Grignard reagents polarized bond

You have already had considerable experience with carbanionic compounds and their applications in synthetic organic chemistry The first was acetyhde ion m Chapter 9 followed m Chapter 14 by organometallic compounds—Grignard reagents for example—that act as sources of negatively polarized carbon In Chapter 18 you learned that enolate ions—reactive intermediates generated from aldehydes and ketones—are nucleophilic and that this property can be used to advantage as a method for carbon-carbon bond formation... 

The conversion of a nitrile R —CN into a ketone R —CO—R demonstrates that polarized multiple bonds other than C=0 also react with Grignard reagents, and that such reactions are synthetically useful. Esters 22 and acid chlorides can react subsequently with two equivalents of RMgX the initially formed tetravalent product from the first addition reaction can decompose to a ketone that is still reactive, and reacts with a second RMgX. The final product 23 then contains two substituents R, coming from the Grignard reagent ... 

We saw in Section 17.5 that treatment of an aldehyde or ketone with a Grignard reagent, RMgX, yields an alcohol by nucleophilic addition of a carbon anion, or carbanion. A carbon-magnesium bond is strongly polarized, so a Grignard reagent reacts for all practical purposes as R - +MgX. 

Indeed, it is endothermic by over 130 kJmoU This documents that the polar/ionic bond between magnesium and oxygen is exceptionally strong, a fact we already surmised by the vigor of the reaction of Grignard reagents with air and water. 

The most important synthetic use of Grignard reagents and organolithium reagents is to form new carbon-carbon bonds by addition to polar multiple bonds, particularly carbonyl bonds. An example is the addition of methyl-magnesium iodide to methanal ... 

The Hg-C bond is very weak, partly on account of the large size of the Hg atom and the low polarity of the bond (the mercury dialkyls are molecular substances, with linear C—Hg-C skeletons). Despite the low atomisation enthalpy of mercury compared with other metallic elemental substances, reactions of this kind are usually exothermic. The mercury dialkyls are relatively insensitive to air and moisture and are therefore convenient to store and use. They are prepared from Grignard reagents ... 

---