Ch_322a_6.023

Ch_322a_6.023 - potassium hydroxide in ethanol (to...

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Elimination Reactions of Alkyl Halides In an elimination reaction , the atoms or groups X and Y are lost from adjacent carbons forming a multiple bond. C C X Y C=C (- XY ) The Dehydrohalogenation Reaction A standard synthesis of alkenes is the dehydrohalogenation reaction of alkyl halides . C C H X (- HX ) C=C alkene alkyl halide Example: The Dehydrobromination of tert -Butyl Bromide CH 3 C-Br CH 3 CH 3 tert -butyl bromide + Na + - OCH 2 CH 3 CH 2 =C CH 3 CH 3 + HOCH 2 CH 3 + Na + Br - isobutene
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A Beta- or 1,2-Elimination Reaction This reaction is described as a beta-elimination or 1,2-elimination indicating the positions of the lost atoms or groups. C C Br H H H CH 3 CH 3 ! position " position The ! or 1 position is the carbon with the halogen leaving group. (- HBr ) CH 2 =C CH 3 CH 3 ! " or CH 2 =C CH 3 CH 3 1 2 The Role of Base in Dehydrohalogenation Reactions A number of different bases may be used in the dehydrohalogenation reaction. Typical bases are
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Unformatted text preview: potassium hydroxide in ethanol (to solubilize the alkyl halide) or sodium ethoxide in ethanol. Potassium tert-butoxide is another oxygen base that is often used in dehydrohalogenation reactions. KOH NaOEt KOBu-t Some Oxygen Bases Preparation of Alkoxide Bases Although most alkoxide bases are available from commercial sources, they may be "freshly" prepared from the rapid redox reaction of the "dry" (no water) alcohol with an alkali metal. 2CH 3 CH 2 OH + 2Na 2CH 3 CH 2 O-Na + + H 2 sodium ethoxide 2 CH 3 COH CH 3 CH 3 + 2K 2 CH 3 CO-K + CH 3 CH 3 + H 2 potassium tert-butoxide Sodium alkoxides also may be prepared by reacting alcohols with the strong base sodium hydride : RO-H + Na:H sodium hydride RO-Na + a sodium alkoxide + H 2 pK a ~ 16 pK a = 35...
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Ch_322a_6.023 - potassium hydroxide in ethanol (to...

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