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  Alkenes Oxidations Reactions Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Alkenes Oxidations Reactions

(i) Hydroxylation

Dihydroxylated products (glycols) are acquired in pyridine solution, by reaction with aqueous osmium tetroxide or potassium permanganate (pH > 8). Both reactions come out to proceed by similar mechanism; in the osmium reaction the metallocyclic intermediate can be isolated. In fundamental solution the purple permanganate anion is reduced to the green manganate ion, which is providing a nice color test for the double bond functional group. From the mechanism displayed here we would suppose syn-stereoselectivity in the bonding to oxygen and regioselectivity is not an issue.
When viewed in context with the earlier consider addition reactions, hydroxylation reaction might seem implausible. Osmium and Permanganate tetroxide have identical configurations, in which the metal atom take the center of a tetrahedral grouping of negatively charged oxygen atoms. A possible description is that an empty d-orbital of the electrophilic metal atom expands well further than the surrounding oxygen atoms and starts electron transfer from the double bond to the metal, in much similar approach noted above for platinum. The nucleophilic oxygens' Back-bonding to the antibonding π*-orbital completes this interaction. The product is formation of a metallocyclic intermediate, as displayed below.

2288_Functional Group Reactions Oxidation Homework Help.jpg

(ii) Epoxidation

Some oxidation reactions of alkenes give cyclic ethers in which both carbons of a double bond become bonded to similar oxygen atom. These products are called oxiranes or epoxides . A significant technique for preparing epoxides is by reaction with peracids, RCO3H. The oxygen-oxygen bond of this type of peroxide derivatives is not only weak (ca. 35 kcal/mole), but in this case is polarized so that the hydroxyl group is positive and the acyloxy group is negative. If we predict electrophilic character for the OH moiety, the equation may be written as:

65_Functional Group Reactions Oxidation Homework Help 1.jpg

It is not likely that a dipolar intermediate, as displayed above, is in fact formed. The epoxidation reaction is believed to happen in a single step with a transition state incorporating all of the bonding events displayed in the equation. Subsequently, epoxidations by peracids all the time have syn-stereoselectivity, and rarely give structural rearrangement. Presumably the electron shifts point outs by the blue arrows induce a charge separation that is directly neutralized by the green arrow electron shifts.

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