Chapter 3. Alkena dan Alkuna: Nomenklatur dan Reaksinya

Tutik Dwi WahyuningsihJurusan Kimia FMIPA UGM

2011

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Introduction: kegunaan alkenaStruktur alkenaNomenklatur Alkena & AlkunaNomenklatur E/Z Jenis/tipe ikatan rangkap duaReaksi pada Alkena

Adisi Substitusi Diels Alder Pemutusan

Alkena dan Alkuna

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Example : a mixture of and bonds, but no triple bonds

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Commercial Uses: Ethylene

=>

5

Commercial Uses: Propylene

=>

6

Other Polymers

=>

7

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 =>

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Alkenes

Geometrical isomers are possible since there is no rotation about a C=C bond.

Cis- and trans- isomers possible.

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Functional Group

• Pi bond is the functional group.• More reactive than sigma bond.

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Orbital Description

• Sigma bonds around C are sp2 hybridized.• Angles are approximately 120 degrees.• No nonbonding electrons.• Molecule is planar around the double bond.

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Pi Bond• Sideways overlap of parallel p orbitals.• No rotation is possible without breaking

the pi bond (63 kcal/mole).• Cis isomer cannot become trans without

a chemical reaction occurring.

=>

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IUPAC Nomenclature

• Parent is longest chain containing the double bond.

• -ane changes to -ene. (or -diene, -triene)• Number the chain so that the double

bond has the lowest possible number.• In a ring, the double bond is assumed to

be between carbon 1 and carbon 2. =>

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Name These AlkenesCH2 CH CH2 CH3

CH3 C

CH3

CH CH3

CH3

CHCH2CH3H3C

1-butene

2-methyl-2-butene

3-methylcyclopentene

2-sec-butyl-1,3-cyclohexadiene

3-n-propyl-1-heptene =>

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Alkene Substituents

= CH2

methylene(methylidene)

- CH = CH2

vinyl(ethenyl)

- CH2 - CH = CH2

allyl(2-propenyl)

Name: =>

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Common Names

• Usually used for small molecules.• Examples:

CH2 CH2

ethylene

CH2 CH CH3

propylene

CH2 C CH3

CH3

isobutylene=>

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Cis-trans Isomerism

• Similar groups on same side of double bond, alkene is cis.

• Similar groups on opposite sides of double bond, alkene is trans.

• Cycloalkenes are assumed to be cis.• Trans cycloalkenes are not stable

unless the ring has at least 8 carbons. =>

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Name these:

C CCH3

H

H

CH3CH2

C CBr

H

Br

H

trans-2-pentene cis-1,2-dibromoethene

=>

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E-Z Nomenclature

• Use the Cahn-Ingold-Prelog rules to assign priorities to groups attached to each carbon in the double bond.

• If high priority groups are on the same side, the name is Z (for zusammen).

• If high priority groups are on opposite sides, the name is E (for entgegen). =>

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Example, E-Z

C CH3C

H

Cl

CH2C C

H

H

CH CH3

Cl1

2

1

2

2Z

2

1

1

2

5E

(2Z, 5E)-3,7-dichloro-2,5-octadiene =>

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Definisi

• Ikatan rangkap dua terkonjugasi : dipisahkan oleh satu ikatan tunggal.

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• Ikatan rangkap dua terisolasi : dipisahkan oleh dua atau lebih ikatan tunggal.

• Ikatan rangkap dua terakumulasi : ikatan rangkap dua berdekatan.Contoh : 1,2-pentadiena

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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. < tetra sub.

• Alkyl group stabilizes the double bond.• Alkene less sterically hindered.

=>

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Alkenes

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Disubstituted Isomers• Stability: cis < geminal < trans isomer• Less stable isomer is higher in energy, has

a more exothermic heat of hydrogenation.

27.6 kcalTrans-2-butene

28.0 kcal (CH3)2C=CH2Isobutylene

28.6 kcalCis-2-butene CH3C C

CH3

H H

HC C

CH3

CH3 H=>

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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. =>

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Polarity Examples

= 0.33 D = 0

=>

cis-2-butene, bp 4°C

C CH

H3C

H

CH3

trans-2-butene, bp 1°C

C CH

H

H3C

CH3

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ADDITION REACTIONADDITION REACTION

An addition reaction is one in which the tworeactants add together to make the product

A + B AB

with no other pieces lost or left over.

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ELECTROPHILIC ADDITION TO DOUBLE BONDSELECTROPHILIC ADDITION TO DOUBLE BONDS

EXAMPLES:

conc.

conc.

OSO3HC C

H

C CH

OH

C CH

Cl

+

+

+

C C

C C

C C

C CE

XC C + EX

H2SO4H2O

H2SO4

HCl

0 oC

electrophilicreagent

explainedlater

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Addition Reactions of Alkenes and Alkynes

A common addition reaction is hydrogenation:

CH3CH=CHCH3 + H2 CH3CH2CH2CH3

Hydrogenation requires high temperatures and pressures as well as the presence of a catalyst (e.g. Ni).

Note: hydrogenation forms alkanes from alkenes.

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Addition Reactions of Alkenes and Alkynes

It is possible to cause hydrogen halides and water to add across bonds:

CH2=CH2 + HBr CH3CH2Br ( a bromide)

CH2=CH2 + H2O CH3CH2OH (an alcohol)

The addition of water is usually catalysed by H2SO4.

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Addition Reactions of Alkenes and Alkynes

The most dominant reaction for alkenes and alkynes involves the addition of something to the two atoms which form the double bond:

Note that the C-C bond has been replaced by two C-Br bonds.

H2C CH2 + Br2 H2C CH2Br Br

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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.

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Addition of HX (1)

Protonation of double bond yields the most stable carbocation. Positive charge goes to the carbon that was not protonated.

X =>

+ Br_

+

+CH3 C

CH3

CH CH3

H

CH3 C

CH3

CH CH3

H

H Br

CH3 C

CH3

CH CH3

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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 =>

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Reaksi Adisi via Intermediet KarbokationHidrasiHidrasi

Adisi Hidrogen halidaAdisi Hidrogen halida R CH CH2

H+R CH CH3

+

secondarycarbocation(primary R+

not formed)

H2O

X-

R CH CH3

OH

R CH CH3

alcohol

X

alkyl halidewhere X = Cl, Br, & I

Reaction products are examples of Markovnikov addition

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CCH3

CH3CH2 CCH3

CH3CH3

Cl+ CHCH3 CH2

Cl

CH3HCl

major minor

A REGIOSELECTIVE REACTIONA REGIOSELECTIVE REACTION

One of the possible products is formed in larger amounts than the other one(s).

Compare

REGIOSPECIFICREGIOSPECIFIC Only one of the possible products is formed (100%).

REGIOSELECTIVEREGIOSELECTIVE

THIS IS

>90% <10%

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Regiospecificity• Markovnikov’s Rule: The proton of an

acid adds to the carbon in the double bond that already has the most H’s. “Rich get richer.”

• More general Markovnikov’s Rule: In an electrophilic addition to an alkene, the electrophile adds in such a way as to form the most stable intermediate.

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

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MARKOVNIKOFF RULEMARKOVNIKOFF RULE

CH2

+ HCl

CH3Cl

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

..... conversely, the anion X adds to the most highly substituted carbon ( the carbon with most alkyl groups attached).

majorproduct

PREDICTING THE MAJOR PRODUCT

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Markovnikoff formulated his rule by observingthe results of hundreds of reactions that heperformed.

AN “EMPIRICAL” RULEAN “EMPIRICAL” RULE

EMPIRICAL = DETERMINED BY OBSERVATION

He had no idea why the reaction worked thisway, only that as a general rule it did give the stated result.

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SOME ADDITIONAL EXAMPLESSOME ADDITIONAL EXAMPLES

CH3

+ HCl

CH3Cll

CH2

+ HCl

CH3Cl

CH CH2 CH CH3Cl+ HCl

Only the major product is shown - all are regioselective.

All these reactions follow the Markovnikoff Rule.

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ANOTHER WAY TO STATE THE RULE

When the reaction forms the carbocation intermediate,the most highly substituted carbocation is favored : tertiary > secondary > primary.

MARKOVNIKOFF RULEMARKOVNIKOFF RULE

methyl carbocation

primary carbocation

secondary carbocation

tertiary carbocation

leastfavored

mostfavored

CR

R

R+

R CH R+

R CH2+

CH3+

(lowest energy)

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Addition Reactions of Alkenes and Alkynes

Reactions of alkynes resemble those of alkenes:

CH3CH2C CCH2CH3

HCl

CH3CH2CH CClHCH3CH3

Cl

H

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Addition Reactions of Alkenes and Alkynes

CH3CH2CH CClHCH3CH3

HCl

Cl

H

Cl

ClH

H

3,3-dichlorohexane

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Alkene SynthesisOverview

• E2 dehydrohalogenation (-HX)• E1 dehydrohalogenation (-HX)• Dehalogenation of vicinal dibromides (-X2)

• Dehydration of alcohols (-H2O) =>

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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+

=>

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End of Chapter 3