Alkenes - CnH2n - Naming.

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Chapter 6

• Alkenes - CnH2n - Naming.
• Degree of unsaturation.
• Stereoisomers - cis, trans, E, Z.
• Alkene stability.
• Electrophilic Addition.
• The Hammond Postulate.

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Alkene - CnH2n
ethene or ethylene
propene or propylene
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Alkene - CnH2n
trans-2-butene
cis-2-butene
2-Methlypropene
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Alkene - CnH2n
1-Pentene
2-Pentene
2-Pentene
?-
?-
2-Methyl-1-butene
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Alkene - CnH2n
4,7-Dimethyl-2,5-octadiene
3-Methylcyclohexene
3,4-Diisopropyl-2,5-dimethyl-3--hexene
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Alkene - CnH2n
2,3-Dimethylcyclopentene
4,4-Dimethylheptene
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Chapter 6

• Alkenes - CnH2n - Naming.
• Degree of unsaturation.
• Stereoisomers - cis, trans, E, Z.
• Alkene stability.
• Electrophilic Addition.
• The Hammond Postulate.

8
Degree of Unsaturation.
Page 270, problem 7.44
Compound A has the formula C8H8. It reacts
rapidly with KMnO4 to give CO2 and a carboxylic
acid, B (C7H6O2), but reacts with only 1 molar
equivalent of H2 on catalytic hydrogenation over
a palladium catalyst. On hydrogenation under
conditions that reduce aromatic rings, 4
equivalents of H2 are taken up, and hydrocarbon C
(C8H16) is produced. What are the structures of
A, B, and C? Write the reactions.
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Degree of Unsaturation.
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Rules for Degree of Unsaturation.
See page 191.
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etc.
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Given the oUS many possibilities.
Given the oUS composition possibilities
reduced.
Given the oUS, composition, chemical
information often compounds identity can be
determined.
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Degree of Unsaturation.
Page 270, problem 7.44
Compound A has the formula C8H8. It reacts
rapidly with KMnO4 to give CO2 and a carboxylic
acid, B (C7H6O2), but reacts with only 1 molar
equivalent of H2 on catalytic hydrogenation over
a palladium catalyst. On hydrogenation under
conditions that reduce aromatic rings, 4
equivalents of H2 are taken up, and hydrocarbon C
(C8H16) is produced. What are the structures of
A, B, and C? Write the reactions.
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Chapter 6

• Alkenes - CnH2n - Naming.
• Degree of unsaturation.
• Stereoisomers - cis, trans, E, Z.
• Alkene stability.
• Electrophilic Addition.
• The Hammond Postulate.

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Stereoisomers - cis, trans
p 196. Disustituted means two substituents other
than hydrogen that are bonded to the double-bond
carbons.
trans
cis
neither
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Stereoisomers - E, Z.
Trisubstituted
Tetrasubstituted
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Stereoisomers - E, Z.
high
high
low
high
low
low
low
high
E
Z
ze zame zide
(Z)-3-Methyl-2-pentene
(E)-2,2,3,4-Tetramethyl-3-hexene
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Stereoisomers - E, Z.
Assign E or Z configuration
low
high
high
low
high
low
Z
low
high
Z
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Chapter 6

• Alkenes - CnH2n - Naming.
• Degree of unsaturation.
• Stereoisomers - cis, trans, E, Z.
• Alkene stability.
• Electrophilic Addition.
• The Hammond Postulate.

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Alkene Stability
Evidence heats of hydrogenation
Reasons hyperconjugation bond strength
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Hyperconjugation
p 205.
interaction between the unfilled antibonding
C C? bond orbital and a filled C - H ? bond
orbital on a neighboring substituent...
The more subtituents that are present, the more
opportunities exist for hyperconjugation, and the
more stable the alkene.
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Bond Strength
p 206.
More highly substituted alkenes always have a
higher ratio of sp3 - sp2 to sp3 - sp3 bonds than
less highly substituted alkenes and are therefore
more stable.
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Heats of Hydrogenation - cis
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Heats of Hydrogenation - trans
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Heats of Hydrogenation - cis trans
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Heats of Hydrogenation - cis trans reaction
profile
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Chapter 6

• Alkenes - CnH2n - Naming.
• Degree of unsaturation.
• Stereoisomers - cis, trans, E, Z.
• Alkene stability.
• Electrophilic Addition.
• The Hammond Postulate.

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Electrophilic Addition - symmetric
2o carbocation
nucleophile
electrophile
2o carbocation
(Lewis base)
(Lewis acid)
nucleophile
(Lewis base)
electrophile
only one product !!!!
(Lewis acid)
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Electrophilic Addition - asymmetric
3o carbocation
nucleophile
electrophile
2o carbocation
(Lewis acid)
(Lewis base)
electrophile
(Lewis acid)
Markonikovs Rule
(Major)
nucleophile
(Lewis base)
regiospecific
(Minor)
electrophile
(Lewis acid)
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Electrophilic Addition - examples
Predict major product
ether
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Electrophilic Addition - examples
Predict major product
ether
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Electrophilic Addition - examples
Predict major product
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Electrophilic Addition - examples
Predict major product
ether
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Electrophilic Addition - examples
Predict major product
ether
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Electrophilic Addition - examples
Predict major product
ether
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Stability of Carbocation
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Evidence of Carbocation
minor
major
H-shift
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Electrophilic Addition - examples
Predict all products
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H-shift
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Stability of Carbocation

• Inductive effect.
• Hyperconjugation

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Inductive Effect
R is an alkyl group, larger and polarizable.
C
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Hyperconjugation
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Chapter 6

• Alkenes - CnH2n - Naming.
• Degree of unsaturation.
• Stereoisomers - cis, trans, E, Z.
• Alkene stability.
• Electrophilic Addition.
• The Hammond Postulate.

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Experimental Data
heat
CH3CH2CH3 2Cl2 ? CH3CHClCH3 CH3CH2CH2Cl 2HCl
?
?
heat
CH3CH2CH3 2Br2 ? CH3CHBrCH3 CH3CH2CH2Br 2HBr
?
?
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Experimental Data
heat
CH3CH2CH3 2Cl2 ? CH3CHClCH3 CH3CH2CH2Cl 2HCl
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heat
CH3CH2CH3 2Br2 ? CH3CHBrCH3 CH3CH2CH2Br 2HBr
8
92
Regiospecificity stronger in Bromine and
Chlorine. Why?
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Hammond Postulate
CH3CH3 Cl ? CH3CH2 HCl ?H -21 kJ/mol
CH3CH3 Br ? CH3CH2 HBr ?H 42 kJ/mol
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Hammond Postulate
CH3CH2 ------- H -- Br
CH3CH2 -- H ------- Cl
CH3CH2 HBr
E
E
CH3CH3 Cl
-21kJ/mol
42 kJ/mol
CH3CH2 HCl
CH3CH3 Br
RC
RC
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Carbocation
Ease of breaking 3o gt 2o gt 1o
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Hammond Postulate Carbocation
E
RC
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Chapter 6

• Alkenes - CnH2n - Naming.
• Degree of unsaturation.
• Stereoisomers - cis, trans, E, Z.
• Alkene stability.
• Electrophilic Addition.
• The Hammond Postulate.