The Grignard reaction welds two carbon fragments into one new carbon–carbon bond.
A Grignard's carbon adds to a carbonyl; workup reveals an alcohol.
1. A Grignard Is a Carbon Nucleophile — an Umpolung Carbanion
Carbon holds the C–Mg electrons, making it a nucleophilic carbanion — the umpolung reverse of an electron-poor carbonyl carbon.
2. You Make One from an Alkyl Halide and Magnesium in Dry Ether
Stir an alkyl or aryl halide (R–X) with magnesium in dry ether (Et2O or THF) and Mg inserts into the C–X bond to give R–Mg–X; only the R group matters downstream.
Each halide plus Mg, Et2O becomes the corresponding R–MgX.
3. Grignards Add to Aldehydes and Ketones to Build C–C Bonds
The Grignard carbon adds to C=O to make an alkoxide; a separate acidic workup (H3O+) then reveals the alcohol.
4. The Carbonyl You Start From Fixes the Class of Alcohol
One R always adds, so the starting carbonyl's substitution decides the alcohol class:
- Formaldehyde (H2C=O) → 1° alcohol.
- Any other aldehyde → 2° alcohol.
- Ketone → 3° alcohol.
Formaldehyde → primary alcohol (ethanol).
Acetaldehyde → secondary alcohol (isopropanol).
Acetone → tertiary alcohol (tert-butanol).
5. CO2 Gives a Carboxylic Acid; Epoxides Add Two Carbons
Adding to CO2 caps R with a –COOH (one carbon longer); opening an epoxide gives an alcohol two carbons longer.
CO2 → carboxylic acid (acetic acid), one carbon longer.
Ethylene oxide → primary alcohol, two carbons longer (propan-1-ol).
6. Esters and Acid Chlorides Add Twice to Give 3° Alcohols
The first addition expels the leaving group to unmask a ketone, which is more reactive, so a second equivalent adds — giving a 3° alcohol with two identical R groups.
Methyl acetate + two equivalents of CH3MgBr → a tertiary alcohol (tert-butanol).
7. Any Protic Hydrogen (O–H, N–H, ≡C–H) Destroys a Grignard
This strong base grabs any acidic proton faster than it adds — one drop of water turns R–MgX into useless R–H, as do alcohols, amines, and terminal alkynes.
Keep everything dry, add the acidic workup last, and protect any O–H, N–H, or terminal alkyne on your substrate.
8. Summary
R–MgX from R–X + Mg in dry ether · umpolung carbanion · adds to carbonyls for a new C–C bond · formaldehyde → 1°, aldehydes → 2°, ketones → 3° · CO2 → carboxylic acid · epoxides add two carbons · esters/acid chlorides add twice → 3° · dies on any protic H.
Worked example
- The Grignard carbon is a strong nucleophile (δ−); it adds to formaldehyde's carbonyl carbon.
- Formaldehyde (H2C=O) always gives a 1° alcohol after workup.
- Carbons added: ethyl (2) + the formaldehyde carbon = a three-carbon 1° alcohol.
Answer. Propan-1-ol.
Quiz yourself
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Carbon is more electronegative than magnesium, so the C–Mg bonding electrons sit on carbon, giving it a partial negative charge (δ–). This polarity reversal — umpolung — makes the carbon a carbanion-like nucleophile and strong base, the opposite of an electron-poor carbonyl carbon.
A ketone already has two carbon groups on the carbonyl carbon, so adding the Grignard's R gives a tertiary alcohol. Formaldehyde has only H's, so it gives a primary alcohol. Any other aldehyde falls in between and gives a secondary alcohol.
The Grignard is a very strong base and instantly deprotonates any O–H, N–H, or terminal ≡C–H. Doing so converts R–MgX into the useless alkane R–H and consumes your reagent. That is why the setup must be dry and the acidic workup is added only at the very end.
It adds to CO2 to give a carboxylate that workup protonates to a carboxylic acid. It is useful because it extends the chain by exactly one carbon and installs a –COOH group in a single, predictable step.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.