Nucleophilic Addition of Hydride and Grignard Reagents Alcohol Formation

Nucleophilic Addition of Grignard and Hydride Reagents Alcohol Formation 

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 carbanioii. A carbon-magnesium bond is strongly polarized, so a Grignard reagent reacts for all practical purposes as K MgX. 

A Grignard reaction begins with an acid-base complexation of to the carbonyl oxygen atom of the aldehyde or ketone, thereby making the carbonyl group a better electropliile. Nucleophilic addition of R then produces a tetrahedral magnesium alkoxide intermediale, and protonation by addition of water 

Mechanism of the Grignard reaction. Nucleophilic addition of a carbanion to an aldehyde or ketone, followed by protonation of the alkoxide intermediate, yields an alcohol. 

Process to view an animation of the addition of an amine to a carbonyl compound to form an imine. 

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Nucleophilic Addition of Hydride and Grignard Reagents Alcohol Formation... 

Pyrones also add Grignard nucleophiles at the carbonyl carbon, C-4 dehydration of the inunediate tertiary alcohol product with mineral acid provides an important route to 4-mono-substituted pyrylium salts." More vigorous conditions lead to the reaction of both 2- and 4-pyrones with two mole equivalents of organometallic reagent and the formation of 2,2-disubstituted-2H- and 4,4-disubstituted-4//-pyrans, respectively." Perhaps surprisingly, hydride (lithium aluminium hydride) addition to 4,6-dimethyl-2-pyrone takes place, in contrast, at C-b." ... 

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