WWU -- Chemistry Alkenes and Alkynes I. Addition Reactions Chapter Eight.

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Alkenes and Alkynes I.Alkenes and Alkynes I.Addition ReactionsAddition Reactions

Chapter Eight

Types of AdditionsTypes of Additions

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Sect 8.1: Addition Sect 8.1: Addition ReactionsReactions

C C X Y C C

X

Y

+


Addition of H-XAddition of H-X

C

H

H

C

H

H

CH

H

C

H

H

H

X

H

H

X

H

H

H

H X

H X


Addition of WaterAddition of Water

C

H

H

C

H

H

H

O

H

H+ catalyst

CH

H

C

H

H

H

OH

H

H

OH

H

H

H

H

O

H

H+ catalyst


Sect. 8.2: Introduction Sect. 8.2: Introduction to to MechanismsMechanisms

• mechanism• two step• rate determining step (i.e. slow

step)• energy diagram• transition states• intermediates


MECHANISMMECHANISMSTEP-BY-STEP ACCOUNT OF WHAT HAPPENS

C C

E+

C C

E+

C C

E

Xstep 2step 1

intermediates areformed during areaction but are not products

:X-

Intermediate


product

H

intermediate

TS2

TS1

ENERGY PROFILEENERGY PROFILEtwo step reaction

ENERGY

step 1 step 2C C

E+

X -

+C C

C C

E

X

E


ACTIVATED COMPLEXESACTIVATED COMPLEXEScorrespond to transition states for each

step

ACTIVATED COMPLEXES

C C

E

XX-

+C C

E

E+

C C

C C

E

C C

E

X

intermediate

show bonds in the process of breaking or forming(bonds are half formed or half broken)

+

+

-

+


Sect 8.3: Electrophilic Sect 8.3: Electrophilic Addition to a Double BondAddition to a Double Bond

CH2 CH2 X Y X CH2 CH2 Y+


HyperconjugationHyperconjugation

C CH

H

HH

H

Emptyp-orbital


CARBOCATION STABILITYCARBOCATION STABILITYHYPERCONJUGATION

CC

H

H

H

+

R

Relectrons in an adjacent

C-H bond help to stabilizethe positive charge of thecarbocation by proximity(overlap)

CR

R

R+R CH R

+R CH2

+

tertiary secondary primary

> >Most stable

Least stable

CR

R

R+ R CH R+

R CH2+

..


Can you recognize the Can you recognize the following carbocations? 1following carbocations? 1oo, ,

22oo, 3, 3oo+

+ +

+ +

+

1 o 2 o 2 o

3 o 2 o

2 o


Sect 8.4: Addition of Sect 8.4: Addition of Hydrogen Halides: Hydrogen Halides: Markovnikov’s ruleMarkovnikov’s rule

H3C

C

CH2

CH3

H3C

C

C

CH3

H3C

C

C

CH3

H

Br

H

H

major product

minor product

H

H

HBr

H-Br


Markovnikov’s RuleMarkovnikov’s RuleIn the ionic addition of an acid to the carbon-carbon double bond of an alkene, the hydrogen of the acid attaches itself to the carbon atom which already holds the greater number of hydrogens.

–“Them that has, gets!”

–“The richer get richer!”

(V. W. Markovnikov -- 1838 - 1904)


MarkovnikovMarkovnikov


MARKOVNIKOV RULEMARKOVNIKOV RULE

CH2

+ HCl

CH3

Cl

When adding HX to a double bond thehydrogen of HX goes to the carbonwhich already has the most hydrogens

..... conversely, the anion X adds to the mosthighly substituted carbon ( the carbon withmost alkyl groups attached).


REGIOSELECTIVEREGIOSELECTIVE REACTION REACTION

CCH3

CH3

CH2 CCH3

CH3

CH3

Cl

+ CHCH3 CH2

Cl

CH3HCl

major minor

one of the possible products is formedin larger amounts than the other one

Compare

REGIOSPECIFICREGIOSPECIFIConly one of the possible products isformed (100%).


Mechanism (Markovnikov)Mechanism (Markovnikov)

R CH CH2 R CH-CH2

R CH CH2 H

Br

1)+

+

slow

2)_ fast

+

+

H

Br

H

R CH-CH2

+H

Electrophile

Nucleophile

Secondary C+

Major product


Mechanism (anti-Mechanism (anti-Markovnikov)Markovnikov)

R CH CH2R CH CH2

H

R CH CH2

H

R CH CH2 Br

H

1)+ slow

+2)

_ fast

+

+

H

Br

+

Minor!

Primary carbocation


COMPETING PATHWAYSCOMPETING PATHWAYS

lower energyintermediate

higher energyintermediate

rate-determininng(slow) step

rate-determining step

faster

slower

1 o

2 o


Markovnikov Addition to Markovnikov Addition to an Alkenean Alkene

C CH2

CH3

CH3

H X CH3 C CH2

CH3

X

H CH3 C CH2

CH3

H

X+ +

major product minor productX = Cl, Br, I


MechanismMechanism

C CH21)+

+

slow

2)_ fast

+

+

H

Electrophile

Nucleophile

CH3

CH3

CH CH2

CH3

CH3 H

+C CH2

CH3

CH3 H

C CH2CH3

CH3 H

X

X

3 o C+


SOME ADDITIONAL EXAMPLESSOME ADDITIONAL EXAMPLES

CH3

+ HCl

CH3

Cl

CH2

+ HCl

CH3

Cl

CH CH2 CH CH3

Cl+ HCl

only major product is shown


Sect 8.5 Addition of Sect 8.5 Addition of Sulfuric Acid to an AlkeneSulfuric Acid to an Alkene

CH CH2R R CH CH2 H

O

S

OH

OO

+ H2SO4

(cold, concentrated)

an alkyl hydrogen sulfate


H O S O

O

O

H

C C C C

H+ C C

H

OSO3 H

O S O

O

O

H

alkyl hydrogen sulfate

cold

-

ALKYL HYDROGEN SULFATESALKYL HYDROGEN SULFATES

C C

H

OHwaterroom temp

FOLLOWS MARKOVNIKOFFRULE

SLOW


Addition of Water to an Addition of Water to an AlkeneAlkene

R CH CH2 R CH CH2 H

OH

+ H2OH2SO4

an alcohol


Mechanism of HydrationMechanism of Hydration

R CH CH2 R CH CH2 H

R CH CH2 H R CH CH2 H

OH2

R CH CH2 H

OH2

R CH CH2 H

OH

O S OH

O

O

H

1) + H2SO4

2)

+

++ H2O

slow

+

3)

+

+

+ HSO4

_

HSO4

_

+


Sect 8.6 Addition of Sect 8.6 Addition of Bromine to an AlkeneBromine to an Alkene

R CH CH R R CH CH R

Br

Br

+ Br2

CCl4

or CH2Cl2


ADDITION OF BROMINEADDITION OF BROMINE

C C

Br

Br

+C C

Br

C C

Br Br

BrCCl4

Br Br:+ -

alkene polarizes bromine

SLOW


H

H

Br

Br

H

H

Br

H

Br

HBr

H

H

Br

Br

Br

cis compound trans compound

NOT OBSERVED

THE REACTION IS STEREOSPECIFICTHE REACTION IS STEREOSPECIFIC

ACTUAL PRODUCT

ANTI ADDITION

anti

syn

antisyn

+

“open“ carbocationwould give bothcis and trans

-


WHAT WOULD EXPLAIN FORMATIONWHAT WOULD EXPLAIN FORMATION OF ONLY THE OF ONLY THE transtrans PRODUCT ? PRODUCT ?

..... A BRIDGED OR CYCLIC INTERMEDIATE


+Br

HH

CYCLIC BROMONIUM IONCYCLIC BROMONIUM ION

note size ofbromine

+


HH

-Br

HH

Br

Br

Br+

BRIDGED BROMONIUM IONBRIDGED BROMONIUM ION

bridgingblocks approachfrom this side


Br

H

H

Br

Br

H

H

BrH

Br

Br

H

ENANTIOMERS

symmetricintermediate

+

FORMATION OF ENANTIOMERSFORMATION OF ENANTIOMERS

Addition could also startfrom the top with bromideattacking the bottom.


CH3

H H

CH3CH3

H CH3

H

CH3 CH CH CH3

+

Br2

CH3 CH CH CH3

Br Br

ADDITION OF BROMINE TO 2-BUTENEADDITION OF BROMINE TO 2-BUTENE

* *

cis-2-butene trans-2-butene

WILL THESE STEREOISOMERS GIVE THE SAME PRODUCTS?

2n possible stereoisomers


CH3

H H

CH3CH3

H CH3

H

BrBr

CH3 CH3

HH

BrBr

H CH3

HCH3

BrBr

CH3 HCH3H+

Br2 / CCl4 Br2 / CCl4

enantiomers meso

NO ! THEY GIVE DIFFERENT PRODUCTSNO ! THEY GIVE DIFFERENT PRODUCTS

These results can only be explained by stereospecific anti addition.

cis trans


Br

Br

H3CCH3

HH

H3CCH3

HH

Br+

BrBr

H3CCH3

H

H

Br

Br

H3C

CH3

H

H

Br

Br

Br Br

H3C HH

CH3

Br Br

H CH3

H3C H

ROTATEROTATE

ENANTIOMERS

ADD TO RIGHT

ADD TO LEFT

cis-

-


Br

Br

H3CH

HCH3

H3CH

HCH3

Br

BrBr

H3CH

H

CH3

Br

Br

H3C

H

H

CH3

Br

Br

Br Br

H HH3C

CH3

Br Br

H3CCH3

HH

ROTATEROTATE

ADD TO RIGHT

ADD TO LEFT

MESO

IDENTICAL (also meso)

trans_

_

+


Bromination of an Bromination of an Unsymmetrical AlkeneUnsymmetrical Alkene

R CH CH2 Br Br R CH CH2

Br

R CH CH2

BrC C

Br

BrH

H

R

H

1)

+slow

+

_

2)+

+ Br: :. _

.

anti-addition(stereospecific)

+ Br


Stereochemistry of Stereochemistry of Bromination of Alkenes Bromination of Alkenes

• Simple alkenes: Addition of bromine or chlorine goes exclusively anti, with the formation of a bridged ion

• If a resonance-stabilized open-chain carbocation is possible, there may be a mixture of mechanisms, with some molecules reacting via a bridged ion and some molecules reacting via an open-chain carbocation


Stereochemistry of Stereochemistry of Bromination of AlkenesBromination of Alkenes

--Part Two--Part Two• In cases where a resonance-stabilized

carbocation is possible, if the solvent is made more polar (acetic acid or nitromethane), the proportion of molecules reacting via an open-chain carbocation increases.

• For simple alkenes, changing solvents has little or no effect on stereochemistry.


Sect 8.7 Halohydrin Sect 8.7 Halohydrin FormationFormation

Br2

H2O

a bromohydrin

OH

+ HBrR C C

Br

R-CH=CH-R HH R

Br2 + H2O

HO-Br

+ HBr


MechanismMechanism

R CH CH2 + Br Brslow

R CH CH2

Br

R CH CH2

Br

OH H

R CH CH2

Br

OH H

R CH CH2

Br

OH H

R CH CH2

Br

O

H

+

+ Br

H


Sect. 8.8: Carbocation Sect. 8.8: Carbocation RearrangementsRearrangements

CHCl

C

major

rearranged!!

+ C

minor

not rearranged!!

CH3

CH3

CH3

CH CH2 CH3

CH3

CH3

CH CH2

H

Cl

CH3

CH3

CH3

CH CH2

Cl

H


Sect 8.10 Free Radical Sect 8.10 Free Radical Addition of HBr to Alkenes Addition of HBr to Alkenes

((antianti-Markovnikov!)-Markovnikov!)

R CH CH2R CH CH2 Br

H

+ HBrROOR


ADDITION OF HBrADDITION OF HBr

CH2

conc. HBr

CH3

Br

CH2

conc. HBr

CH2BrH

Markovnikov Addition

Anti-Markovnikov Addition

Oxygen


R O O R R O O R

R O R O HH Br

CH2 CH R Br CH2 CH R

Br CH2 CH R H Br Br CH2 CH R

H

1)

2)

3)

4)

light or

heat+..

.. ..

... .

..

... + ..

..+ Br:

..

...

Br:..

... + .

. + + Br: ...

.

hydrogen abstraction

hydrogen abstraction

slow

...


STABILITY OF CARBON RADICAL INTERMEDIATESSTABILITY OF CARBON RADICAL INTERMEDIATES

Radicals are electron-deficient just like carbocationsand have the same stability order.

lowestenergy

highest energy

R C R

R

R C

R

H R C H2 C H3< < <

tertiary secondary primary methyl

and they are stabilized by resonance and / or hyperconjugation.

CH2

( )

( )

( ) etc.CH2

( )

( )( )

( )


Sect. 8.11 and 8.12: Hydrogenation Sect. 8.11 and 8.12: Hydrogenation of Alkenes and alkynesof Alkenes and alkynes

+ H2

Pt, Pd,

or Ni

syn-addition

(stereospecific)

CCH2

lindlar catalyst

R

H H

R CH CH R

R

R CH CH R

H H

R

R


Hydrogenation of AlkenesHydrogenation of Alkenes

Also PtO2 -- sometimes Ru, Rh, or Re

R CH CH R R CH CH R

H H

+ H2

Pt, Pd,

or Ni

syn-addition

(stereospecific)


HYDROGEN ADSORBS ON THE SURFACE OF THE CATALYSTHYDROGEN ADSORBS ON THE SURFACE OF THE CATALYST

n H2 + Pt Pt(H )2n

H H H.Hydrogen Adsorbed onCatalyst Surface

. .finely-dividedparticlesdispersed insolution

Pt Pt PtPt Pt

.H

.H

Pt

.H

Pt Pt Pt Pt Pt

.


MECHANISM OF HYDROGENATIONMECHANISM OF HYDROGENATION

.H.

H

.H

.H .

H.H.

H

H H

CATALYST


.H.

H

.H

.H .

H.H.

H

.H

.H

HYDROGENADSORBS




.H.

H

.H

.H .

H.H.

H

.H

RR

R R

. .

.H

ALKENEAPPROACHES



.H.

H

.H

.H .

H.H.

H

.H

.H

RR

R R

. .ALKENE PICKS UPTWO HYDROGENS



.H.

H

.H

.H .

H.H.

H

RR

R R

. .H

..H

ALKANE ISFORMED


BOTH HYDROGENS ADD TO THE SAME BOTH HYDROGENS ADD TO THE SAME SIDE OF THE DOUBLE BONDSIDE OF THE DOUBLE BOND

CH3

CH3

CH3

CH3

CH3

CH3

H

H

HH syn

addition

antiaddition

X

notobserved


EXAMPLESEXAMPLES

CH3 + H2 CH3Pt

CH3 CH CH CH3Pt

H2+ CH3 CH2 CH2 CH3

CH2

PtH2+

CH3


Sect. 8.14: Addition Sect. 8.14: Addition PolymersPolymers

Ccatalyst

C

ethylene polyethylene

a macromolecule with very high molecular weight

H

H

H

H

C C

H

H H

H


Polymerization of propenePolymerization of propene

C Ccatalyst three different forms

syndiotactic, isotactic and atactic

CH3CH3 CH3CH3

CH3 CH3CH3

CH3

CH3

H

H

H

isotactic

syndiotactic

Indirect HydrationIndirect Hydration

• Oxymercuration-Demercuration– Markovnikov product formed– Anti addition of H-OH– No rearrangements

• Hydroboration– Anti-Markovnikov product formed– Syn addition of H-OH

HydroborationHydroboration

• Borane, BH3, adds a hydrogen to the most substituted carbon in the double bond.

• The alkylborane is then oxidized to the alcohol which is the anti-Mark product.

C C(1) BH3

C

H

C

BH2

(2) H2O2, OH-

C

H

C

OH

Borane ReagentBorane Reagent

• Borane exists as a dimer, B2H6, in equilibrium with its monomer.

• Borane is a toxic, flammable, explosive gas.• Safe when complexed with tetrahydrofuran.

THF THF . BH3

O B2H6 O+

B-

H

H

H

+2 2

Predict the ProductPredict the Product

Predict the product when the given alkene reacts with borane in THF, followed by oxidation with basic hydrogen peroxide. CH3

D

(1)

(2)

BH3, THF

H2O2, OH-

syn addition

HCH3

DOH

EpoxidationEpoxidation

• Alkene reacts with a peroxyacid to form an epoxide (also called oxirane).

• Usual reagent is peroxybenzoic acid.

CC + R C

O

O O H CC

O

R C

O

O H+

One-Step ReactionOne-Step Reaction

• To synthesize the glycol without isolating the epoxide, use aqueous peroxyacetic acid or peroxyformic acid.

• The reaction is stereospecific.

CH3COOH

O

OH

H

OH

H

Chapter 8Chapter 8 68

Syn Hydroxylation Syn Hydroxylation of Alkenesof Alkenes

• Alkene is converted to a cis-1,2-diol,• Two reagents:

– Osmium tetroxide (expensive!), followed by hydrogen peroxide or

– Cold, dilute aqueous potassium permanganate, followed by hydrolysis with base

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WWU -- Chemistry Alkenes and Alkynes I. Addition Reactions Chapter Eight.

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