Transcript Alkenes - Gadjah Mada University

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
Substitusi
Diels Alder
Pemutusan
2
Example : a mixture of  and  bonds, but no
triple bonds
3
Commercial Uses:
Ethylene
=>
4
Commercial Uses:
Propylene
=>
5
Other Polymers
=>
6
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
=>
7
Alkenes
Geometrical isomers are possible since there is no
rotation about a C=C  bond.
Cis- and trans- isomers possible.
8
Functional Group
• Pi bond is the functional group.
• More reactive than sigma bond.
9
Orbital Description
•
•
•
•
Sigma bonds around C are sp2 hybridized.
Angles are approximately 120 degrees.
No nonbonding electrons.
Molecule is planar around the double bond.
10
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.
=>
11
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.
=>
12
Name These Alkenes
CH2
CH CH2
CH3
1-butene
CHCH2CH3
CH3
C CH CH3
CH3
H3C
2-sec-butyl-1,3-cyclohexadiene
2-methyl-2-butene
CH3
3-methylcyclopentene
3-n-propyl-1-heptene
=>
13
Alkene Substituents
= CH2
methylene
(methylidene)
Name:
- CH = CH2
vinyl
(ethenyl)
- CH2 - CH = CH2
allyl
(2-propenyl)
=>
14
Common Names
• Usually used for small molecules.
• Examples:
CH3
CH2
CH2
ethylene
CH2
CH CH3
propylene
CH2
C CH3
=>
isobutylene
15
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.
=>
16
Name these:
H
CH3
Br
C C
CH3CH2
Br
C C
H
trans-2-pentene
H
H
cis-1,2-dibromoethene
=>
17
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).
=>
18
Example, E-Z
1
1
H3C
Cl
C C
H
2Z
1
H
CH2
2
2
Cl
2
CH CH3
C C
H
2
1
5E
(2Z, 5E)-3,7-dichloro-2,5-octadiene
=>
19
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.
=>
22
Alkenes
23
Disubstituted Isomers
• Stability: cis < geminal < trans isomer
• Less stable isomer is higher in energy, has
a more exothermic heat of hydrogenation.
Cis-2-butene
CH3
C C
H
Isobutylene
Trans-2-butene
CH3
H
(CH3)2C=CH2
H
CH3
28.6 kcal
C C
CH3
28.0 kcal
27.6 kcal
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 dipoledipole interactions occur.
 Alkyl groups are electron-donating toward the pi
bond, so may have a small dipole moment.
=>
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Polarity Examples
H3C
CH3
H
C C
H
CH3
C C
H
cis-2-bu te n e , bp °C
4
 = 0.33 D
H3C
H
trans-2-bu te n e , bp°C
1
=0
=>
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ADDITION REACTION
An addition reaction is one in which the two
reactants 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 BONDS
X
C C
C C
+ EX
E
electrophilic
reagent
EXAMPLES:
C C
+
conc.
+
C C
HCl
explained
later
C C
Cl
H2O
H
H2SO4
OH
C C
H
C C
+
conc.
H2SO4
0 oC
OSO3H
C C
H
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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:
H2C CH2 + Br2
H2C CH2
Br Br
Note that the C-C  bond has been replaced by two
C-Br  bonds.
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Electrophilic Addition
• Step 1: Pi electrons attack the electrophile.
E
C
C
+
+
E
C
C +
• Step 2: Nucleophile attacks the carbocation.
E
C C+
+
_
Nuc:
E Nuc
C C
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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.
CH3
CH3 C CH CH3
+
H
CH3
CH3 C CH CH3
H Br
X
+ Br
_
CH3
CH3 C CH CH3
+
H
=>
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Addition of HX (2)
CH3
CH3 C CH CH3
H Br
CH3
CH3 C CH CH3
+
H
_
Br
CH3
CH3 C CH CH3
+
H
+ Br
_
CH3
CH3 C CH CH3
=>
Br H
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Reaksi Adisi via Intermediet Karbokation
Hidrasi
Adisi Hidrogen halida
OH
R
R
CH CH2
+
H
+
R CH CH3
secondary
carbocation
(prim ary R+
not form ed)
alcohol
H2 O
X
-
X
CH CH3
R
CH CH3
alkyl halide
where X = Cl, Br, & I
Reaction products are examples of Markovnikov addition
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THIS IS
A REGIOSELECTIVE REACTION
CH3
CH3
C CH2
CH3
HCl
CH3
>90%
C CH3 + CH3 CH CH2
Cl
major
REGIOSELECTIVE
CH3
<10%
Cl
minor
One of the possible products is
formed in larger amounts than
the other one(s).
Compare
REGIOSPECIFIC
Only one of the possible products
is formed (100%).
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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.
=>
37
MARKOVNIKOFF RULE
PREDICTING THE MAJOR PRODUCT
When adding HX to a double bond,
the hydrogen of HX goes to the carbon
which already has the most hydrogens
CH2
CH3
+ HCl
Cl
major
product
..... conversely, the anion X adds to the
most highly substituted carbon
( the carbon with most alkyl groups attached).
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AN “EMPIRICAL” RULE
Markovnikoff formulated his rule by observing
the results of hundreds of reactions that he
performed.
EMPIRICAL = DETERMINED BY OBSERVATION
He had no idea why the reaction worked this
way, only that as a general rule it did give the
stated result.
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SOME ADDITIONAL EXAMPLES
Only the major product is shown - all are regioselective.
CH 3
+ HCl
CH 2
+ HCl
CH CH 2
+ HCl
CH 3
Cll
CH 3
Cl
CH CH 3
Cl
All these reactions follow the Markovnikoff Rule. 40
MARKOVNIKOFF RULE
ANOTHER WAY TO STATE THE RULE
When the reaction forms the carbocation intermediate,
the most highly substituted carbocation is favored :
tertiary > secondary > primary.
least
favored
methyl carbocation
primary carbocation
secondary carbocation
most
tertiary carbocation
favored
(lowest energy)
+ CH3
R
CH2
+
R CH R
+
R
R
C
+
R
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Addition Reactions of Alkenes and Alkynes
Reactions of alkynes resemble those of alkenes:
CH3CH2C
CCH2CH3
HCl
Cl
CH3CH2CH CClHCH3CH3
H
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Addition Reactions of Alkenes and Alkynes
Cl
CH3CH2CH CClHCH3CH3
H
HCl
H
H
Cl
Cl
3,3-dichlorohexane
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Alkene Synthesis
Overview
•
•
•
•
E2 dehydrohalogenation (-HX)
E1 dehydrohalogenation (-HX)
Dehalogenation of vicinal dibromides (-X2)
Dehydration of alcohols (-H2O)
=>
44
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+
=>45
End of Chapter 3
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