Ch_322a_11.018

Ch_322a_11.018 - The rate of epoxidation increases with the...

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Epoxides Epoxides are three-membered cyclic ethers that are also called oxiranes . C C O an epoxide or oxirane C C O H H H H oxirane (IUPAC) (ethylene oxide) Preparation Epoxides are often prepared from reacting alkenes with organic peroxy acids ("peracids") in a process called epoxidation . RCH=CHR' + R''CO-OH O = peroxy acid RCH CHR' O + R''COOH epoxide
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Stereochemistry of Epoxidation Epoxidation is a syn-addition . C=C CH 3 CH 3 H H cis-2-butene + RCO-OH O = C C O CH 3 CH 3 H H cis-2,3-dimethyloxirane C=C CH 3 CH 3 H H trans-2-butene + RCO-OH O C C O CH 3 CH 3 H H trans-2,3-dimethyloxirane
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Peroxyacids CO-OH O = Cl m-chloroperoxybenzoic acid In past years, this was the reagent of choice for epoxidation reactions. CO-OH O CO 2 - 2 Mg 2+ magnesium monoperoxyphthalate (MMPP) MMPP is now used because of its greater safety.
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Reactivity of Alkenes in the Epoxidation Reaction
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Unformatted text preview: The rate of epoxidation increases with the number of alkyl groups around the double bond. CH 2 =CH 2 CH 3 CH=CH 2 (CH 3 ) 2 C=CH 2 (CH 3 ) 2 C=CHCH 3 relative rate 1 22 484 5082 It appears that increasing !-electron density enhances the rate of the epoxidation reaction. Mechanism of Epoxidation It is believed that epoxidation proceeds through a complex between the !-electrons of the alkene and the electron-deficient oxygen of the peroxyacid. C C Ar C=O O-O-H ! + !" C C Ar C=O O O H complex C C Ar C=O O O H complex The electron-deficient oxygen adds to the alkene in a syn-addition . C C Ar C=O O O H C C O Ar C O O H syn-addition...
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Ch_322a_11.018 - The rate of epoxidation increases with the...

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