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  Transformation of alkenes, Chemistry tutorial

Introduction:

There is a diversity of oxidation reactions in which C=C bonds add oxygen or are cleaved to the oxygenated products. Whenever oxygen adds to C=C bonds, the products are Epoxides or 1,2-diols. Some of the most significant reactions of alkenes comprise oxidation. Whenever we discuss of oxidation, we generally mean reactions which form carbon-oxygen bonds. (Halogens are the oxidizing agents and the addition of a halogen molecule across a double bond is generally an oxidation as well.) Oxidations are specifically significant as most of the common functional groups include oxygen, and alkene oxidations are some of the best techniques for introducing oxygen into organic molecules.

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Fig: Transformation of Alkenes

Conversion of alkenes to Epoxides (epoxidation):

The epoxide is three-membered cyclic ether, as well termed as an oxirane or oxacyclopropane. Epoxides are important synthetic intermediates employed for transforming alkenes to a variety of other functional groups. The alkene is transformed to an epoxide via a peroxyacid, a carboxylic acid that consists of an additional oxygen atom in a -O-O- (peroxy) linkage.  

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Fig: Conversion of alkenes to Epoxides

The epoxidation of an alkene is obviously an oxidation, as oxygen atom is added. Peroxyacids are extremely selective oxidizing agents. A peroxyacid epoxidizes the alkene via a concerted electrophilic reaction in which some bonds are broken and some others are formed at similar time. Beginning by the alkene and the peroxyacid, a one-step reaction provides the epoxide and the acid directly, devoid of any intermediates. The reaction procedure is a single step (concerted) transfer of an oxygen atom to the C=C. For illustration, by employing peroxycarboxylic acids like m-chloroperbenzoic acid (MCPBA), perbenzoic acid or peracetic acid, 2,3-dimethyl-2-butene is transformed to the respective epoxide.

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Fig: Example of epoxidation

Similar to hydrogen peroxide, H2O2, peroxycarboxylic acids are oxidizing agents and are frequently employed for that purpose. Peroxycarboxylic acids are usually unstable and should be stored in the cold or, rather, be ready as required. The significant exception is 3-chloroperoxybenzoic acid, a remarkably stable crystalline solid now available commercially. This reagent gives a simple and expedient one-step route to Epoxides.

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Fig: 3-chloroperoxybenzoic acid-epoxidation

Conversion of alkenes to syn-1,2-diols (hydroxylation):

Alkenes are oxidized readily through potassium permanganate, KMnO4; however the products based on the reaction conditions. Cold dilute potassium permanganate reacts by double bonds to provide vicinal diols that are generally known as glycols.

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Fig: Hydroxylation

Reaction conditions require to be cautiously controlled. Outcomes are variable and generally low. The reaction takes place by syn addition and is thought to comprise an intermediate cyclic manganate ester which is rapidly hydrolyzed.

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Fig: Conversion of alkenes to syn-1,2-diols

The cis-diol can be isolated from the osmate ester by H2S, however a more convenient (and less expensive) process comprises the combination of hydrogen peroxide by a catalytic amount of osmium tetroxide. The osmate ester is made up however is transformed via the peroxide to the cis-diol. Osmium tetroxide is continually regenerated, in such a way that merely a small amount need be utilized.

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Fig: Isolation of cis-diol

Whenever more concentrated solutions of potassium permanganate are employed in the oxidation of alkenes, the initially made glycol is oxidized further. The product is a mixture of ketones or carboxylic acids, based on the extent of replacement of the double bond.

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Fig: Potassium permanganate used in the oxidation of alkenes

This is not a general reaction in organic synthesis as the results are generally low. Oxidative cleavage of the double bond can usually be acquired in better yield via reaction with ozone.

Ozonolysis of alkenes (oxidative cleavage):

The reactions of alkenes by ozone are generally carried out via passing ozone-containing air via a solution of the alkene in an inert solvent at low temperatures (generally-80oC). Reaction is fast and completion of reaction is found out by testing the effluent gas with potassium iodide. Unreacted ozone reacts to provide iodine. Appropriate solvents for Ozonization comprise methylene chloride, alcohol and ethylacetate. The primary made addition product, the molonozonide, rearranges fast, even at low temperatures, to the ozonide structure:

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Fig: Ozonolysis of alkenes

In several cases polymeric structures are obtained. Some of the ozonides, particularly the polymeric structures, decompose by explosive violence on heating; therefore, the ozonides are usually not isolated however are decomposed directly to the desired products. Hydrolysis with water takes place readily to provide carbonyl compounds and hydrogen peroxide.

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Fig: Hydrolysis with water

Aldehydes are oxidized via hydrogen peroxide to carboxylic acids. Therefore, reductions are often employed in decomposing the ozonides. These conditions comprise zinc dust and acetic acid, catalytic hydrogenation and dimethyl sulphide.

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Fig: Aldehydes oxidized by hydrogen peroxide

The treatment of ozonide by sodium borohydride provides the corresponding alcohols.

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Fig: Treatment of the ozonide

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