Química Orgânica I - ualg.ptw3.ualg.pt/~abrigas/QOI7a_alken.pdf · 1 AFB QO I 2007/08 1 Química...
Transcript of Química Orgânica I - ualg.ptw3.ualg.pt/~abrigas/QOI7a_alken.pdf · 1 AFB QO I 2007/08 1 Química...
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Química Orgânica I
Ciências Farmacêuticas
Bioquímica
Química
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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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Adaptado de:
� Organic Chemistry, 6th Edition; L. G. Wade, Jr.
� Organic Chemistry, 6th edition; McMurry’s
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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.
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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
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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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Heteroatoms
� Halogens take the place of hydrogens, so add their number to the number of H’s.
� Oxygen doesn’t change the C:H ratio, so ignore oxygen in the formula.
� Nitrogen is trivalent, so it acts like half a carbon.
C
H
H
C
H
H
N C
H
HH
=>
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Structure for C6H7N?
� Since nitrogen counts as half a carbon, the number of H’s if saturated is 2(6.5) + 2 = 15.
� Number of missing H’s is 15 – 7 = 8.
� Element of unsaturation is 8 ÷ 2 = 4.
e.g. anilina=>
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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-pentene
trans-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-octadiene
3,7-dichloro-(2Z, 5E)-octa-2,5-diene
=>
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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.
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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. < 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=>
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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.
=>
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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
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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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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.
=>
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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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E2: Cyclohexanes
Leaving groups must be trans diaxial. =>
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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
=>
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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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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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