docOrganometallic Compounds Preparation of Organomagnesium
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Organometallic Compounds Preparation of Organomagnesium
δ+ R Organometallic Compounds δ– X Carbon-Metal Bonds δ– R δ+ M Nucleophilic carbon Organometallic Compounds Organolithium Compounds normally prepared by reaction of alkyl halides with lithium R X + 2Li 2Li R Li + LiX LiX same for Ar— Ar—X is an oxidation-reduction reaction: carbon is reduced Examples Preparation of Organomagnesium Compounds diethyl ether (CH3)3CCl + 2Li 2Li (CH3)3CLi + LiCl LiCl –10° 10°C (75%) diethyl ether Br + 2Li 2Li Li + LiBr LiBr 35° 35°C (95-99%) Grignard Reagents prepared by reaction of alkyl halides with magnesium R X + Mg Examples diethyl ether Cl + Mg MgCl MgCl –10° 10°C RMgX MgX (96%) diethyl ether same for Ar— Ar—X Diethyl ether is most often used solvent. Tetrahydrofuran is also used. Br + Mg MgBr MgBr 35° 35°C (95%) Alkyl halides, vinyl halides, and aryl halides can all be used to form organolithium and organomagnesium compounds However, these organometallic compounds cannot be prepared from compounds containing acidic groups (OH, NH2, NHR, SH, C=CH, CO2H) Cannot use H2O, CH3OH, CH3CH2OH, etc. as solvents Cannot prepare from substances such as HOCH2CH2Br, etc. Coupling Reactions Coupling Reactions Formation of carbon–carbon bonds Coupling Reactions Brø Brønsted basicity Organolithium and Organomagnesium Compounds as Brønsted Bases δ– R H δ+ R • OR' • •• M M H + –• •• • OR' •• Grignard reagents (M (M = MgX) MgX) and organolithium reagents (M (M = Li) are strong bases. Example Example CH3CH2CH2CH2Li + H2O CH3CH2CH2CH3 + MgBr MgBr + CH3OH LiOH LiOH + CH3OMgBr MgBr (100%) (100%) Approximate Acidities Compound (CH3)3CH CH3CH3 CH4 Ethylene Benzene Ammonia Acetylene Water pK a 71 62 60 45 43 36 26 16 Hydrocarbons are very weak acids. Their conjugate bases are very strong bases. Grignard reagents and organolithium reagents are strong bases. Acetylenic Grignard Reagents are prepared by an acid-base reaction CH3CH2MgBr MgBr + CH3CH3 weaker acid + HC CH stronger acid HC CMgBr MgBr Question • Select the best method to prepare CH3CH2C≡CMgI. • A) CH3CH2C≡C-I + Mg (in diethyl ether) • B) I-CH2CH2C≡CH + Mg(OH)2 (in diethyl ether) • C) CH3CH2C≡CH + CH3MgI (in diethyl ether) • D) CH3CH2C≡CH + Mg (in diethyl ether)