Organometallic — double addition
Grignard addition to an ester (2 equivalents → tertiary alcohol)
How the Grignard addition to an ester (2 equivalents → tertiary alcohol) works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Ester (methyl benzoate), 2+ equiv. CH₃MgBr, then H₃O⁺ workup. Conditions: Dry Et₂O or THF, 0 °C → 25 °C; then aqueous NH₄Cl or dilute acid. Bonds formed: 2 × C–C; O–H (workup); bonds broken: C=O π; C–OMe. Stereochemistry: Product has two identical groups: no stereocentre at carbinol C. Rate and selectivity: Ketone intermediate is more reactive than the ester, so no stop at the ketone.
At a glance
Step 1: First addition, then loss of methoxide
A Grignard reagent attacks the ester's carbonyl carbon, and the tetrahedral intermediate collapses by expelling alkoxide — the same addition-elimination sequence as LiAlH₄ reduction of an ester, just delivering an alkyl group from carbon instead of hydride.
Step 2: Second addition to the more reactive ketone
This generates a ketone in situ, which is immediately attacked by a second equivalent of the same Grignard reagent — ketones are more electrophilic than esters, so this second addition happens readily and can't be stopped at the ketone stage under normal conditions.
Step 3: Aqueous workup
Aqueous workup protonates the resulting alkoxide, giving a tertiary alcohol bearing two identical new alkyl groups from the Grignard reagent — this double addition is why controlling a Grignard reaction at the ketone stage requires a specialized reagent like a Weinreb amide instead.
Abbreviations and terms
- OMe
- Methoxy. –OCH₃.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- Et2O
- Diethyl ether. Low-boiling ether solvent, classic for Grignard reagents.
- LiAlH4
- Lithium aluminium hydride. Powerful hydride (H⁻) donor; reduces esters, acids, amides to alcohols/amines.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
How it compares with related reactions
| Grignard addition to an ester (2 equivalents → tertiary alcohol) | Grignard addition to acetaldehyde | Nozaki-Hiyama-Kishi (NHK) reaction | |
|---|---|---|---|
| Reagents | Ester (methyl benzoate), 2+ equiv. CH₃MgBr, then H₃O⁺ workup | CH₃MgBr in Et₂O or THF; then H₃O⁺ | Vinyl/aryl iodide or triflate, CrCl₂ (excess), cat. NiCl₂, aldehyde |
| Conditions | Dry Et₂O or THF, 0 °C → 25 °C; then aqueous NH₄Cl or dilute acid | Dry ether, 0 °C → 25 °C; strictly anhydrous | DMF or DMSO, 25 °C, inert atmosphere |
| Stereochemistry | Product has two identical groups: no stereocentre at carbinol C | Planar C=O: both faces attacked (racemic if new stereocentre) | High anti diastereoselectivity via chair Zimmerman–Traxler TS |
| Rate / selectivity | Ketone intermediate is more reactive than the ester, so no stop at the ketone | Fast with aldehydes; RMgX is also a strong base — incompatible with acidic H | Ni/Cr transmetalation is turnover-limiting; Cr(III) species do not enolise aldehydes |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Grignard addition to an ester (2 equivalents → tertiary alcohol)?
Ester (methyl benzoate), 2+ equiv. CH₃MgBr, then H₃O⁺ workup. Conditions: Dry Et₂O or THF, 0 °C → 25 °C; then aqueous NH₄Cl or dilute acid.
Which bonds are formed and which are broken?
Formed: 2 × C–C; O–H (workup). Broken: C=O π; C–OMe.
What is the stereochemical outcome?
Product has two identical groups: no stereocentre at carbinol C
What controls the rate or the selectivity?
Ketone intermediate is more reactive than the ester, so no stop at the ketone
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam