14.3 Preparation of Organolithium Compounds 14.3 PREPARATION OF ORGANOLITHIUM COMPOUNDS Before we describe the applications of organometallic reagents to organic synthesis, let us examine their preparation. Organolithium compounds and other Group I organometal lic compounds are prepared by the reaction of an alkyl halide with the appropriate metal alide with lithium was cited RX 2M RM +mtX arlier(Section 2. 16) Group I reduction Group I metals a organometallic halide owerful reducing agents. CHacCI 2Li →(CH3)3CLi+LiCl tert-Butyl chloride Lithium terl-Butyllithium Lithium (75%) chloride The alkyl halide can be primary, secondary, or tertiary. Alkyl iodides are the most reac tive, followed by bromides, then chlorides. Fluorides are relatively unreactive Unlike elimination and nucleophilic substitution reactions, formation of organo- lithium compounds does not require that the halogen be bonded to sp-hybridized carbon Compounds such as vinyl halides and aryl halides, in which the halogen is bonded to hybridized carbon, react in the same way as alkyl halides, but at somewhat slower rates >Br+2Li→ Li+ liBr Bromobenzene Lith Phenyllithium Lithium 95-99%) Organolithium compounds are sometimes prepared in hydrocarbon solvents such as pentane and hexane, but normally diethyl ether is used. It is especially important that the solvent be anhydrous. Even trace amounts of water or alcohols react with lithium to form insoluble lithium hydroxide or lithium alkoxides that coat the surface of the metal and prevent it from reacting with the alkyl halide. Furthermore, organolithium reagent are strong bases and react rapidly with even weak proton sources to form hydrocarbons We shall discuss this property of organolithium reagents in Section 14.5 PROBLEM 14.2 Write an equation showing the formation of each of the fol owing from the appropriate bromide SAMPLE SOLUTION (a) In the preparation of organolithium compounds from organic halides, lithium becomes bonded to the carbon that bore the halogen Therefore, isopropenyllithium must arise from isopropenyl bromide ether CH2=CCH3 2Li CH=CC Br Isopropenyl bromide Lithium Reaction with an alkyl halide takes place at the metal surface. In the first step, ar electron is transferred from the metal to the alkyl halide. R:X:+ L IR.X Alkyl halide Lithium Anion radical Lithium cation Back Forward Main MenuToc Study Guide ToC Student o MHHE Website14.3 PREPARATION OF ORGANOLITHIUM COMPOUNDS Before we describe the applications of organometallic reagents to organic synthesis, let us examine their preparation. Organolithium compounds and other Group I organometal￾lic compounds are prepared by the reaction of an alkyl halide with the appropriate metal. The alkyl halide can be primary, secondary, or tertiary. Alkyl iodides are the most reac￾tive, followed by bromides, then chlorides. Fluorides are relatively unreactive. Unlike elimination and nucleophilic substitution reactions, formation of organo￾lithium compounds does not require that the halogen be bonded to sp3 -hybridized carbon. Compounds such as vinyl halides and aryl halides, in which the halogen is bonded to sp2 - hybridized carbon, react in the same way as alkyl halides, but at somewhat slower rates. Organolithium compounds are sometimes prepared in hydrocarbon solvents such as pentane and hexane, but normally diethyl ether is used. It is especially important that the solvent be anhydrous. Even trace amounts of water or alcohols react with lithium to form insoluble lithium hydroxide or lithium alkoxides that coat the surface of the metal and prevent it from reacting with the alkyl halide. Furthermore, organolithium reagents are strong bases and react rapidly with even weak proton sources to form hydrocarbons. We shall discuss this property of organolithium reagents in Section 14.5. PROBLEM 14.2 Write an equation showing the formation of each of the fol￾lowing from the appropriate bromide: (a) Isopropenyllithium (b) sec-Butyllithium SAMPLE SOLUTION (a) In the preparation of organolithium compounds from organic halides, lithium becomes bonded to the carbon that bore the halogen. Therefore, isopropenyllithium must arise from isopropenyl bromide. Reaction with an alkyl halide takes place at the metal surface. In the first step, an electron is transferred from the metal to the alkyl halide. Li Lithium Lithium cation Li Alkyl halide R X Anion radical [R ] X CH2œCCH3 W Br Isopropenyl bromide 2Li Lithium W Li CH2œCCH3 Isopropenyllithium LiBr Lithium bromide diethyl ether diethyl ether 35°C Br Bromobenzene 2Li Lithium Li Phenyllithium (95–99%) LiBr Lithium bromide RX Alkyl halide 2M Group I metal MX Metal halide RM Group I organometallic compound (CH3)3CCl tert-Butyl chloride 2Li Lithium LiCl Lithium chloride (CH3)3CLi tert-Butyllithium (75%) diethyl ether 30°C 14.3 Preparation of Organolithium Compounds 549 The reaction of an alkyl halide with lithium was cited earlier (Section 2.16) as an example of an oxidation– reduction. Group I metals are powerful reducing agents. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website