Transcript Chapter 5

Reactions of Alkenes and Alkynes.
Chapter 5
Chapter 5
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Contents of Chapter 4
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Electrophilic Addition Reactions
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Carbocations
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Various Addition Reactions
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Mechanism for Electrophilic
Addition to Alkenes
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Reaction of 2-butene with hydrogen bromide is
typical of electrophilic addition to alkenes
The reaction starts with the slow addition of an
electrophile to an sp2 carbon, resulting in
formation of a carbocation
The next step is the rapid addition of a
nucleophile to the other sp2 carbon
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Addition of Hydrogen Halides
to Alkenes
The more substituted carbocation is
preferred
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Stability of Carbocations
Alkyl groups (“R”s) tend to stabilize the
positive charge on the sp2 carbon of a
carbocation
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Transition State Stability
Formation of a tertiary carbocation should be
faster than formation of a primary carbocation
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Markovnikov’s Rule
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Modern equivalent statement:
The electrophile adds to the sp2 carbon
that forms the least stable carbocation
The other carbon forms the carbocation
intermediate.
Predicting product: put + and – signs
under carbons and electrophile and
nucleophile and swap partners.
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Addition of Water
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Addition of Alcohol
Initiating the reaction: generating a carbocation
electrophilic attack of H +
H 3C
H
H+
H 3C
CH2
C CH2 + CH3CH2 OH
slow
H 3C
2-methylpropene
CH3
carbocation
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Addition of Alcohol
The resulting carbocation is attacked by
the nucleophilic alcohol
H
nucleophilic attack of ethanol
H 3C
H
CH2
CH3
+ CH3CH2 OH
fast
H3C CH2
CH3
H O
H2C CH3
ethanol
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Product Analysis
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Alkynes
Common names of alkynes are based
on substitution of the simplest alkyne,
acetylene
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IUPAC Nomenclature of
Alkynes
• Find the longest chain containing the triple
bond and change the corresponding “-ane”
ending to “-yne”
• The chain is numbered in direction that
gives the triple bond, the lower number
• If the same number for the triple bond is
obtained in numbering from both directions,
the number for the substituent nearest the
chain end is minimized
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IUPAC Multifunctional
Compound Nomenclature
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IUPAC Multifunctional
Compound Nomenclature
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The longest chain has to go past the highest-priority
functional group
High prio group has lowest possible number
If not highest priority NH2 is amino and OH is hydroxy
substituent
General form is n-substit-n-alken-n-yn-n-groupsuffix
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Reactivity Considerations
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The hydrohalogenation product is an alkene
which can undergo a second electrophilic
addition reaction
First halogen follows alkene hydrohalogenation
regioselectivity rules
Second halogen goes on same carbon as first
halogen
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Relative Stabilities of
Carbocations
Vinyl cations are one level less
stable than alkyl cations
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Halogen Addition to Alkynes
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Halogens add to alkynes as well as to
alkenes
Excess halogen leads to the addition of a
second equivalent
Cl2
CH3CH2C CCH3
CH2Cl2
Br2
CH3C CH
CH2Cl2
Cl
CH3CH2C CCH3
Cl
Br
CH3C CH
Br
Chapter 5
Cl2
CH2Cl2
Br2
CH2Cl2
Cl Cl
CH3CH2C CCH3
Cl Cl
Br Br
CH3C CH
Br Br
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Addition of Water to Alkynes
Water adds to alkynes in the presence
of acid to yield an enol
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Addition of Water to Alkynes
However the initially formed enol reacts
further to produce a ketone
Such isomers, differing only in the
placement of a hydrogen atom, are called
tautomers
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Addition of Hydrogen to an
Alkyne
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Acidity of a Hydrogen Bonded
to an sp Carbon
The conjugate bases have the following
relative base strength because the
stronger the acid, the weaker the
conjugate base
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Syntheses Using Acetylide
Ions
Alkylation reactions work best with primary
alkyl halides and methyl halides
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Introduction to Multistep
Synthesis
The thought process is known as
retrosynthetic analysis and is indicated
by using open arrows
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Introduction to Multistep
Synthesis - Considerations
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Alkene products made from alkynes
Carbonyl products made from alkynes
Alkane products made from alkenes
Understand issues involved in proper
choice of reagents thoroughly
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Introduction to Multistep
Synthesis – Reagent Issues
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Regioselectivity of HX + alkene/alkyne rxn
Regioselectivity of 2 HX + alkyne rxn
Stereospecificity of H2/Lindlar reduction
H2 with Pd/C, Pt/C, or Ni does complete reduction
RX + NaNH2 + H-C=C-R rxn works best with primary
halide
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Introduction to Multistep
Synthesis – Practice
Use retrosynthetic analysis to make these:
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