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Lab # 2 The Grignard Reaction.doc

Lab # 2 The Grignard Reaction.doc - CH2630 Lab 2 Grignard...

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CH2630: Lab # 2 - Grignard Reaction Name: Megan Hopkins Date: September 28,2017 Objectives: Grignard Reaction: Synthesis of triphenylmethanol followed by treating the product with concentrated sulfuric acid to test for the presence of triphenylmethyl carbocation. The experimental components are: 1. Synthesis of the Grignard reagent phenylmagnesium bromide from bromobenzene and magnesium turnings in anhydrous ether; 2. Synthesis of triphenylmethoxymagnesium bromide by nucleophilic addition of the Grignard reagent across the carbonyl group of benzophenone; 3. Synthesis of triphenylmethanol from hydrolysis of the intermediate triphenylmethoxymagnesium bromide; 4. Testing for the presence of the bright orange triphenylmethyl carbocation by treating a small sample of the triphenylmethanol with concentrated sulfuric acid. References: 1. T.W.G. Solomons, C.B. Fryhle and S.A. Snyder, Organic Chemistry, 12 th Edition, Wiley, 2016, pages 548-560. 2. T.W.G. Solomons, C.B. Fryhle and S.A. Snyder, Organic Chemistry, 11 th Edition, Wiley, 2014, pages 558-569. 3. T.W.G. Solomons and C.B. Fryhle, Organic Chemistry, 10 th Edition, Wiley, 2011, pages 563-573. Introduction Organomagnesium halides were discovered by the French chemist Victor Grignard in 1900. Grignard received the Nobel Prize for his discovery in 1912, and organomagnesium halides are now called Grignard reagents in his honour. Grignard reagents have great use in organic syntheses. Grignard reagents are prepared by the reaction of organic halides with magnesium metal in anhydrous ether solvent. An oxygen free atmosphere is maintained by a blanket of ether during synthesis of the Grignard reagent due to ether vapor being more dense than oxygen. © Dr. John Purdie Sloan; CH2610 & CH 2630 Organic Chemistry Experiments, Fall 2017 & Winter 2018. Page 1 of 26
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R X + Mg Et 2 O Anhydrous ether R Mg X Ar X + Mg Et 2 O Anhydrous ether Ar Mg X Where R = Alkyl and Ar = Aryl Groups, and X = I, Br and Cl. The order of reactivity of Alkyl halides is R-I > R-Br > R-Cl. Aryl Grignard reagents are more easily prepared from aryl bromides and aryl iodides than from aryl chlorides. Once prepared, Grignard reagents are usually used directly in subsequent reactions. The actual structures of Grignard reagents are more complex than indicated by the structure R-Mg-X. Complex formation with molecules of ether is an important factor in the formation and stability of Grignard reagents. Grignard reagents have electron-rich nucleophilic carbon atoms resulting in them being strong bases that easily decompose in the presence of acidic hydrogen atoms of water, alcohols and amines. This is consistent with stronger acids reacting with stronger bases giving weaker conjugate bases and weaker conjugate acids e.g.: R Mg X + CH 3 CH 2 O H R H + CH 3 CH 2 O Mg X stronger base stronger acid weaker acid weaker base Synthesis of Primary, Secondary and Tertiary Alcohols from Grignard Reagents and Aldehydes and Ketones: Grignard reagents are strong nucleophiles and easily attack the partially positive carbonyl
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