carbo cation

7/30/2019 Carbo Cation

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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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6-7

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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6-11

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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6-13

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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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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6-20

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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6-21

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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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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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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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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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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6-31


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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6-32

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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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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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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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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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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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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6-39


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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6-40


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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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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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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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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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 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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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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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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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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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



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Mechanism of catalytic hydrogenation



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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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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



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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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reaction of cis-2-butene with bromine forms bridged

bromonium ions which are meso and identical



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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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reaction with bromine forms bridged bromonium ion

intermediates which are enantiomers



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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



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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)



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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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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+



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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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carbo cation

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