Download Reactions of Alkenes and Alkynes

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8.9
Addition of Carbenes to Alkenes:
Cyclopropane Synthesis
A carbene, R2C:, is a neutral molecule containing a
divalent carbon with only six electrons in its valence
shell
• One simple method for generating dichlorocarbene is by
treatment of CHCl3 with KOH
• Carbenes behave as electrophiles, adding to alkenes to
yield cyclopropanes
CHP 8 Problems 8.1-22, 29-54, 58, 61-63.
Addition of Carbenes to Alkenes: Cyclopropane
Synthesis
Mechanism of the formation of dichlorocarbene
Addition of Carbenes to Alkenes: Cyclopropane
Synthesis
• Dichlorocarbene carbon atom is sp2-hybridized with a
vacant p orbital extending above and below the plane of the
three atoms with an unshared pair of electrons occupying
the third sp2 lobe
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Addition of Carbenes to Alkenes: Cyclopropane
Synthesis
• Reaction of dichlorocarbene with an alkene results in a
dichlorocyclopropane
• Addition is stereospecific, meaning that only a single
stereoisomer is formed as product
8.10 Radical Addition to Alkenes: Alkene
Polymers
Radicals add to alkene double bonds
• Radicals remove one electron from double bond
• One electron left behind yielding a new radical
Polymer
• A large molecule built up by repetitive bonding together of
many smaller molecules called monomers
• Cellulose (glucose polymer)
Radical Addition to Alkenes: Alkene Polymers
• Proteins (amino acid polymers)
• Nucleic acid (nucleotide polymer)
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Radical Addition to Alkenes: Alkene Polymers
Simplest polymerization
• Result when an alkene is treated with a small
amount of a radical as an initiator
Radical Addition to Alkenes: Alkene Polymers
Initiation
1.
2.
3.
Small amount of benzoyl peroxide catalyst is heated breaking
weak O-O bonds and yielding radicals
Benzoyloxy radical adds to C=C bond of ethylene forming a
carbon radical
a) One electron from C=C bond pairs up with electron of
benzoyloxy radical to form C-O bond
b) Other electron remains on carbon (a carbon-centered
radical)
Radical Addition to Alkenes: Alkene Polymers
Propagation
• Polymerization occurs when the carbon radical adds to
another ethylene molecule to yield another radical
Termination
• Chain process ends by a reaction that consumes a radical
•
Combination of two growing chains
2-R–CH2CH2 →
R–CH2CH2CH2CH2–R
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Radical Addition to Alkenes: Alkene Polymers
Vinyl monomers
•
Substituted ethylene
Undergo polymerization to yield polymer with substituted
groups regularly spaced in alternating carbon atom long chain
•
•
Polypropylene
•
Styrene
Radical Addition to Alkenes: Alkene Polymers
Polymerization of unsymmetrically substituted vinyl
monomers
Propylene or Styrene
• Radical addition steps can take place at either end of the
double bond to yield:
•
•
•
•
A primary radical intermediate (RCH2.)
A secondary radical (R2CH.)
Similar to electrophilic addition reaction
More highly substituted, secondary radical is formed
Worked Example 8.4
Predicting the Structure of a Polymer
Show the structure of poly(vinyl chloride), a polymer
made from H2C=CHCl, by drawing several
repeating units
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8.11 Biological Additions of Radicals to
Alkenes
Radical vs. Electrophilic Addition Reactions
Electrophilic addition
•
•
Reaction occurs once
Intermediate is then quenched and reaction stops.
Biological Additions of Radicals to Alkenes
Radical vs. Electrophilic Addition Reactions
Radical addition
•
•
•
Difficult to control
Limited use in the laboratory
Reaction intermediate is not quenched so reaction continues
Biological Additions of Radicals to Alkenes
Biological Reactions
• Only one substrate molecule at a time is present in
the active site of the enzyme where the reaction
occurs (necessary reactant groups nearby)
• More controlled
• More common than laboratory radical reactions
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Biological Additions of Radicals to Alkenes
Step 1 – formation of a carbon radical at C13
Step 2 – C13 radical reacts with O2 at C11 through resonance
form
Biological Additions of Radicals to Alkenes
Step 3 – Oxygen radical reacts with C8-C9 double bond
forming carbon radical at C8
Step 4 – C8 radical adds to C12-C13 double bond forming
carbon radical at C13
Biological Additions of Radicals to Alkenes
Step 5 – Resonance form of C13 carbon radical adds at C15
to a second O2 molecule
Step 6 – Reduction of O-O bond gives prostaglandin H 2
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