CHAPTER 14 ORGANOMETALLIC COMPOUNDS O rganometallic compounds are compounds that have a carbonmetal bond; they lie at the place where organic and inorganic chemistry meet. You are already familiar with at least one organometallic compound, sodium acetylide (NaCPCH), which has an ionic bond between carbon and sodium. But just because a compound contains both a metal and carbon isnt enough to classify it as organometal- lic. Like sodium acetylide, sodium methoxide (NaOCH3) is an ionic compound. Unlike sodium acetylide, however, the negative charge in sodium methoxide resides on oxygen, not carbon. The properties of organometallic compounds are much different from those of the other classes we have studied to this point. Most important, many organometallic com- pounds are powerful sources of nucleophilic carbon, something that makes them espe- cially valuable to the synthetic organic chemist. For example, the preparation of alkynes by the reaction of sodium acetylide with alkyl halides (Section 9.6) depends on the pres- ence of a negatively charged, nucleophilic carbon in acetylide ion. Synthetic procedures that use organometallic reagents are among the most impor- tant methods for carboncarbon bond formation in organic chemistry. In this chapter you will learn how to prepare organic derivatives of lithium, magnesium, copper, and zinc and see how their novel properties can be used in organic synthesis. We will also fi nish the story of polyethylene and polypropylene begun in Chapter 6 and continued in Chapter 7 to see the unique way that organometallic compounds catalyze alkene polymerization. Sodium acetylide (has a carbon-to-metal bond) Na?CPCH ? Sodium methoxide (does not have a carbon-to-metal bond) Na?OCH3 ? 546 BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website 14.1ORGANOMETALLIC NOMENCLATURE Organometallic compounds are named as substituted derivatives of metals. The metal is the base name, and the attached alkyl groups are identifi ed by the appropriate prefi x. When the metal bears a substituent other than carbon, the substituent is treated as if it were an anion and named separately. PROBLEM 14.1Both of the following organometallic reagents will be encoun- tered later in this chapter. Suggest a suitable name for each. (a) (CH3)3CLi(b) SAMPLE SOLUTION(a) The metal lithium provides the base name for (CH3)3CLi. The alkyl group to which lithium is bonded is tert-butyl, and so the name of this organometallic compound is tert-butylithium. An alternative, equally correct name is 1,1-dimethylethyllithium. An exception to this type of nomenclature is NaCPCH, which is normally referred to as sodium acetylide. Both sodium acetylide and ethynylsodium are acceptable IUPAC names. 14.2CARBONMETAL BONDS IN ORGANOMETALLIC COMPOUNDS With an electronegativity of 2.5 (Table 14.1), carbon is neither strongly electropositive nor strongly electronegative. When carbon is bonded to an element more electronegative than itself, such as oxygen or chlorine, the electron distribution in the bond is polarized H MgCl CH3MgI Methylmagnesium iodide (CH3CH2)2AlCl Diethylaluminum chloride Li H Cyclopropyllithium CH2CHNa Vinylsodium (CH3CH2)2Mg Diethylmagnesium 14.2CarbonMetal Bonds in Organometallic Compounds547 TABLE 14.1Electronegativities of Some Representative Elements Element F O Cl N C H Cu Zn Al Mg Li Na K 4.0 3.5 3.0 3.0 2.5 2.1 1.9 1.6 1.5 1.2 1.0 0.9 0.8 Electronegativity BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website so that carbon is slightly positive and the more electronegative atom is slightly negative. Conversely, when carbon is bonded to a less electronegative element, such as a metal, the electrons in the bond are more strongly attracted toward carbon. Figure 14.1 uses electrostatic potential maps to show how different the electron distri- bution is between methyl fl uoride (CH3F) and methyllithium (CH3Li). An anion that contains a negatively charged carbon is referred to as a carbanion. Covalently bonded organometallic compounds are said to have carbanionic character. As the metal becomes more electropositive, the ionic character of the carbonmetal bond becomes more pronounced. Organosodium and organopotassium compounds have ionic carbonmetal bonds; organolithium and organomagnesium compounds tend to have covalent, but rather polar, carbonmetal bonds with signifi cant carbanionic character. It is the carbanionic character of such compounds that is responsible for their usefulness as synthetic reagents. CM ? M is less electronegative than carbon CX ? X is more electronegative than carbon 548CHAPTER FOURTEENOrganometallic Compounds (a) Methyl fluoride (b) Methyllithium FIGURE 14.1 Electro- static potential maps of (a) methyl fl uoride and of (b) methyllithium. The elec- tron distribution is reversed in the two compounds. Car- bon is electron-poor (blue) in methyl fl uoride, but electron- rich (red) in methyllithium. BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website 14.3PR