chapter 3. alkena dan alkuna: nomenklatur dan reaksinya tutik dwi wahyuningsih jurusan kimia fmipa...
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Chapter 3. Alkena dan Alkuna: Nomenklatur dan Reaksinya
Tutik Dwi Wahyuningsih
Jurusan Kimia FMIPA UGM
2011
2
Introduction: kegunaan alkenaStruktur alkenaNomenklatur Alkena & AlkunaNomenklatur E/Z Jenis/tipe ikatan rangkap duaReaksi pada Alkena
Adisi Substitusi Diels Alder Pemutusan
Alkena dan Alkuna
7
Industrial Methods
• Catalytic cracking of petroleumLong-chain alkane is heated with a catalyst to
produce an alkene and shorter alkane.Complex mixtures are produced.
• Dehydrogenation of alkanesHydrogen (H2) is removed with heat, catalyst.
Reaction is endothermic, but entropy-favored.
• Neither method is suitable for lab synthesis =>
8
Alkenes
Geometrical isomers are possible since there is no rotation about a C=C bond.
Cis- and trans- isomers possible.
10
Orbital Description
• Sigma bonds around C are sp2 hybridized.• Angles are approximately 120 degrees.• No nonbonding electrons.• Molecule is planar around the double bond.
11
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.
=>
12
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. =>
13
Name These Alkenes
CH2 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 =>
14
Alkene Substituents
= CH2
methylene(methylidene)
- CH = CH2
vinyl(ethenyl)
- CH2 - CH = CH2
allyl(2-propenyl)
Name: =>
15
Common Names
• Usually used for small molecules.
• Examples:
CH2 CH2
ethylene
CH2 CH CH3
propylene
CH2 C CH3
CH3
isobutylene=>
16
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. =>
18
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). =>
19
Example, E-Z
C C
H3C
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 =>
21
• Ikatan rangkap dua terisolasi : dipisahkan oleh dua atau lebih ikatan tunggal.
• Ikatan rangkap dua terakumulasi : ikatan rangkap dua berdekatan.Contoh : 1,2-pentadiena
22
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.
=>
24
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=>
25
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. =>
26
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
27
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.
28
ELECTROPHILIC ADDITION TO DOUBLE BONDSELECTROPHILIC ADDITION TO DOUBLE BONDS
EXAMPLES:
conc.
conc.
OSO3H
C C
H
C C
H
OH
C C
H
Cl
+
+
+
C C
C C
C C
C C
E
X
C C + EX
H2SO4H2O
H2SO4
HCl
0 oC
electrophilicreagent
explainedlater
29
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.
30
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.
31
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 CH2
Br Br
32
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.
33
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
34
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
=>
35
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 halide
where X = Cl, Br, & I
Reaction products are examples of Markovnikov addition
36
CCH3
CH3
CH2 CCH3
CH3
CH3
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
REGIOSPECIFICREGIOSPECIFICOnly one of the possible products is formed (100%).
REGIOSELECTIVEREGIOSELECTIVE
THIS IS
>90% <10%
37
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. =>
38
MARKOVNIKOFF RULEMARKOVNIKOFF RULE
CH2
+ HCl
CH3
Cl
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
39
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.
40
SOME ADDITIONAL EXAMPLESSOME ADDITIONAL EXAMPLES
CH3
+ HCl
CH3
Cll
CH2
+ HCl
CH3
Cl
CH CH2 CH CH3
Cl+ HCl
Only the major product is shown - all are regioselective.
All these reactions follow the Markovnikoff Rule.
41
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)
42
Addition Reactions of Alkenes and Alkynes
Reactions of alkynes resemble those of alkenes:
CH3CH2C CCH2CH3
HCl
CH3CH2CH CClHCH3CH3
Cl
H
43
Addition Reactions of Alkenes and Alkynes
CH3CH2CH CClHCH3CH3
HCl
Cl
H
Cl
ClH
H
3,3-dichlorohexane
44
Alkene SynthesisOverview
• E2 dehydrohalogenation (-HX)
• E1 dehydrohalogenation (-HX)
• Dehalogenation of vicinal dibromides (-X2)
• Dehydration of alcohols (-H2O) =>
45
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+ =>