Química Orgânica I - UAlgw3.ualg.pt/~abrigas/QOI0809A_ALKEN.pdf · reaction occurring. => 4 ... w3.ualg.pt\~abrigas QOI 0809 ALKEN 37 Hofmann Product ... • May get rearrangement

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1

w3.ualg.pt\~abrigas QOI 0809 ALKEN 1

Química Orgânica I

2008/09


Hidrocarbonetos insaturados

Terpenes: antiviral,

antiseptic, anti-inflammatory.

OLEFINS“oil-forming gas”

2


• first algal pheromone (Müller DG et al., Science 1971,

171, 815).

• 117 y


Functional Group

• Pi bond is the functional group.

• More reactive than sigma bond.

• Bond dissociation energies:

– C=C BDE 146 kcal/mol

– C-C BDE -83 kcal/mol

– Pi bond 63 kcal/mol

=>

3


Bond Lengths and Angles

• Hybrid orbitals have more s character.

• Pi overlap brings carbon atoms closer.

• Bond angle with pi orbital increases.

– Angle C=C-H is 121.7°

– Angle H-C-H is 116.6° =>


Pi Bond

• Sideways overlap of parallel p orbitals.

• No rotation is possible without breaking the pi bond

(264 kJ/mole).

• Cis isomer cannot become trans without a chemical

reaction occurring.

=>

4


Elements of Unsaturation

• A saturated hydrocarbon: CnH2n+2

• Each pi bond (and each ring) decreases the number of H’s by two.

• Each of these is an element of unsaturation.

• To calculate: – find number of H’s if it were saturated,

– subtract the actual number of H’s,

– then divide by 2. =>


6.2 Degree of Unsaturation

5


Propose a Structure:

• First calculate the number of elements of unsaturation.

• Remember:

– A double bond is one element of unsaturation.

– A ring is also one element of unsaturation.

– A triple bond is two elements of unsaturation.

– =>

for C5H8


Propose a Structure:

• Isopreno

– nH “saturado”, CnH2n+2; neste caso é 12

– 12-8 = 4

– elementos de insaturação 4/2 =2

for C5H8

http://physics.nist.gov/PhysRefData/MolSpec/Hydro/Html/Tables/C5H8.html

6


Cycloalkene nomenclature

CH3 CH3CH31

2

1

2

3

1

2

4

1-methylcyclohexene 3-methylcyclohexene 4-methylcyclohexene


Name These Alkenes

CH2 CH CH2 CH3

CH3 C

CH3

CH CH3

CH3

CHCH2CH3

H3C

1-butene

but-1-ene

2-methyl-2-butene

2-methylbut-2-ene

3-methylcyclopentene

2-sec-butyl-1,3-cyclohexadiene

2-sec-butylcyclohexa-1,3-diene

7


6-vinyltridecane

4-isopropyldecane

4-vinyloctane


Alkene Substituents

= CH2

methylene

(methylidene)

- CH = CH2

vinyl

(ethenyl)

- CH2 - CH = CH2

allyl

(2-propenyl)

Name: =>

methylenecyclohexane ethenylbenzene

8


In a ring, the double bond is assumed to be

between carbon 1 and carbon 2.

Br

(Z)-7-bromo-4-ethyl-3-methylcyclohept-1-ene


Common Names

• Usually used for small molecules.

• Examples:

CH2 CH2

ethylene

CH2 CH CH3

propylene

CH2 C CH3

CH3

isobutylene

=>

9



Name these:

C C

CH3

H

H

CH3CH2

C C

Br

H

Br

H

trans-2-pentenetrans-pent-2-ene

cis-1,2-dibromoethene

=>

10


Example, E-Z

C C

H3C

H

Cl

CH2

C C

H

H

CH CH3

Cl1

2

1

2

2Z

2

1

1

2

5E

3,7-dichloro-(2Z, 5E)-2,5-octadiene3,7-dichloro-(2Z, 5E)-octa-2,5-diene

=>


Commercial Uses: Ethylene

=>

11


Commercial Uses: Propylene

=>


Other Polymers

=>

12


Stability of Alkenes

• Measured by heat of hydrogenation:

Alkene + H2 → Alkane + energy

• More heat released, higher energy alkene.


Substituent Effects

• More substituted alkenes are more stable.H2C=CH2 < R-CH=CH2 < R-CH=CH-R < R-CH=CR2 < R2C=CR2 unsub. < monosub. < disub. < trisub. < tetrasub.

• Alkyl group stabilizes the double bond.

• Alkene less sterically hindered.

=>

13


Disubstituted Isomers

• Stability: cis < geminal < trans isomer

• Less stable isomer is higher in energy, has a

more exothermic heat of hydrogenation.

-116 kJTrans-2-butene

-117 kJ(CH3)2C=CH2Isobutylene

-120 kJCis-2-butene CH3C C

CH3

H H

HC C

CH3

CH3 H=>


Relative Stabilities

14


Hyperconjugation

• Electrons in neighboring filled σ orbital stabilize vacant antibonding π orbital – net positive interaction

• Alkyl groups are more stabilizing than H


Bond strengths/hybridization effects

• sp3-sp3 bond is weaker than sp3-sp2, sp2-sp2

15


Cycloalkene Stability

• Cis isomer more stable than trans.

• Small rings have additional ring strain.

• Must have at least 8 carbons to form a stable trans double bond.

• For cyclodecene (and larger) trans double bond is almost as stable as the cis.

=>


Bredt’s Rule

• A bridged bicyclic compound cannot have a

double bond at a bridgehead position unless one of

the rings contains at least eight carbon atoms.

• Examples:

Unstable.Violates Bredt’s rule Stable. Double bond

in 8-membered ring.=>

16



Physical Properties

• Low boiling points, increasing with mass.

• Branched alkenes have lower boiling points.

• Less dense than water.

• Slightly polar

– Pi bond is polarizable, so instantaneous dipole-dipole interactions occur.

– Alkyl groups are electron-donating toward the pi

bond, so may have a small dipole moment.

=>

17


Polarity Examples

µ = 0.33 D µ = 0

=>

cis-2-butene, bp 4°C

C C

H

H3C

H

CH3

trans-2-butene, bp 1°C

C C

H

H

H3C

CH3


Alkene SynthesisOverview

• E2 dehydrohalogenation (-HX)

• E1 dehydrohalogenation (-HX)

• Dehalogenation of vicinal dibromides (-X2)

• Dehydration of alcohols (-H2O)

=>

18


Removing HX via E2

• Strong base abstracts H+ as X- leaves

from the adjacent carbon.

• Tertiary and hindered secondary alkyl

halides give good yields.

• Use a bulky base if the alkyl halide

usually forms substitution products.

=>


Some Bulky Bases

C

CH3

H3C

CH3

O_

tert-butoxide

(CH3CH2)3N :triethylamine

=>

N

H

CH(CH3)2

CH(CH3)2

diisopropylamine

N CH3H3C

2,6-dimethylpyridine

19


Hofmann Product

• Bulky bases abstract the least hindered H+

• Least substituted alkene is major product.

=>


E2: Diastereomers

Stereospecific reaction: (S, R) produces only trans product, (R, R) produces only cis.

Ph

Br H

H CH3

Ph

≡≡≡≡

H

Ph CH3Br

PhHH

Ph

CH3

Ph

HBr

CH3Ph

PhH =>

20


E2: Cyclohexanes

Leaving groups must be trans diaxial. =>


E2: Vicinal Dibromides• Remove Br2 from adjacent carbons.

• Bromines must be anti-coplanar (E2).

• Use NaI in acetone, or Zn in acetic acid.

I-

Br

CH3H

Br

CH3H

C CCH3

H

H

H3C =>

21


Removing HX via E1

• Secondary or tertiary halides

• Formation of carbocation intermediate

• May get rearrangement

• Weak nucleophile

• Usually have substitution products too

=>


Dehydration of Alcohols

• Reversible reaction

• Use concentrated sulfuric or phosphoric

acid, remove low-boiling alkene as it forms.

• Protonation of OH converts it to a good

leaving group, HOH

• Carbocation intermediate, like E1

• Protic solvent removes adjacent H+

=>

22


Dehydration Mechanism

=>


Industrial Methods

• Catalytic cracking of petroleum

– Long-chain alkane is heated with a catalyst to produce

an alkene and shorter alkane.

– Complex mixtures are produced.

• Dehydrogenation of alkanes

– Hydrogen (H2) is removed with heat, catalyst.

– Reaction is endothermic, but entropy-favored.

• Neither method is suitable for lab synthesis

=>

23


REACTIONS


Reactivity of C=C

• Electrons in pi bond are loosely held.

• Electrophiles are attracted to the pi electrons.

• Carbocation intermediate forms.

• Nucleophile adds to the carbocation.

• Net result is addition to the double bond.

=>

C C

C C+

24


Electrophilic Addition

• Step 1: Pi electrons attack the electrophile.

C C+ E

+C

E

C +

C

E

C + + Nuc:

_

C

E

C

Nuc

=>

• Step 2: Nucleophile attacks the carbocation.


Types of Additions

=>

25


Addition of HX (1)

Protonation of double bond yields the

most stable carbocation.

X

=>

+ Br_

+

+CH3 C

CH3

CH CH3

H

CH3 C

CH3

CH CH3

H

H Br

CH3 C

CH3

CH CH3


Addition of HX (2)

CH3 C

CH3

CH CH3

H Br

CH3 C

CH3

CH CH3

H+

+ Br_

CH3 C

CH3

CH CH3

H+

Br_

CH3 C

CH3

CH CH3

HBr

=>

26


Addition of HX

http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu

_organik/alkene/hx_addition.vlu/Page/vsc/en/ch/12/oc/alkene/markovnikov/markovnikov.vscml.html


Regiospecificity

• In an electrophilic addition to an alkene, the electrophile adds in such a way as to form the most stable intermediate. Markovnikov

• HCl, HBr, and HI add to alkenes to form Markovnikov products.

27


Free-Radical Addition of HBr

• In the presence of peroxides, HBr adds

to an alkene to form the “anti-

Markovnikov” product.

• Only HBr has the right bond energy.

• HCl bond is too strong.

• HI bond tends to break heterolytically to

form ions.

=>


Free Radical Initiation

• Peroxide O-O bond breaks easily to form

free radicals.

+R O H Br R O H + Br

O OR R +R O O Rheat

• Hydrogen is abstracted from HBr.

28


Propagation Steps

• Bromine adds to the double bond.

+C

Br

C H Br+ C

Br

C

H

Br

C

Br

CC CBr +

• Hydrogen is abstracted from HBr.


Anti-Markovnikov ??

• Tertiary radical is more stable, so that

intermediate forms faster.

CH3 C

CH3

CH CH3 Br+

CH3 C

CH3

CH CH3

Br

CH3 C

CH3

CH CH3

Br

X

29


Hydration of Alkenes

• Reverse of dehydration of alcohol

• Use very dilute solutions of H2SO4 or H3PO4 to drive equilibrium toward hydration.

C C + H2OH

+

C

H

C

OH

alkenealcohol


Mechanism for Hydration

+C

H

C+

H2O C

H

C

O H

H+

+ H2OC

H

C

O H

H+

C

H

C

O

H

H3O++

=>

C C OH H

H

++ + H2OC

H

C+

30


Orientation for Hydration

Markovnikov

+

CH3 C

CH3

CH CH3 OH H

H

++ H2O+

H

CH3CH

CH3

CCH3

H2O

CH3 C

CH3

CH CH3

HOH H

+

H2OCH3 C

CH3

CH CH3

HOH


Indirect Hydration

• Oxymercuration-Demercuration– Markovnikov product formed

– Anti addition of H-OH

– No rearrangements

• Hydroboration– Anti-Markovnikov product formed

– Syn addition of H-OH

=>

31


Oxymercuration (1)

• Reagent is mercury(II) acetate which dissociates slightly to form +Hg(OAc).

• +Hg(OAc) is the electrophile that attacks the pi bond.

CH3 C

O

O Hg O C

O

CH3 CH3 C

O

O

_

Hg O C

O

CH3

+


Oxymercuration (2)

The intermediate is a cyclic mercurinium

ion, a three-membered ring with a positive

charge.

C C+Hg(OAc) C C

Hg+

OAc

=>

32


Hg

OAc


33


Oxymercuration (3)

• Water approaches the mercurinium ion from the side opposite the ring (anti addition).

• Water adds to the more substituted carbon to form the Markovnikov product.

C C

Hg+

OAc

H2O

C

O+

C

Hg

H

H

OAc

H2O

C

O

C

Hg

H

OAc

=>


Demercuration

Sodium borohydride, a reducing agent,

replaces the mercury with hydrogen.

C

O

C

Hg

H

OAc

4 4 C

O

C

H

H

+ NaBH4 + 4 OH

_+ NaB(OH)4

+ 4 Hg + 4 OAc_

=>

34


Predict the Product

Predict the product when the given alkene

reacts with aqueous mercuric acetate,

followed by reduction with sodium

borohydride.

CH3

D

(1) Hg(OAc)2, H2O

(2) NaBH4

=>

OH

CH3D

H

anti addition


oxymercuration

- reduced ring strain

- higher stabilization of the partial positive charge at the higher-substituted carbon atom through hyperconjugation

http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/alkene/wasseraddition.vlu/Page/vsc/en/ch/12/oc/alkene/oxymercurierung/oxymercurierung.vscml.html

35


demercuration

http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/alkene/wasseraddition.vlu/Page/vsc/en/ch/12/oc/alkene/oxymercurierung/oxymercurierung.vscml.html

almost exclusive Markovnikov additionalkenes can be hydrated in high yieldsmilder conditions than in an acid-catalysed hydrationrearrangement products are rare


Alkoxymercuration - Demercuration

If the nucleophile is an alcohol, ROH, instead of water, HOH, the product is an ether.

C C

(1) Hg(OAc)2,

CH3OH

C

O

C

Hg(OAc)

H3C

(2) NaBH4

C

O

C

H3C

H

=>

36


Hydroboration

• 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 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 =>

37


BH3 Is a Lewis Acid

• Six electrons in outer shell

• Coordinates to oxygen electron pairs in ethers


• The electron-deficient borane adds to the least-substituted carbon.

• The other carbon acquires a positive charge.• H adds to adjacent C on same side (syn).

=>

Mechanism

38


Actually, Trialkyl

Borane prefers least-substituted carbon due to steric hindrance as well as charge distribution.

=>

C C

H3C

H3C

H

H+ BH3

B

CC H

CH3

H3C

H

H

C

CH

HH

CH3

CH3

C

C

H

H

H3C

CH3

H

3


Orientation in Hydration via Hydroboration

• Regiochemistry is opposite to Markovnikovorientation– OH is added to carbon with most H’s

• H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)

39


Oxidation to Alcohol

• Oxidation of the alkyl borane with basic hydrogen peroxide produces the alcohol.

• Orientation is anti-Markovnikov.

CH3 C

CH3

H

C

H

H

B

H2O2, NaOH

H2OCH3 C

CH3

H

C

H

H

OH

=>


Hydroboration

• More stable carbocation is also consistent with steric

preferences

40


Predict 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

H

CH3

D

OH


Hydrogenation

• Alkene + H2 → Alkane

• Catalyst required, usually Pt, Pd, or Ni.

• Finely divided metal, heterogeneous

• Syn addition

=>

41


Catalytic hydrogenation


42


Hydrogenation of fats

• Hydrogenation of edible oils improves the stability of the oil and modifies its texture.

• A partial hydrogenation will improve the stability of vegetable oil by reducing sensitivity to oxidation.

• A complete hydrogenation modifies the texture of the oil by hardening the oil to produce margarine.

http://www.chemguide.co.uk/organicprops/alkenes/hydrogenation.html


Addition of Carbenes

• Insertion of -CH2 group into a double bond

produces a cyclopropane ring.

• Three methods:

– Diazomethane

– Simmons-Smith: methylene iodide and Zn(Cu)

– Alpha elimination, haloform=>

43


Diazomethane

Extremely toxic and explosive.

N N CH2 N N CH2

diazomethane

N N CH2

heat or uv lightN2 +

carbene

C

H

H

C

H

H

C

CC

CC

H

H


Simmons-Smith

Best method for preparing cyclopropanes.

CH2I2 + Zn(Cu) ICH2ZnI

a carbenoid

CH2I2

Zn, CuCl=>

44


Alpha Elimination

• Haloform reacts with base.

• H and X taken from same carbon

CHCl3 + KOH K+ -CCl3 + H2O

CCl

Cl

Cl Cl-

+CCl

Cl

Cl

Cl

CHCl3

KOH, H2O=>


Stereospecificity

Cis-trans isomerism maintained around

carbons that were in the double bond.

C CH

CH3

H

H3C NaOH, H2O

CHBr3C C

H

CH3

H

H3C

BrBr

=>

45


Addition of Halogens

• Cl2, Br2, and sometimes I2 add to a double bond to form a vicinal dibromide.

• Anti addition, so reaction is stereospecific.

CC + Br2 C C

Br

Br

=>


Mechanism for Halogenation

• Pi electrons attack the bromine

molecule.

• A bromide ion splits off.

• Intermediate is a cyclic bromonium ion.

CC + Br Br CC

Br

+ Br =>

46


The Reality of Bromonium Ions

• Bromonium were postulated more than 60 years ago to expain the stereochemical course of the addition (to give the trans-dibromide from a cyclic alkene

• Olah showed that bromonium ions are stable in liquid SO2 with SbF5 and can be studied directly


Bromonium Ion

47


Mechanism (2)

Halide ion approaches from side opposite

the three-membered ring.

CC

Br

Br

CC

Br

Br

=>


Examples of Stereospecificity

=>

48


Test for Unsaturation

• Add Br2 in CCl4 (dark, red-brown color) to an alkene in the presence of light.

• The color quickly disappears as the bromine adds to the double bond.

• “Decolorizing bromine” is the chemical test for the presence of a double bond.

=>

http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/alkene/halogenaddition_an_alkene.vlu/Page/vsc/en/ch/12/oc/alkene/halogen_addition/halogenaddition.vscml.html


Formation of Halohydrin

• If a halogen is added in the presence of water, a halohydrin is formed.

• Water is the nucleophile, instead of halide.

• Product is Markovnikov and anti.

CC

Br

H2O

CC

Br

OH H

H2O

CC

Br

OH

+ H3O+

=>

49


Regiospecificity

The most highly substituted carbon has the most positive charge, so nucleophile

attacks there.

=>


Epoxidation

• 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+

=>

50


Mechanism

One-step concerted reaction. Several

bonds break and form simultaneously.

OC

O

R

H

C

C

OO

H

OC

O

RC

C

+

=>


C C

O

HO

O

C C

O

HO

O

51


Epoxide Stereochemistry

Since there is no opportunity for rotation

around the double-bonded carbons, cis or

trans stereochemistry is maintained.

CCCH3 CH3

H HPh C

O

O O H

CCCH3 CH3

H HO

=>


Opening the Epoxide Ring

• Acid catalyzed.

• Water attacks the protonated epoxide.

• Trans diol is formed.

CC

O

H3O+

CC

O

H

H2O

CC

O

OH

H H H2O

CC

O

OH

H

=>

52


One-pot 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

=>


Syn Hydroxylation of 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

=>

53


Mechanism with OsO4

Concerted syn addition of two oxygens to form a cyclic ester.

C

COs

O O

OO

C

C

O O

OO

Os

C

C

OH

OH+ OsO4

H2O2

=>


Stereospecificity

If a chiral carbon is formed, only one

stereoisomer will be produced (or a pair of

enantiomers).

C

C

CH2CH3

H CH2CH3

C

C

CH2CH3

CH2CH3

OH

OH

H

HH2O2

H

(2)

(1) OsO4

cis -3-hexene meso -3,4-hexanediol

=>

54


Osmium Tetroxide Catalyzed

Formation of Diols

• Hydroxylation - converts to syn-diol

• Osmium tetroxide, then sodium bisulfate

• Via cyclic osmate di-ester


Oxidative Cleavage

• Both the pi and sigma bonds break.

• C=C becomes C=O.

• Two methods:– Warm or concentrated or acidic KMnO4.

– Ozonolysis

• Used to determine the position of a double bond in an unknown.

=>

55


Cleavage with MnO4-

• Permanganate is a strong oxidizing agent.

• Glycol initially formed is further oxidized.

• Disubstituted carbons become ketones.

• Monosubstituted carbons become carboxylic acids.

• Terminal =CH2 becomes CO2.


Example

CCCH3 CH3

H CH3 KMnO4

(warm, conc.)C C

CH3

CH3

OHOH

H3C

H

C

O

H3C

H

C

CH3

CH3

O

C

O

H3COH

+

=>

56


Permangante Oxidation of Alkenes

• Oxidizing reagents other than ozone also cleave

alkenes

• Potassium permanganate (KMnO4) can produce

carboxylic acids and carbon dioxide if H’s are

present on C=C


Ozonolysis

• Reaction with ozone forms an ozonide.

• Ozonides are not isolated, but are

treated with a mild reducing agent like Zn or dimethyl sulfide.

• Milder oxidation than permanganate.

• Products formed are ketones or

aldehydes.

=>

57


Ozonolysis Example

CCCH3 CH3

H CH3 O3C

H3C

H

O O

CCH3

CH3

O

Ozonide

+(CH3)2S

C

H3C

HO C

CH3

CH3

O CH3 S

O

CH3

DMSO

=>


58


Examples of Ozonolysis of Alkenes

• Used in determination of structure of an unknown alkene


Biological Hydration

fumarase sf (fumár(ico)+ase) Biol Enzima

cristalina que ocorre em muitos tecidos

animais e vegetais e catalisa o equilíbrio

entre o ácido fumárico e o ácido málico

pela conversão recíproca de um em outro, por hidratação e desidratação.

Var: fumárase.

59


Krebs


fumarase

http://www.chemgapedia.de/vsengine/supplement/vsc/de/ch/5/bc/chemical_pathways/citronensaeurezyklus/fumarase/fumarase.vscml/Fragment/51b536051713c229fd9e9091c327ed2d-18.html

http://chemistry.gsu.edu/glactone/PDB/Proteins/Krebs/BACKUP/Krebs.html.DBH2

60


Biological Hydration

• Fumarate to malate catalyzed by fumarase

• Specific for trans isomer

• Addition of H, OH is anti

• Fumarase deficiency is an enzyme irregularity that causes severe mental retardation


Polymerization

• An alkene (monomer) can add to another

molecule like itself to form a chain

(polymer).

• Three methods:

– Cationic, a carbocation intermediate

– Free radical

– Anionic, a carbanion intermediate (rare) =>

61


Cationic Polymerization

Electrophile, like H+ or BF3, adds to the least

substituted carbon of an alkene, forming

the most stable carbocation.

C CCH3

H

H

H

OH

H

H

C

H

H

H

C

CH3

H

+ C CCH3

H

H

H

C

H

H

H

C

CH3

H

C

H

H

C

CH3

H=>


Radical Polymerization

In the presence of a free radical initiator, like

peroxide, free radical polymerization

occurs.

C CPh

H

H

H

RO

C

H

RO

H

C

Ph

H

+ C CPh

H

H

H

C

H

RO

H

C

Ph

H

C

H

H

C

Ph

H

=>

62


Chain Branching During Polymerization

• During radical propagation chain can develop forks leading to branching

• One mechanism of branching is short chain branching in which an internal hydrogen is abstracted


Long Chain Branching

• In long chains, a hydrogen from another chain is

abstracted

63


Anionic Polymerization

Nu: needs electron-withdrawing groups

(nitro, cyano, or carbonyl)

C CCOCH3

CN

H

H

O

OH-

C

H

HO

H

C

COCH3

CN

O

+ C CCOCH3

CN

H

H

O

C

H

HO

H

C

C

CN

C

H

C

COCH3

CNH

OO OCH3


polymers

http://www.chemistryland.com/PolymerPlanet/Polymers/PolymerTutorial.htmhttp://ocw.mit.edu/OcwWeb/Materials-Science-and-Engineering/3-051JSpring-2006/CourseHome/index.htm

64


• resumo


65



66


• http://www.chemguide.co.uk/organicprops/

alkenemenu.html#top

• http://www.chemgapedia.de/vsengine/topi

cs/en/vlu/Chemie/Organische_00032Che

mie/Alkene/index.html


Adaptado de:

• Organic Chemistry, 6th Edition; L. G.

Wade, Jr.

• Organic Chemistry, 6th edition; McMurry’s

Documents

Química Orgânica I - UAlgw3.ualg.pt/~abrigas/QOI0809A_ALKEN.pdf · reaction occurring. => 4 ... w3.ualg.pt\~abrigas QOI 0809 ALKEN 37 Hofmann Product ... • May get rearrangement