Addition of Grignard reagents to ketones to give tertiary alcohols

by James

Description: When a ketone is treated with a Grignard reagent, a new C–C bond is formed at the carbonyl carbon. Subsequent addition of acid will then give the alcohol.
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Notes: Note that X in this case is a halide (e.g. Cl, Br, I). There are many ways of depicting the second step (“acid”) including H+, H3O+, and H2O.

Examples



Notes

Note that the byproduct here is the magnesium salt. The last example shows no reaction, because the Grignard (1 equivalent) being a strong base, deprotonates the alcohol instead of adding to the ketone. Grignards are strong bases!

Mechanism

The reaction proceeds through 1,2-addition of the C–MgX bond to the carbonyl (Step 1, arrows A and B) to give a tetrahedral product (alkoxide) which is then protonated with acid (Step 2, arrows C and D) to give the alcohol and a magnesium salt.

Notes: There are many ways of depicting the acid step, including “acid”, H+, H3O+, etc. All of them amount to the same thing.

Video Walkthrough:

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