CHAPTER FOURTEEN Organometallic Compounds Because of their basicity organolithium compounds and grignard reagents can not be prepared or used in the presence of any material that bears a hydroxyl group Nor are these reagents compatible with -NH or-SH groups, which can also con vert an organolithium or organomagnesium compound to a hydrocarbon by proton transter The carbon-metal bonds of organolithium and organomagnesium compounds have appreciable carbanionic character. Carbanions rank among the strongest bases that we'l see in this text. Their conjugate acids are hydrocarbons-very weak acids indeed. The quilibrium constants Ka for ionization of hydrocarbons are much smaller than the kas for water and alcohols Table 14.2 presents some approximate data for the acid strengths of representative hydro- Acidity increases in progressing from the top of Table 14.2 to the bottom. An acid will transfer a proton to the conjugate base of any acid above it in the table. Organo- act like carbanions and will abstract from any substance more acidic than a hydrocarbon. Thus, N-H groups and terminal alkynes(RC=C-H) are converted to their conjugate bases by proton transfer to organolithium and organomagnesium compounds TABLE 14.2 Approximate Acidities of Some Hydrocarbons and Reference Materials pk Conjugate base 2-Methylpropane (CH3)3C-H (CH3)3C Ethane CH3CH2--H CH3CH2 Methane HaC CH2=CH一H10 Benzene H-H 10-43 H H2N一H 10-36 Acetylene 10-26 cHCH2O一H10-16 16 CH3CH2O HO-H 18×10 15.7Ho *The acidic proton in each compound is shaded in red Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteBecause of their basicity organolithium compounds and Grignard reagents can￾not be prepared or used in the presence of any material that bears a hydroxyl group. Nor are these reagents compatible with ±NH or ±SH groups, which can also con￾vert an organolithium or organomagnesium compound to a hydrocarbon by proton transfer. The carbon–metal bonds of organolithium and organomagnesium compounds have appreciable carbanionic character. Carbanions rank among the strongest bases that we’ll see in this text. Their conjugate acids are hydrocarbons—very weak acids indeed. The equilibrium constants Ka for ionization of hydrocarbons are much smaller than the Ka’s for water and alcohols. Table 14.2 presents some approximate data for the acid strengths of representative hydro￾carbons. Acidity increases in progressing from the top of Table 14.2 to the bottom. An acid will transfer a proton to the conjugate base of any acid above it in the table. Organo￾lithium compounds and Grignard reagents act like carbanions and will abstract a proton from any substance more acidic than a hydrocarbon. Thus, N±H groups and terminal alkynes (RCPC±H) are converted to their conjugate bases by proton transfer to organolithium and organomagnesium compounds. C H Hydrocarbon (very weak acid) Proton H C Carbanion (very strong base) 552 CHAPTER FOURTEEN Organometallic Compounds TABLE 14.2 Approximate Acidities of Some Hydrocarbons and Reference Materials Compound 2-Methylpropane Ethane Methane Ethylene Benzene Ammonia Acetylene Ethanol Water 1071 1062 1060 1045 1043 1036 1026 1016 1.8  1016 Ka 71 62 60 45 43 36 26 16 15.7 Formula* pKa (CH3)3C±H CH3CH2±H CH3±H CH2œCH±H H2N±H HCPC±H CH3CH2O±H HO±H H H H H H H Conjugate base H H H H H (CH3)3C H3C HCPC H2N CH3CH2O HO CH3CH2 CH2œCH *The acidic proton in each compound is shaded in red. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website