the alkenes in each set in order of increasing
rate of reaction with HI.
(a) CH3 CH= CH CH 3
(b)
and
CH3
CH3 C= CHCH3
and
6-77
6 Prob 6.17
Write the major product formed on treatment of 2-butene
with each reagent.
(a) H2 O ( H 2 SO 4 )
(b) Br2
(c) Cl 2
(d) Br2 in H2 O
(e) HI
(f) Cl 2 in H 2 O
(g) Hg ( OAc ) 2 , H2 O
(h) pr dt ( g) + N a BH4
6-78
6 Prob 6.18
What alkene undergoes acid-catalyzed hydration to give
each alcohol as the major product?
(a) 3-hexanol
(b) 2-methylcyclohexanol
(c) 2-methylbutan ol
(d) 2-propanol
6-79
6 Prob 6.19
Reaction of 2-methyl-2-pentene with each reagent is
regiospecific. What is the regiospecificity and how it is
accounted for?
(a) HI
(b) HBr
(c) H2 O, H2 SO 4
(d) Br2 in H 2 O
(e) Hg ( OAc ) 2 in H2 O
6-80
6 Prob 6.21
Draw the alkene of indicated molecular formula that gives
the compound shown as the major product.
(a) C5 H1 0 + H2 O
H2 SO 4
OH
Br
Br
(b) C5 H1 0 + Br2
(c) C7 H1 2
CH3
+ HCl
Cl
6-81
6 Prob 6.22
Account for the fact that addition of HCl to 1bromopropene gives 1-bromo-1-chloropropane.
CH3 CH= CHBr + HCl
1-Bromopropene
CH3 CH2 CHBrCl
1-Bromo-1-chloropropane
6-82
6 Prob 6.23
Propenoic acid reacts with HCl to give 3-chloropropanoic
acid. Account for this result.
O
CH2 = CHCOH + HCl
Propenoic acid)
(Acrylic acid)
O
ClCH 2 CH 2 COH
3-Chloropropanoic acid
Cl O
CH3 CHCOH
2-Chloropropanoic acid
(this product is not formed)
6-83
6 Prob 6.24
Draw a structural formula for the alkene of molecular
formula C5H10 that reacts with Br2 to give each product.
(a)
(b)
Br
Br
Br
(c)
Br
Br
Br
6-84
6 Prob 6.26
Draw a structural formula of the cycloalkene of molecular
formula C6H10 that reacts with Cl2 to give each compound.
Cl
Cl
(a)
(b)
Cl
(c)
H3 C
Cl
CH3
Cl
Cl
Cl
(d)
CH2 Cl
6-85
6 Prob 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.
CH3
+ Br2
H
CCl4
Br
CH3
Br
CH3
or
Br
H
(a)
Br
H
(b)
6-86
6 Prob 6.28
Propose a structural formula for terpin. How many
cis,trans isomers are possible for the structural formula
you have proposed?
+ 2 H2 O
H 2 SO 4
C1 0 H2 0 O 2
Terpin
Limonene
6-87
6 Prob 6.29
Propose a mechanism for this reaction.
CH3
CH3 -C= CH2
CH3
+ ICl
CH3 C-CH2 I
Cl
6-88
6 Prob 6.30
Propose a mechanism for this reaction.
CH3
CH3 C= CH2 + CH3 OH
H2 SO 4
CH3
CH3 C-OCH3
CH3
6-89
6 Prob 6.31
Propose a mechanism for the formation of each product.
CH3 CH= CHCH2 CH3
Cl OCH3
Cl 2
CH3 OH
H 3 CO Cl
Cl Cl
CH3 CHCHCH2 CH3 + CH3 CHCHCH2 CH3 + CH3 CHCHCH2 CH3
50%
35%
15%
6-90
6 Prob 6.32
Propose a mechanism for the formation of each product.
+ HBr
O
CH3 COH
Cyclohexene
O
OCCH 3
Br
+
Bromocyclohexane
(85%)
Cyclohexyl acetate
(15%)
6-91
6 Prob 6.33
Propose a mechanism for this reaction.
+ Br2 + H2 O
1-Pentene
OH
Br
+ HBr
1-Bromo-2-pentanol
6-92
6 Prob 6.34
Propose a mechanism for this reaction.
OH + Br
2
4-Penten-1-ol
O
CH2 Br
+ HBr
6-93
6 Prob 6.35
Propose a mechanism for each reaction.
OH
(a)
H2 SO 4
H2 O
H
H
CH3
Br
H
(b)
O
Br2
N aOH
H OC O
H
H
H + N aBr + H2 O
O
C
O
6-94
6 Prob 6.36
Propose a mechanism for this reaction.
Cl
+
1-Methyl-1-vinylcyclopentane
HCl
1-Chloro-1,2-dimethylcyclohexane
6-95
6 Prob 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.
(a)
CH2
CH3
(c) CH3 C= CHCH2 CH 3
(b)
CH3
(d) CH2 = CH( CH2 ) 5 CH3
(e) ( CH3 ) 3 CCH= CH2
6-96
6 Prob 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.
1 . BH 3
2 . H2 O 2 , Na OH
OH
-Pinene
6-97
6 Prob 6.41
Draw a structural formula of the alkene that gives each
set of products.
1 . O3
(a) C7 H1 2
O
O
2 . ( CH3 ) 2 S
1 . O3
(b) C1 0 H 1 8
2 . ( CH3 ) 2 S
1 . O3
(c) C1 0 H 1 8
2 . ( CH3 ) 2 S
O
O
+
O
+
H
O
H
O
H
O
6-98
6 Prob 6.47
State the number and kind of stereoisomers formed when
(R)-3-methyl-1-pentene is treated with each reagent.
H CH3
(R)-3-Methyl-1-pentene
(a) Hg(OAc)2 , H2O followed by NaBH4
(b) H2 / Pt
(c) BH3 followed by H2O2 in NaOH
(d) Br2 in CCl 4
6-99
6 Prob 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)
1 . Hg( OA c ) 2 , H2 O
2 . Na BH 4
OH
Br
(b)
+ Br
2
CCl4
Br
Br
(c)
+ Br 2
CCl4
Br
6-100
6 Prob 6.49 (cont’d)
(d)
+ HCl
Cl
OH
(e)
+ Cl 2 in H 2 O
Cl
OsO 4
(f)
OH
ROOH
OH
(g)
(h)
OH
1 . BH 3
CH3 2 . H2 O 2 , Na OH
CH3
CH3
CH3
+ HBr
Br
6-101
6
Alkenes II
End Chapter 6
6-102