Epoxidation, alkenes peroxyacetic acid

Relative Rates of Epoxidation of Some Representative Alkenes with Peroxyacetic Acid... 

Epoxidation of alkenes is a stereospecific syn addition Which stereoisomer of 2 butene reacts with peroxyacetic acid to give meso 2 3 epoxybu tane Which one gives a racemic mixture of (2/ 3/ ) and (25 35) 2 3 epoxybutane ... 

Another method of making epoxides is the electrophilic reaction of alkenes with a peroxy acid such as peroxyacetic acid (sometimes simply peracetic acid). Thus, cyclohexene may be converted into the epoxide in a single reaction. 

Direct anti hydroxylation of an alkene (without isolation of the epoxide intermediate) is possible by using an acidic aqueous solution of a peroxyacid. As soon as the epoxide is formed, it hydrolyzes to the glycol. Peroxyacetic acid (CH3CO3H) and peroxyformic acid (HCO3H) are often used for the anti hydroxylation of alkenes. 

Oxidation of Alkenes to Oxirans, using Peroxides. The peroxide (15 R = OOH) is a useful oxidant for a number of alkenes, giving epoxides in good to moderate yields and generating (15 R = OH). The reactivity of this peroxide is two orders of magnitude lower than that of peroxyacetic acid but at least one order of magnitude greater than that of a-peroxy-esters and -nitriles. Its selectivity relative to the structure of the alkene is similar to that for peroxyacetic acid. 

The rate of epoxidation of alkenes is increased by alkyl groups and other ERG substituents, and the reactivity of the peroxy acids is increased by EWG substituents." These structure-reactivity relationships demonstrate that the peroxy acid acts as an electrophile in the reaction. Low reactivity is exhibited by double bonds that are conjugated with strongly EWG substituents, and very reactive peroxy acids, such as trifluoroperoxyacetic acid, are required for oxidation of such compounds. " Strain increases the reactivity of alkenes toward epoxidation. Norbornene is about twice as reactive as cyclopentene toward peroxyacetic acid." trani-Cyclooctene is 90 times more reactive than cyclohexene." Shea and Kim found a good correlation between relief of strain, as determined by MM calculations, and the epoxidation rate. ° There is also a correlation with ionization potentials of the alkenes. Alkenes with aryl substituents are less reactive than unconjugated alkenes because of ground state stabilization and this is consistent with a lack of carbocation character in the TS. 

The answer is B. In this reaction, 2-butene is reacted with peroxyacetic acid. This reagent converts the alkene into an epoxide, which is a compound that has a three-sided ring with an oxygen atom in the ring. 

Reagents for epoxidation. Peroxyacetic acid is used in strongly acidic aqueous solutions. Alkenes are epoxidized, then opened to glycols in one step. Weakly acidic peroxyacids, such as peroxybenzoic acid, can be used in nonaqueous solutions to give good yields of epoxides. 

The most common laboratory method for the synthesis of epoxides from alkenes is oxidation with a peroxycarboxylic acid (a peracid), RCO3H. One peracid used for this purpose is peroxyacetic acid ... 

The most general reagents for conversion of simple alkenes to epoxides are peroxycarboxylic acids. w-Chloroperoxybenzoic acid (MCPBA) is a particularly convenient reagent, but it is not commercially available at the present time. The magnesium salt of monoperoxyphthalic acid has been recommended as a replacement. Potassium hydrogen peroxysulfate, which is sold commercially as oxone, is a convenient reagent for epoxidations that can be done in aqueous methanol. Peroxyacetic acid, peroxybenzoic acid, and peroxytrifluoroacetic acid have also been used frequently for epoxidation. All of the peroxycarboxylic acids are potentially hazardous materials and require appropriate precautions. 

The reaction of peroxyacids with alkenes is illustrated by the experiment that reacts cyclopentene with peroxyacetic acid (83). There are two isolated products cyclopentene oxide (88) in 57% yield and acetic acid (89). The yield is only 57% because the remainder of the 100% is unreacted alkene. Labeling experiments (replacing the natural isotope with the less abundant isotope 1 0) show that the oxygen atom of the epoxide in 88 comes from the OH oxygen in the peroxyacid (labeled in red in 83 and 88). 

We recall that epoxides can be synthesized by oxidizing an alkene (Section 9.8). The possible oxidizing agents are peroxyacetic acid (CHjCOsH), tw-chloroperoxy-benzoic acid (MCPBA). The epoxi-dation of alkenes with peroxy acids is stereospecific. The stereochemistry of the groups in the alkene remains cis groups in the alkene remain cis in the epoxide, and tram groups in the alkene remain... 

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