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Unformatted text preview: Lecture Summary 21
October 13, 2006
Chapter 7 - Alkenes: Reactions and Synthesis
Oxidation of Alkenes
There are several oxidizing agents that will oxidize alkenes to varying levels of oxidation states.
The addition of two oxygens to a double bond can be accomplished with osmium tetroxide. The
result is the formation of a 1,2-diol. This reaction is stereospecific to give the cis product. Note that
sodium bisulfite is required to break down the intermediate osmium complex. O
OO a) OsO4 intermediate not isolated OH b) NaHSO3 OH
CIS addition of OH's 1,2-Diols are useful and can be further oxidized with periodic acid. This results in cleaving the
carbon-carbon bond between the two alcohols and forming aldehydes on the end. OH O HIO4 OH O OH
IO O O O carbon-carbon
bond broken The cleavage of a carbon-carbon bond is useful for breaking apart molecules. A more
straightforward method exists to break a double bond directly to give carbonyl functional groups
on either end. This reaction involves the highly reactive ozone molecule. This reaction forms
intermediates that must be broken down with zinc in acetic acid.
O O3 Zn/HOAc O
O OO OO ©2006 Gregory R. Cook North Dakota State University page 1 Chem 341 Potassium permanganate is one of the strongest oxidizing agents. Not only will it cleave apart a
carbon-carbon double bond, but it will continue to oxidize any bonds to hydrogens that were
present on the alkene carbons. Note that if there are no hydrogens, a ketone is produced, if one
hydrogen, it will form a carboxylic acid, and if the alkene is terminal, possessing two hydrogens, it
will become carbon dioxide.
K Mn O 4
O O H + + O OH
H K Mn O 4
H H O C O OH
O K Mn O 4 OH HO
O Synthetic Strategy
One of the basic goals of organic chemists is to make molecules. Usually this requires a multi-step
synthesis starting from materials that are readily available from petroleum products or plant
materials. In order to carry a multi-step synthesis it takes strategic planning. The best way to
approach a synthetic challenge is to analyze the target structure and work backwards one step at
a time until you get back to your starting materials. As we build our repertoire of organic functional
group transformations, we will come back to this issue of synthetic strategy. O Say you were given cyclopentane as a starting material.
How would you prepare 1,5-pentandioic acid? This would
require a multistep synthesis, as there are no methods to
carry out this transformation in one step. O ?
Cl2 KMnO4 light
Cl OH KOH Working backwards, we could see that the diacid could be
prepared in one step from cyclopentene and potassium
permanganate. Cyclopentene can be prepared by elimination of chlorocyclopentane, and this can be prepared from
cyclopentane by free radical chlorination. ©2006 Gregory R. Cook North Dakota State University page 2 Chem 341 Alkene Polymerization
Polymers can be produced from alkenes by chain reactions using either radical or cationic
polymerization. These reactions are initiated by either a radical source, like benzoylperoxide, or a
cation source like a strong acid.
O O O
2 OO = In O radical polymerization is often
initiated by the formation of benzoyl
radicals from benzoyl peroxide. benzoylperoxide H In + In In H + *
polystyrene ©2006 Gregory R. Cook North Dakota State University Cationic polymerizaiton grows chains
in the same fashion. H Cl Cl * The chain grows by sequential
addition of radicals to ethylene. * F Cl
* F F
PVC page 3 F FF F
Teflon Chem 341 ...
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