Grignard reaction mechanism: interactive practice
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You just pushed every arrow of the Grignard reaction mechanism.
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Organic Chemistry 2 · Free interactive demo
The Grignard reaction adds a carbon nucleophile to a carbonyl group and is one of the most useful ways to build a new carbon–carbon bond. This interactive tool has you draw every curved arrow of a real example: ethylmagnesium bromide (CH₃CH₂MgBr) adds to acetone, and water then gives the tertiary alcohol 2-methylbutan-2-ol.
Grignard reagent formation comes first, before the mechanism you draw here. An alkyl halide such as bromoethane reacts with magnesium metal in dry ether or THF, and magnesium inserts into the C–Br bond. The resulting C–Mg bond is strongly polarized toward carbon, so the carbon behaves like a carbanion: a strong nucleophile and a strong base. That is why the reaction must stay dry. Water or any O–H or N–H proton would simply protonate the reagent and destroy it.
Step one is nucleophilic addition. The electrons of the C–Mg bond attack the electrophilic carbonyl carbon and form the new C–C bond, while the C=O π electrons move onto oxygen. Both arrows act together, and the product is a magnesium alkoxide (drawn here as O⁻ with Mg⁺–Br).
Step two is the workup. Only after the addition is complete is water added, in a separate step. The alkoxide oxygen takes a proton from water to give the neutral alcohol. Aldehydes give secondary alcohols, ketones like acetone give tertiary alcohols, and formaldehyde gives primary alcohols.
Each arrow is checked as you draw it, and Hint reveals the next move a little at a time.
Turn your own notes into exercises like this one
With the Arrow Lab student plan you can snap a photo of a reaction from your lecture notes or textbook. Arrow Lab reads the starting materials and reagents, you check that reading, and it builds the mechanism as an exercise like the one above: you draw each arrow, get hints when you are stuck, and see right away whether each move is correct.
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The Grignard reaction mechanism, step by step
The key idea is polarity reversal: in bromoethane the carbon attached to bromine is electrophilic, but once magnesium inserts, that same carbon is nucleophilic. The new C–C bond comes from the C–Mg bonding pair, so start the first arrow at the C–Mg bond, not at the carbon atom.
The 3 steps of the demo, in the exercise's own words. Try drawing each one above before you read it.
Add ethyl to acetone
Start the arrow at the C–Mg bond, not at the neutral carbon. Both arrows act together. The magnesium alkoxide is drawn as O⁻ and Mg⁺–Br; bonds to ether and larger magnesium clusters are not shown. Do not add water yet.
Add water after addition
Add water after the reaction in dry solvent. Water gives a proton to the alkoxide and becomes hydroxide.
Products
2-Methylbutan-2-ol has both methyl groups from acetone and the two-carbon ethyl group. Its OH oxygen comes from the carbonyl and is now neutral with two lone pairs. Water becomes hydroxide, whose negative charge balances Mg⁺–Br; these ions stand for the magnesium-containing residue after adding water. The alcohol is tertiary because its OH-bearing carbon has three carbon neighbors.
How the mechanism is drawn
Acetone and ethylmagnesium bromide (CH₃CH₂–Mg–Br) in dry ether, then water in a separate step. Draw addition followed by proton transfer; we show the magnesium alkoxide as O⁻ and Mg⁺–Br, without bonds to ether or larger magnesium clusters.