Hydroxylation and Oxidation of Alkenes – hydroxylation reaction
Several oxidizing reagents react with alkenes under mild conditions to give, as the overall result, addition of hydrogen peroxide as HO—OH. Of particular importance are alkaline permanganate (MnO4⊖) and osmium tetroxide (OSO4), both of which react in an initial step by a suprafacial cycloaddition mechanism like that postulated for ozone.
Each of these reagents produces cis-1,2-dihydroxy compounds (diols) with cycloalkenes:
Hydroxylation and Oxidation of Alkenes – Oxidation with Peroxidic Compounds/Oxacyclopropane (Oxirane) Formation
Alkenes can be oxidized with peroxycarboxylic acids, RCO3H, to give oxacy-clopropanes (oxiranes, epoxides), which are three-membered cyclic ethers:
The reaction, known as epoxidation, is valuable because the oxacyclopropane ring is cleaved easily, thereby providing a route to the introduction of many kinds of functional groups. In fact, oxidation of alkenes with peroxymethanoic acid (peroxyformic acid), prepared by mixing methanoic acid and hydrogen peroxide, usually does not stop at the oxacyclopropane stage, but leads to ring-opening and the subsequent formation of a diol:
This is an alternative scheme for the hydroxylation of alkenes. However, the overall stereochemistry is opposite to that in permanganate hydroxylation. For instance, cyclopentene gives trans-1,2-cyclopen-tanediol. First the oxirane forms by suprafacial addition and then undergoes ring opening to give the trans product:
The ring opening is a type of SN2 reaction. Methanoic acid is sufficiently acidic to protonate the ring oxygen, which makes it a better leaving group, thus facilitating nucleophilic attack by water. The nucleophile always attacks from the side remote from the leaving group:
The peroxyacids that are used in the formation of oxacyclopropanes include peroxyethanoic (CH3CO3H), peroxybenzoic (C6H5CO3H), and trifluoroper-oxyethanoic (CF3CO3H) acids. A particularly useful peroxyacid is 3-chloro-peroxybenzoic acid, because it is relatively stable and is handled easily as the crystalline solid. The most reactive reagent is trifluoroperoxyethanoic acid, which suggests that the peroxyacid behaves as an electrophile (the electronegativity of fluorine makes the CF3 group strongly electron-attracting). The overall reaction can be viewed as a cycloaddition, in which the proton on oxygen is transferred to the neighboring carbonyl oxygen more or less simultaneously with formation of the three-membered ring:
A reaction of immense industrial importance is the formation of oxacyclopropane itself (most often called ethylene oxide) by oxidation of ethene with oxygen over a silver oxide catalyst at 300°:
Oxacyclopropane is used for many purposes, but probably the most important reaction is ring opening with water to give 1,2-ethanediol (ethylene glycol, bp 197°). This diol, mixed with water, is employed widely in automotive cooling systems to provide both a higher boiling and lower freezing coolant than water alone:
Propene and higher alkenes are not efficiently epoxidized by oxygen and Ag2O in the same way as ethene is because of competing attack at other than the double-bond carbons. Hydroxylation and oxidation of alkenes are reactions that produces alcohol.
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