Lecture21 - Lecture Summary 21 Chapter 7 Alkenes Reactions...

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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 O Os 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. O3 then Zn/HOAc O + 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 ? HO 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 heat 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 F polyvinylchloride PVC page 3 F FF F * * F FFF Teflon Chem 341 ...
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