Chapter 14 Organometallic Compounds

14.1 Organometallic Nomenclature 7

Methylmagnesium iodide Metal is the parent Li Cyclopropyllithium Vinylsodium H2C CHNa CH3CH2MgCH2CH3 Diethylmagnesium CH3MgI Methylmagnesium iodide 8

14.2 Carbon-Metal Bonds in Organometallic Compounds

Table 14.1 (p 547) F 4.0 H 2.1 O 3.5 Cu 1.9 N 3.0 Zn 1.6 C 2.5 Al 1.5 H 2.1 Mg 1.2 Li 1.0 Na 0.9 K 0.8 5

organometallics are a source of nucleophilic carbon Polarity of Bonds d+ d– d– d+ R X R X organometallics are a source of nucleophilic carbon 4

Polarity of Bonds CH3F CH3Li 4

14.3 Preparation of Organolithium Compounds 9

Organolithium Compounds normally prepared by reaction of alkyl halides with lithium R X + 2Li R Li + LiX same for Ar—X is an oxidation-reduction reaction: carbon is reduced 10

Examples diethyl ether (CH3)3CCl + 2Li (CH3)3CLi + LiCl –10°C (75%) Br + 2Li Li + LiBr 35°C (95-99%) 11

Electron Bookkeeping R X + Li [R X] + Li+ Li• R Li R• + X– • • – • • 14

14.4 Preparation of Organomagnesium Compounds: Grignard Reagents 15

prepared by reaction of alkyl halides with magnesium Grignard Reagents prepared by reaction of alkyl halides with magnesium R X + Mg RMgX same for Ar—X Diethyl ether is most often used solvent. Tetrahydrofuran is also used. 10

Examples diethyl ether Cl MgCl + Mg –10°C (96%) diethyl ether Br + Mg MgBr 35°C (95%) 11

Electron Bookkeeping R X + Mg [R X] + Mg+ Mg+ R Mg+ X– R• + X– • • • • – + Mg+ Mg+ • • R Mg+ • X– R• + • X– 14

I > Br > Cl >> F Order of Reactivity I > Br > Cl >> F RX > ArX 18

certain groups cannot be present in the solvent Forbidden Groups certain groups cannot be present in the solvent the halide from which the Grignard reagent is prepared the substance with which the Grignard reagent reacts 19

Anything with an OH, SH, or NH group Forbidden Groups Anything with an OH, SH, or NH group therefore cannot use H2O, CH3OH, CH3CH2OH, etc. as solvents cannot prepare Grignard reagent from substances such as HOCH2CH2Br, etc. 20

14.5 Organolithium and Organomagnesium Compounds as Brønsted Bases 22

Brønsted basicity d– d+ R H R M H OR' OR' – M + •• • • – M + • • •• Grignard reagents (M = MgX) and organolithium reagents (M = Li) are strong bases. 23

water is a stronger acid than butane Example CH3CH2CH2CH2Li + H2O water is a stronger acid than butane CH3CH2CH2CH3 + LiOH (100%) 11

methanol is a stronger acid than benzene Example MgBr + CH3OH methanol is a stronger acid than benzene + CH3OMgBr (100%) 11

Table 14.2 Approximate Acidities of Hydrocarbons Hydrocarbon pKa (CH3)3CH 71 CH3CH3 62 CH4 60 Ethylene 45 Benzene 43 Ammonia 36 Acetylene 26 Water 16 Hydrocarbons are very weak acids. Their conjugate bases are very strong bases. Grignard reagents and organolithium reagents are strong bases. 25

Acetylenic Grignard Reagents are prepared by an acid-base reaction CH3CH2MgBr + HC CH stronger acid CH3CH3 + HC CMgBr weaker acid 26