chapter 3. alkena dan alkuna: nomenklatur dan reaksinya
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Chapter 3. Alkena dan Alkuna: Nomenklatur dan Reaksinya. Tutik Dwi Wahyuningsih Jurusan Kimia FMIPA UGM 2011. Alkena dan Alkuna. Introduction: kegunaan alkena Struktur alkena Nomenklatur Alkena & Alkuna Nomenklatur E/Z Jenis/tipe ikatan rangkap dua Reaksi pada Alkena Adisi - PowerPoint PPT PresentationTRANSCRIPT
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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
=>
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Commercial Uses: Propylene
=>
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Other Polymers
=>
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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