Question

How do Grignard reagent react with an acetone?

Answer 1

A tertiary alcohol would result after workup.

`(H_3C)_2C=O + RMgX rarr (H_3C)_2R C-O^(-)rarr(H_3C)_2RCOH`

Explanation:

The Grignard reagent features a more or less direct bond between a carbon centre and a metal centre: `RH_2C^(delta-)""^(+delta)MgX`, or even as a full-blown carbanion, `RH_2C^(-)""^(+)MgX`. Given this formulation, its interaction with a carbonyl species may be rationalized.

The Grignard will react with an aldehyde to form (after workup) a `2^@` alcohol, with a ketone to give a `3^@` alcohol, with carbon dioxide, and ethylene oxide, to give a carboxylic acid, or a primary alcohol respectively. All of these are examples of prized `C-C` bond forming reactions.

Of course, the ipso carbon on each substrate is somewhat electron-poor, i.e. electrophilic, viz., `""^(delta-)O=C^(+delta)=O^(delta-)`. Upon addition of the Grignard, (which you can usually do by pouring the ethereal solution of `RMgX` directly onto dry ice) a direct `C-C` bond is formed between the Grignard residue and the carbonyl. This gives a carboxylate salt, and after workup, a carboxylic acid that is 1 carbon longer than the original Grignard residue.

With a ketone, acetone, a `3^@` alcohol is formed:

`R_3C^(delta-)""^(+delta)MgX +R'_2C(=O) rarr R'_2(""^(-)O)C-CR_3`

If it were acetone, then the alcohol product would be `(H_3C)_2C(OH)R`.

All of this demands fast-working, being on the ball, and reasonably dry solvents. Today, you can even buy pre-prepared solutions of Grignard and lithium reagents for direct use in synthesis.

Can you predict the product between acetone and methyl magnesium chloride?