Question

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

Answer 1

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.