Química Orgânica I - UAlgw3.ualg.pt/~abrigas/QOI0809A_ALKEN.pdf · reaction occurring. => 4 ......
Transcript of Química Orgânica I - UAlgw3.ualg.pt/~abrigas/QOI0809A_ALKEN.pdf · reaction occurring. => 4 ......
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 1
Química Orgânica I
2008/09
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Hidrocarbonetos insaturados
Terpenes: antiviral,
antiseptic, anti-inflammatory.
OLEFINS“oil-forming gas”
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• first algal pheromone (Müller DG et al., Science 1971,
171, 815).
• 117 y
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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
=>
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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° =>
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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.
=>
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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. =>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 8
6.2 Degree of Unsaturation
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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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 10
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
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Cycloalkene nomenclature
CH3 CH3CH31
2
1
2
3
1
2
4
1-methylcyclohexene 3-methylcyclohexene 4-methylcyclohexene
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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
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6-vinyltridecane
4-isopropyldecane
4-vinyloctane
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Alkene Substituents
= CH2
methylene
(methylidene)
- CH = CH2
vinyl
(ethenyl)
- CH2 - CH = CH2
allyl
(2-propenyl)
Name: =>
methylenecyclohexane ethenylbenzene
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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
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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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Name these:
C C
CH3
H
H
CH3CH2
C C
Br
H
Br
H
trans-2-pentenetrans-pent-2-ene
cis-1,2-dibromoethene
=>
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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
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 20
Commercial Uses: Ethylene
=>
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Commercial Uses: Propylene
=>
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Other Polymers
=>
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Stability of Alkenes
• Measured by heat of hydrogenation:
Alkene + H2 → Alkane + energy
• More heat released, higher energy alkene.
w3.ualg.pt\~abrigas QOI 0809 ALKEN 24
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.
=>
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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.
-116 kJTrans-2-butene
-117 kJ(CH3)2C=CH2Isobutylene
-120 kJCis-2-butene CH3C C
CH3
H H
HC C
CH3
CH3 H=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 26
Relative Stabilities
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Hyperconjugation
• Electrons in neighboring filled σ orbital stabilize vacant antibonding π orbital – net positive interaction
• Alkyl groups are more stabilizing than H
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Bond strengths/hybridization effects
• sp3-sp3 bond is weaker than sp3-sp2, sp2-sp2
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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.
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 30
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.=>
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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
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 34
Alkene SynthesisOverview
• E2 dehydrohalogenation (-HX)
• E1 dehydrohalogenation (-HX)
• Dehalogenation of vicinal dibromides (-X2)
• Dehydration of alcohols (-H2O)
=>
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 35
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.
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 36
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
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Hofmann Product
• Bulky bases abstract the least hindered H+
• Least substituted alkene is major product.
=>
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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 =>
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 39
E2: Cyclohexanes
Leaving groups must be trans diaxial. =>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 40
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 =>
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Removing HX via E1
• Secondary or tertiary halides
• Formation of carbocation intermediate
• May get rearrangement
• Weak nucleophile
• Usually have substitution products too
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 42
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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Dehydration Mechanism
=>
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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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REACTIONS
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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+
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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.
w3.ualg.pt\~abrigas QOI 0809 ALKEN 48
Types of Additions
=>
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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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 50
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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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
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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.
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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.
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 54
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.
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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.
w3.ualg.pt\~abrigas QOI 0809 ALKEN 56
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
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29
w3.ualg.pt\~abrigas QOI 0809 ALKEN 57
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 58
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+
![Page 30: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/30.jpg)
30
w3.ualg.pt\~abrigas QOI 0809 ALKEN 59
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 60
Indirect Hydration
• Oxymercuration-Demercuration– Markovnikov product formed
– Anti addition of H-OH
– No rearrangements
• Hydroboration– Anti-Markovnikov product formed
– Syn addition of H-OH
=>
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31
w3.ualg.pt\~abrigas QOI 0809 ALKEN 61
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
+
w3.ualg.pt\~abrigas QOI 0809 ALKEN 62
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
=>
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32
w3.ualg.pt\~abrigas QOI 0809 ALKEN 63
Hg
OAc
w3.ualg.pt\~abrigas QOI 0809 ALKEN 64
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33
w3.ualg.pt\~abrigas QOI 0809 ALKEN 65
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
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 66
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_
=>
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34
w3.ualg.pt\~abrigas QOI 0809 ALKEN 67
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 68
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
![Page 35: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/35.jpg)
35
w3.ualg.pt\~abrigas QOI 0809 ALKEN 69
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 70
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
=>
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36
w3.ualg.pt\~abrigas QOI 0809 ALKEN 71
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
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 72
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 =>
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37
w3.ualg.pt\~abrigas QOI 0809 ALKEN 73
BH3 Is a Lewis Acid
• Six electrons in outer shell
• Coordinates to oxygen electron pairs in ethers
w3.ualg.pt\~abrigas QOI 0809 ALKEN 74
• 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
![Page 38: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/38.jpg)
38
w3.ualg.pt\~abrigas QOI 0809 ALKEN 75
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 76
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)
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39
w3.ualg.pt\~abrigas QOI 0809 ALKEN 77
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
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 78
Hydroboration
• More stable carbocation is also consistent with steric
preferences
![Page 40: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/40.jpg)
40
w3.ualg.pt\~abrigas QOI 0809 ALKEN 79
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 80
Hydrogenation
• Alkene + H2 → Alkane
• Catalyst required, usually Pt, Pd, or Ni.
• Finely divided metal, heterogeneous
• Syn addition
=>
![Page 41: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/41.jpg)
41
w3.ualg.pt\~abrigas QOI 0809 ALKEN 81
Catalytic hydrogenation
w3.ualg.pt\~abrigas QOI 0809 ALKEN 82
![Page 42: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/42.jpg)
42
w3.ualg.pt\~abrigas QOI 0809 ALKEN 83
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 84
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=>
![Page 43: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/43.jpg)
43
w3.ualg.pt\~abrigas QOI 0809 ALKEN 85
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 86
Simmons-Smith
Best method for preparing cyclopropanes.
CH2I2 + Zn(Cu) ICH2ZnI
a carbenoid
CH2I2
Zn, CuCl=>
![Page 44: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/44.jpg)
44
w3.ualg.pt\~abrigas QOI 0809 ALKEN 87
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=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 88
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
=>
![Page 45: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/45.jpg)
45
w3.ualg.pt\~abrigas QOI 0809 ALKEN 89
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
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 90
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 =>
![Page 46: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/46.jpg)
46
w3.ualg.pt\~abrigas QOI 0809 ALKEN 91
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 92
Bromonium Ion
![Page 47: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/47.jpg)
47
w3.ualg.pt\~abrigas QOI 0809 ALKEN 93
Mechanism (2)
Halide ion approaches from side opposite
the three-membered ring.
CC
Br
Br
CC
Br
Br
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 94
Examples of Stereospecificity
=>
![Page 48: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/48.jpg)
48
w3.ualg.pt\~abrigas QOI 0809 ALKEN 95
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 96
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+
=>
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49
w3.ualg.pt\~abrigas QOI 0809 ALKEN 97
Regiospecificity
The most highly substituted carbon has the most positive charge, so nucleophile
attacks there.
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 98
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+
=>
![Page 50: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/50.jpg)
50
w3.ualg.pt\~abrigas QOI 0809 ALKEN 99
Mechanism
One-step concerted reaction. Several
bonds break and form simultaneously.
OC
O
R
H
C
C
OO
H
OC
O
RC
C
+
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 100
C C
O
HO
O
C C
O
HO
O
![Page 51: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/51.jpg)
51
w3.ualg.pt\~abrigas QOI 0809 ALKEN 101
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
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 102
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
=>
![Page 52: 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](https://reader035.fdocuments.in/reader035/viewer/2022082208/5a9ec0657f8b9a89178bc70c/html5/thumbnails/52.jpg)
52
w3.ualg.pt\~abrigas QOI 0809 ALKEN 103
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
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 104
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
=>
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53
w3.ualg.pt\~abrigas QOI 0809 ALKEN 105
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
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 106
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
=>
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54
w3.ualg.pt\~abrigas QOI 0809 ALKEN 107
Osmium Tetroxide Catalyzed
Formation of Diols
• Hydroxylation - converts to syn-diol
• Osmium tetroxide, then sodium bisulfate
• Via cyclic osmate di-ester
w3.ualg.pt\~abrigas QOI 0809 ALKEN 108
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.
=>
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 109
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.
w3.ualg.pt\~abrigas QOI 0809 ALKEN 110
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
+
=>
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 111
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 112
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.
=>
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 113
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
=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 114
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 115
Examples of Ozonolysis of Alkenes
• Used in determination of structure of an unknown alkene
w3.ualg.pt\~abrigas QOI 0809 ALKEN 116
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.
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 117
Krebs
w3.ualg.pt\~abrigas QOI 0809 ALKEN 118
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
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 119
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 120
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) =>
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 121
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=>
w3.ualg.pt\~abrigas QOI 0809 ALKEN 122
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
=>
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 123
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 124
Long Chain Branching
• In long chains, a hydrogen from another chain is
abstracted
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 125
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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 126
polymers
http://www.chemistryland.com/PolymerPlanet/Polymers/PolymerTutorial.htmhttp://ocw.mit.edu/OcwWeb/Materials-Science-and-Engineering/3-051JSpring-2006/CourseHome/index.htm
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 127
• resumo
w3.ualg.pt\~abrigas QOI 0809 ALKEN 128
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 129
w3.ualg.pt\~abrigas QOI 0809 ALKEN 130
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w3.ualg.pt\~abrigas QOI 0809 ALKEN 131
• 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
w3.ualg.pt\~abrigas QOI 0809 ALKEN 132
Adaptado de:
• Organic Chemistry, 6th Edition; L. G.
Wade, Jr.
• Organic Chemistry, 6th edition; McMurry’s