Reactions of Alkenes and Alkynes.
Chapter 5
Chapter 5
1
Contents of Chapter 4

Electrophilic Addition Reactions

Carbocations

Various Addition Reactions
Chapter 5
2
Mechanism for Electrophilic
Addition to Alkenes



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
Chapter 5
3
Addition of Hydrogen Halides
to Alkenes
The more substituted carbocation is
preferred
Chapter 5
4
Stability of Carbocations
Alkyl groups (“R”s) tend to stabilize the
positive charge on the sp2 carbon of a
carbocation
Chapter 5
5
Transition State Stability
Formation of a tertiary carbocation should be
faster than formation of a primary carbocation
Chapter 5
6
Markovnikov’s Rule




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.
Chapter 5
7
Addition of Water
Chapter 5
8
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
Chapter 5
9
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
Chapter 5
10
Product Analysis
Chapter 5
11
Alkynes
Common names of alkynes are based
on substitution of the simplest alkyne,
acetylene
Chapter 5
12
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
Chapter 5
13
IUPAC Multifunctional
Compound Nomenclature
Chapter 5
14
IUPAC Multifunctional
Compound Nomenclature
•
•
•
•
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
Chapter 5
15
Reactivity Considerations



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
Chapter 5
16
Relative Stabilities of
Carbocations
Vinyl cations are one level less
stable than alkyl cations
Chapter 5
17
Halogen Addition to Alkynes


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
18
Addition of Water to Alkynes
Water adds to alkynes in the presence
of acid to yield an enol
Chapter 5
19
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
Chapter 5
20
Addition of Hydrogen to an
Alkyne
Chapter 5
21
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
Chapter 5
22
Syntheses Using Acetylide
Ions
Alkylation reactions work best with primary
alkyl halides and methyl halides
Chapter 5
23
Introduction to Multistep
Synthesis
The thought process is known as
retrosynthetic analysis and is indicated
by using open arrows
Chapter 5
24
Introduction to Multistep
Synthesis - Considerations




Alkene products made from alkynes
Carbonyl products made from alkynes
Alkane products made from alkenes
Understand issues involved in proper
choice of reagents thoroughly
Chapter 5
25
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
Chapter 5
26
Introduction to Multistep
Synthesis – Practice
Use retrosynthetic analysis to make these:
Chapter 5
27