Grignard reagents polar carbon-magnesium bond

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. 

A Grignard reagent will undergo transmetallation (metal exchange) if it is added to a metal halide whose metal is more electronegative than magnesium. In other words, metal exchange will occur if it results in a less polar carbon-metal bond. For example, cadmium... 

The carbon-magnesium bond of Grignard reagents is polar covalent, with a partial negative charge on carbon, making it nucleophile and basic. 

Methyl magnesium iodide - a Grignard reagent - and polarization of the carbon-magnesium bond. Red indicates areas of partial positive charge areas of partial negative charge are blue... 

The polarity of the carbon-magnesium bond is opposite that of the carbon—halogen bond of haloalkanes. Because the carbon atom in a Grignard reagent has a partial negative charge, it resembles a carb-anion, and it reacts with electrophilic centers such as the carbonyl carbon atom of aldehydes, ketones, and esters. We will discuss this chemistry extensively in later chapters. 

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 ... 

In the majority of cases, organolithium compounds and Grignard reagents contain polarized but covalent carbon—metal bonds. Lithioalkanes, -alkenes, and -aromatics, on the one hand, and alkyl, alkenyl, and aryl magnesium halides, on the other hand, are therefore formulated with a hyphen between the metal and the neighboring C atom. Only lithiated alkynes and alkynyl Grignard reagents are considered to be ionic—that is, species with carbon, metal bonds similar to those in LiCN or Mg(CN)2. 

Organocoppers are softer than Grignard reagents because copper is less electropositive than magnesium, so the C-Cu bond is less polarized than the C-Mg bond, giving the carbon atom less of a partial negative... 

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