6-1 6 Organic Chemistry William H. Brown & Christopher S. Foote

6-1

66

Organic Organic Chemistry Chemistry

William H. Brown &William H. Brown &

Christopher S. FooteChristopher S. Foote

6-2

66

Alkenes Alkenes IIIIChapter 6Chapter 6

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



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



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


(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:: ::


++

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


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


+ +

+

1-Pentene

1-Bromo-2-pentanol

Br2 H2 O

HBrBrOH

6-93


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


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

6-102

66

Alkenes IIAlkenes II

End Chapter 6End Chapter 6

Documents

6-1 6 Organic Chemistry William H. Brown & Christopher S. Foote