Transcript Structure and Bonding

Chapter 5 Alkenes and Alkynes
•Alkenes: hydrocarbons that contain a carbon-
carbon double bond. [CnH2n]
•CC=C
propene
•Alkynes: hydrocarbons containing a carbon-
carbon triple bond.
•CCCCC
•[CnH2n-2]
2-pentyne
Chapter 5 Alkenes and Mechanisms of Electrophilic
Addition
Hydrocarbons containing double bonds
C
C
double bond
the functional group
center of reactivity
P-Bonds as Nucleophiles:
4
Name These Alkenes
Alkene Nomenclature
7
Cycloalkene nomenclature
8
Configurational isomers(geometric isomers)
（构型异构体—几何异构体）---烯烃和环烷烃
Definition
Configurational isomers have the same
molecular formula and the same bonds.
However, some of the atoms are arranged
differently in space with respect to each other,
and the isomers cannot be interconverted
without breaking a covalent bond.
Substituted alkenes and cycloalkanes can exist as
configurational isomers.
Alkenes – cis andb trans isomerism
The cis and trans isomers of an alkene are configurational
isomers (also called geometric isomers) because they have
different shapes and cannot interconvert since the double
bond of an alkene cannot rotate. Therefore, the substituents
are ‘fixed’ in space relative to each other.
The methyl groups can be on the same side of the double bond
(the cis isomer or on opposite sides the trans isomer)
The cis and trans nomenclature for alkenes is an old method
of classifying the configurational isomers of alkenes and is
still commonly used.
Ranking Priorities: Cahn-Ingold-Prelog
Rules 系统命名法
• Must rank atoms that are connected at
comparison point
• Higher atomic number gets higher priority
In this case,The higher priority
groups are opposite:
(E )-1-bromo-1-chloro-propene
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Br
C=C
H
Br
Br
H
H
CIS - same side
H
H3C
C=C
H
Br
TRANS - across
H
C C
H
No cis-trans isomers possible
=>
Alkenes 烯烃的制备
Alkenes can be obtained by the transformation of various
functional groups such as the reduction of alkynes, the
elimination of alkyl halides,or the elimination of alcohols, the
elimination of from vicinal dibromides.
1. RX
-C=C-
2. ROH
3.
C C
-C=C-
C C
Electrophilic addition to alkenes
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Curved Arrows in Reaction Mechanisms
Movement of a pair of electrons
Movement of one electron
Formation of a Carbocation Intermediate
• HBr, a Lewis acid, adds to the  bond
• This produces an intermediate with a positive
charge on carbon - a carbocation
• This is ready to react with bromide
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The arrows track “electron movement”
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Reaction Diagram for Addition of HBr to
Ethylene
• Two separate
steps, each with
a own transition
state
• Energy
minimum
between the
steps belongs
to the
carbocation
reaction
intermediate.
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机理
机理？
Sometimes the reaction conditions used in this reaction are
too harsh（苛刻） since heating is involved and
rearrangement reactions can take place.
A milder method which gives better results is to treat the
alkene with mercuric acetate [Hg(OAc)2] then sodium
borohydride
Utilization of Curved Arrows
The arrows track “electron movement”
Carbocation Stabilities
Rearrangement of Carbocation
1,2-hydride shift(1,2-氢迁移）
a more stable
carbocation
Rearrangement of Carbocation
1,2-methyl shift
a more stable
carbocation
Addition of Borane
Hydroboration–Oxidation
Anti-Markovnikov’s rule in product formation
Anti-Markovnikov’s rule
Markovnikov’s rule
Branched polymers are more flexible
Addition of Hydrogen to Alkenes
Catalytic Hydrogenation of an Alkene
Alkenes to alkanes
Alkenes are converted to alkanes by treatment with
hydrogen over a finely alkanes divided metal catalyst such
as palladium, nickel, or platinum. This is an addition
reaction since it involves the addition of hydrogen atoms to
each end of the double bond. It is also called a catalytic
hydrogenation or a reduction reaction.
The catalyst is crucial since the reaction will not take place at
room temperature in its absence.
.
Note that the
hydrogen
molecule is
split once it
has been
added to
the catalyst.
Alkenes to aldehydes and ketones
臭氧化分解反应
Alkenes to carboxylic acids and ketones
Alkenes to1,2-diols
√
A Highly toxic and expensive reagent and
has to be handled with care.
cold alkaline
Alkenes to epoxides
The reaction is unusual in that there is no carbocation
intermediate, and involves a one-step process without
intermediates.
氧化
构型保持