Organometallic — carbonyl addition
Grignard addition to acetaldehyde
How the Grignard addition to acetaldehyde 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: CH₃MgBr in Et₂O or THF; then H₃O⁺. Conditions: Dry ether, 0 °C → 25 °C; strictly anhydrous. Bonds formed: C–C σ; later O–H; bonds broken: C=O π; C–Mg. Stereochemistry: Planar C=O: both faces attacked (racemic if new stereocentre). Rate and selectivity: Fast with aldehydes; RMgX is also a strong base — incompatible with acidic H.
At a glance
Step 1: Polarised C–Mg bond
The C–Mg bond in methylmagnesium bromide is highly polarized: carbon holds most of the electron density, making it behave like a carbanion — a powerful nucleophile.
Step 2: Carbon attacks the C=O
That carbanion-like carbon attacks the electrophilic carbonyl carbon. The C=O π pair shifts entirely onto oxygen, which coordinates to magnesium as the new bond forms.
Step 3: Aqueous workup
Aqueous workup protonates the magnesium alkoxide, delivering a secondary alcohol — the same core step used to build C–C bonds in countless syntheses.
Abbreviations and terms
- 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.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
How it compares with related reactions
| Grignard addition to acetaldehyde | Grignard addition to an ester (2 equivalents → tertiary alcohol) | Nozaki-Hiyama-Kishi (NHK) reaction | |
|---|---|---|---|
| Reagents | CH₃MgBr in Et₂O or THF; then H₃O⁺ | Ester (methyl benzoate), 2+ equiv. CH₃MgBr, then H₃O⁺ workup | Vinyl/aryl iodide or triflate, CrCl₂ (excess), cat. NiCl₂, aldehyde |
| Conditions | Dry ether, 0 °C → 25 °C; strictly anhydrous | Dry Et₂O or THF, 0 °C → 25 °C; then aqueous NH₄Cl or dilute acid | DMF or DMSO, 25 °C, inert atmosphere |
| Stereochemistry | Planar C=O: both faces attacked (racemic if new stereocentre) | Product has two identical groups: no stereocentre at carbinol C | High anti diastereoselectivity via chair Zimmerman–Traxler TS |
| Rate / selectivity | Fast with aldehydes; RMgX is also a strong base — incompatible with acidic H | Ketone intermediate is more reactive than the ester, so no stop at the ketone | 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 acetaldehyde?
CH₃MgBr in Et₂O or THF; then H₃O⁺. Conditions: Dry ether, 0 °C → 25 °C; strictly anhydrous.
Which bonds are formed and which are broken?
Formed: C–C σ; later O–H. Broken: C=O π; C–Mg.
What is the stereochemical outcome?
Planar C=O: both faces attacked (racemic if new stereocentre)
What controls the rate or the selectivity?
Fast with aldehydes; RMgX is also a strong base — incompatible with acidic H
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