What are Grignard's reagents? How are they prepared?

1 Answer
anor277
Aug 1, 2016

Grignards are the classic organometallic reagent; easy to prepare, highly reactive, and very useful for making #C-C# bonds directly and straightfowardly.

Explanation:

In an aprotic, anhydrous donor solvent, an hydrocarbyl halide may be metallated directly with magnesium turnings:

#R-X+MgrarrR-MgX#

#R-MgX#, the Grignard reagent, is usually used directly

See this old answer for tips on preparation.

The classic way to make a carboxylic acid is to pour your Grignard reagent onto dry ice:

#R-MgX + CO_2(s) rarr R-C(=)O^(-)""^+(MgX)#

Grignard react with aldehydes and ketones to give #2""^@# and #3""^@# alcohols:

#R-MgX + H(O=)CR' rarr RC(-O^(-)Mg^+X)R'#

And with ethylene oxide to give an alkoxide 2 carbons longer than the Grignard chain:

#R-MgX + C_2H_4O rarr RCH_2CH_2O^(-)""^(+)MgX#

Grignard reactions do not tolerate water, as they are strongly carbanionic.

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