Chapter 4
Reactions of Alkenes and Alkynes
1. The most important reaction of alkenes is the
addition to the C=C double-bond of various
reagents X-Y to yield saturated products
2. A second characteristic reaction of alkenes is
the formation of chain-growth polymers
Reactions of alkenes
Electrophilic addition reactions
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Addition of HX (Hydrohalogenation)
Addition of H2O
Addition of X2
Addition of H2
Hydroxylation with KMnO4
Oxidative cleavage of alkenes with acidic
KMnO4
Polymerization of alkenes
Addition of HX to Alkenes:
Hydrohalogenation
The of halogen acids, HX, to alkenes is a general
reaction that allows chemists to prepare a variety
of halo-substituted alkane products
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• A regiospecific reaction: The reactions are
regiospecific (regioselective) when only one of
two possible directions of addition occurs
Cl H
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CH3 — C — CH2
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CH3
Orientation of Alkene Addition Reactions:
Markovnilov’s Rule
• In the addition of HX to an alkene, the H attaches
to the carbon with fewer alkyl substituents, and
the X attaches to the carbon with more alkyl
substituents
• Electrophile; H+
H Cl
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CH3 — C — CH2
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CH3
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Carbocation Structure and Stability
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1.
2.
3.
The electronic structure of a carbocation
Bond angles about the positively charged carbon are 120°
Carbon uses sp2 hybrid orbitals to form sigma bonds to the
three attached groups
The unhybridized 2p orbital lies perpendicular to the sigma
bond framework and contains no electrons
• More highly substituted carbocation are more stable
• Alkyl groups tend to donate electrons to the positively
charged carbon atom
• The more alkyl groups there are, the more electron donation
there is and the more stable the carbocation
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Addition of H2O to Alkenes: Hydration
• Addition of water is called hydration
• Acid-catalyzed hydration of an alkene is
regioselective - H adds to the less
substituted carbon of the double bond
• Require high temperature and strongly
acidic condition
Other methods
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Addition of X2 to Alkenes: Halogenation
• Carried out with either the pure reagents or in an
inert solvent such as CCl4 or CH2Cl2
A test for a double bond Br2 (red) → no color
Anti stereochemistry
• Stereoselective reaction: a reaction in which
one stereoisomer is formed or destroyed in
preference to all others than might be formed
or destroyed
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Addition of H2 to Alkenes:
Hydrogenation
• Most alkenes react with H2 in the presence of a
transition metal catalyst to give alkanes
– commonly used catalysts are Pt, Pd, Ru, and Ni
• The process is called catalytic reduction or
catalytic hydrogenation
• Oxidation: the loss of electrons
• Reduction: the gain of electrons
Syn stereochemistry
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Oxidation of Alkenes: Epoxidation,
Hydroxylation and Cleavage
• The addition of oxygen
• Alkenes are oxidized to give epoxides on
treatment with a peroxyacid, RCOOOH
• Epoxides undergo an acid-catalyzed ring-opening
reaction with water (a hydrolysis) to give the
corresponding dialcohol, or diol, also called a
glycol
• Hyrdoxylation, the addition of an -OH group
• The hydroxylation of the alkene can also be
carried out by reaction with potassium
permanganate, KMnO4, in basic solution
• The reaction occurs with syn stereochemistry and
yields a 1,2-dialcohol, or cis-diol, product (also
called glycol)
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• When oxidation of the alkene is carried out with
KMnO4 in acidic solution, cleavage of the double
bond occurs and carbonyl-containing products are
obtained
• The double bond carbons
– contain two substutuents: the products are
ketone
– contain one substutuent: the products are
carboxylic acid
– contain two hydrogens: the products are CO2
Addition of Radical to Alkenes:
Polymers
• A polymer is a large molecule built up by
repetitive bonding together of many smaller
molecules (called monomer)
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–
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–
Cellulose (sugar)
Proteins (amino acid)
Nucleic acid (nucleotide)
Synthetic polymers
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• Many simple alkenes undergo rapid
polymerization when treated with a small amount
of a radical as catalyst
• High pressure (1000-3000 atm)
• High temperature (100-250℃)
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Radical polymerization of an alkene involves
three kinds of steps:
1. Initiation
2. Propagation
3. Termination
In the mechanism, a curved half-arrow, or “fishhook,”
is used to show the movement of a single electron
(several thousand monomers)
Step 1 Initiation: Reaction begins when a few radicals
are generated by the catalyst
• Benzoyloxy peroxide is used as initiator, the O-O
bond is broken on heating to yield benzoyloxy
radicals
• The benzoyloxy radicals then adds to the C=C
bond of ethylene to generate a carbon radical
Step 2 Propagation:
• Polymerization occurs when the carbon radical
formed in step 1 adds to another ethylene
molecule
• Repetition of this step for hundreds or thousands
of times builds the polymer chain
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Step 3 Termination:
Polymerization eventually stops when a reaction that
consumes the radical occurs
Combination of two growing chains is one possible
chain-terminating reaction
2 R-CH2CH2· → R-CH2CH2CH2CH2-R
Conjugated Dienes
A compound has altering single and double bonds – so-called
conjugated compound -– If the double bonds are well separated in a molecule, they react
independently, but they are close together, they may interact with
one another
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There is an electronic interaction between the two
double bonds of a conjugated diene because of p
orbital overlap across the central single bond
This interaction of p orbitals across a single bond
gives conjugated dienes some unusual properties
Buta-1,3-diene is a conjugated diene, whereas penta-1,4-diene
is a non-conjugated diene with isolated double bonds
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HX adds to a conjugated diene, mixtures of
products are often obtained
3-Bromobut-1-ene is the typical Markovnikov
product of 1,2-addition, but 1-bromobut-2-ene
appears unusual (1,4-addition)
Stability of Allylic Carbocations:
Resonance
Allylic carbocation
–
–
Next to the double bond
More stable than nonallylic
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All three carbon atoms are sp2-hybridized, and
each has a p orbital
The p orbital on the central carbon can overlap
equally well with p orbitals on either of the two
neighboring carbons
The two electrons are free to move about over the
entire three-orbital array
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The two individual structures of an allylic
carbocation are called resonance forms
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The only difference between the resonance forms is the
position of the bonding electrons
The atoms remain in exactly the same place in both
resonance forms – connections and 3-D shapes
An allylic carbocation has a single, unchanging
structure called a resonance hybrid that is blend of
the two individual forms
The greater the number of possible resonance forms,
the greater the stability – resonance leads to stability
Drawing and Interpreting Resonance Forms
The lengths of the two C-O
bonds are identical
The acetate ion is simply a
resonance hybrid of the
two resonance forms, with
both oxygens sharing the π
electrons and the negative
charge equally
1. Individual resonance forms are imaginary.
─ The real structure is a resonance hybrid of the
different resonance forms
2. Resonance forms differ only in the
placement of their π or non-bonding
electrons
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3. Different resonance forms of a substance
don’t have to be equivalent
1. Resonance forms must be valid Lewis
structures and obey normal rules of valency
2. Resonance leads to stability
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Localized electrons
– restricted to a particular locality
– belong to a single atom or stay in a bond
between two atoms
• Delocalized electrons
– not localized on a single atom, nor localized
between two atoms
− π or non-bonding electrons can be moved to
near atoms (sp2 atoms)
The greater the number of resonance forms, the
more stable of the substance
•
1. Toward a positive charge
2. Toward a π bond
3. Toward the more electronegative of the atoms (only π
electrons)
•
A compound with delocalized electrons is said to
have resonance
Alkynes and Their Reactions
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Alkynes are hydrocarbons that contain a carboncarbon triple bond
C≣C bond results from the overlap of two sphybridized carbon atoms and consists of one spsp σ bond and two p-p π bonds
The general formula is CnH2n-2
Alkynes are named by general rules similar to
those used for alkanes and alkenes
The suffix –yne
Internal alkynes and terminal alkynes
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• Common names: prefix the substituents on the
triple bond to the name “acetylene”
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Compounds containing both double and triple bonds are
called enynes (not ynenes)
Numbering of the hydrocarbon chain starts from the end
nearer the first multiple bond,whether double or triple
If there is a choice in numbering, double bond receive
lower number than triple bond
Addition of H2
• Lindlar’s catalyst can be prepared by precipitating
palladium on calcium carbonate and treating it with
lead acetate and quinoline
• Syn-addition
• Converted into trans alkenes using Na or Li in
liquid ammonia
IUPAC
CH3C≣CH
Propyne
CH3C≣CCH3 CH2=CHC≣CH
But-2-yne
But-1-en-3-yne
Common
Methylacetylene
Dimethylacetylene
Vinylacetylene
Addition of HX
• Stopped after addition of 1 equivalent of HX
• An excess of HX leads to formation of a dihalide
product
Addition of X2
• Anti-addition
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Addition of H2O
• The enol product rearranges to a more stable
isomer, a ketone
Formation of acetylide anions
• When a terminal alkyne is treated with a strong
base such as sodium amide (NaNH2), the terminal
hydrogen is removed and an acetylide anion is
formed
• Acetylide anions are both acidic and nucleophilic
• A mixture of both possible ketones results when an
internal alkyne is hydrated
• Only a single product is formed from reaction of a
terminal alkyne
• Acetylide anion react with alkyl halides to subsitute
for the halogen and yield a new alkyne product
• It is a very useful method for preparing large
alkyne from small alkyne precursors
• Chapter 7
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