Organolithium and Organomagnesium Compounds as Bronsted Bases

PROBLEM 14.3 Write the structure of the Grignard reagent formed from each of the following compounds on reaction with magnesium in diethyl ether  

SAMPLE SOLUTION (a) Of the two halogen substituents on the aromatic ring, bromine reacts much faster than fluorine with magnesium. Therefore, fluorine is left Intact on the ring, while the carbon-bromine bond is converted to a carbon-magnesium bond. 

The formation of a Grignard reagent is analogous to that of organolithium reagents except that each magnesium atom can participate in two separate one-electron transfer steps  

Organolithium and organomagnesium compounds find their chief use in the preparation of alcohols by reaction with aldehydes and ketones. Before discussing these reactions, let us first examine the reactions of these organometaUic componnds with proton donors. 

Organolithium and organomagnesium compounds are stable species when prepared in suitable solvents such as diethyl ether. They are strongly basic, however, and react instantly with proton donors even as weakly acidic as water and alcohols. A proton is transferred from the hydroxyl group to the negatively polarized carbon of the organometaUic compound to form a hydrocarbon. 

The carbon-metal bonds of organolithium and organomagnesium compounds have appreciable carbanionic character. Carbanions rank among the strongest bases that we ll 

Use curved arrows to show the flow of electrons in the reaction of butyllithium with water, and that of phenylmagnesium bromide with methanol. 

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, with pica s in the 25-70 range. 

TABLE 14.1 Approximate Acidities of Some Hydrocarbons and Reference Materials 

Methyllithium Ammonia (stronger base) (stronger acid p/fa = 36) 

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