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Why is the preference of attack of Grignard reagent on oxygen atoms in the following order: alcohols> ketones> esters?

Why is the preference of attack of Grignard reagent on oxygen atoms in the following order: alcohols> ketones> esters?


1 Answers

Suraj Prasad IIT Patna
askIITians Faculty 286 Points
7 years ago
As we learned previously, Grignard reagents are created by reacting magnesium metal with an alkyl halide (aka haloalkanes). The magnesium atom then gets between the alkyl group and the halogen atom with the general reaction:

R-X + Mg → R-Mg-X

The general mechanism of a Grignard reagent reacting with a carbonyl (except esters) involves the creation of a 6-membered ring transition state. The pi bond of the oxygen attacks a neighboring magnesium bromide which in turn, releases from its R group leaving a carbocation. At the same time, the magnesium bromide ion from another Grignard molecule is attacked by the carbocation and has its magnesium bromide ion stolen (restoring it to its original state as a Grignard reagent). The second molecule's carbocation is then free to attack the carbanion resulting from the vacating pi bond, attaching the R group to the carbonyl.

At this point, there is a magnesium bromide on the oxygen of what was a carbonyl. The proton from the acidic solvent easily displaces this magnesium bromide ion and protonates the oxygen, creating a primary alcohol with formaldehyde, a secondary alcohol with an aldehyde and a tertiary alcohol with a ketone.

With esters, the mechanism is slightly different. Two moles of Grignard are required for each mole of the ester. Initially, the pi bond on the carbonyl oxygen attacks the magnesium bromide ion. This opens up the carbon for attack from the R group of the Grignard. This part of the reaction is slow because of the dual oxygens off of the carbon providing some resonance stabilization. The oxygen's pi bond then re-forms, expelling the O-R group of the ester which then joins with the magnesium bromide, leaving R-O-MgBr and a ketone. The R-O-MgBr is quickly protonated from the acidic solution and the ketone is then attacked by Grignard reagent via the mechanism described earlier.

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