Addition of Grignard reagents to esters to give tertiary alcohols
Description: Grignard reagents add twice to esters, giving tertiary alcohols (after addition of acid)
- It’s possible to show the “acid” step in many ways, such as “H+”, “H3O(+)”, etc. They all amount to the same thing.
- Note that X in this case is a halide (e.g. Cl, Br, I) and the identity of it does not affect the reaction.
- Finally it’s important to note that it’s not possible to get the reaction to stop after addition of just one equivalent of Grignard reagent.
Note that there are many ways to show the “acid workup” step. They all amount to the same thing. Also be sure to understand example #3 – many students have a hard time with this.
Addition of Grignard reagent to the ester (Step 1, arrows A and B) followed by 1,2-elimination of alkoxide (Step 2, arrows C and D) provides a ketone. A second 1,2-addition of Grignard reagent to the ketone (Step 3, arrows E and F) furnishes the tertiary alkoxide, which is then protonated upon addition of acid (Step 4, arrows G and H).
Again note that addition of only 1 equivalent of Grignard reagent will lead to 0.,5 equivalent tertiary alcohol and 0.5 equivalent of starting ester – the Grignard is more reactive with the ketone formed after step 2 than it is towards the starting ester!