Grignard Stoichiometry Calculator
Grignard reactions (discovered by Victor Grignard, Nobel Prize 1912) are among the most important carbon-carbon bond-forming reactions in organic synthesis. Calculating the correct equivalents of Grignard reagent is critical: too few and the reaction is incomplete; too many and purification becomes harder. Aldehydes and ketones require 1 equivalent of Grignard reagent; esters and CO2 require 2 equivalents. Enter the masses and molecular weights of both reagents, choose the stoichiometry, and this calculator identifies the limiting reagent and the theoretical moles of product.
Grignard stoichiometry formula
moles = mass / MW
Grignard equiv = moles(RMgX) / (moles(electrophile) x stoich ratio)
Limiting reagent: whichever gives fewer product moles
The electrophile is limiting if moles(RMgX) / stoich ratio is less than moles(electrophile). Otherwise the Grignard is in excess and the electrophile is limiting. Theoretical product moles = min(moles(RMgX) / stoich, moles(electrophile)).
Common Grignard reagent molecular weights
- Methylmagnesium bromide (MeMgBr): 119.24 g/mol.
- Ethylmagnesium bromide (EtMgBr): 133.27 g/mol.
- Phenylmagnesium bromide (PhMgBr): 181.31 g/mol.
- Allylmagnesium bromide: 145.29 g/mol.
- Vinylmagnesium bromide: 131.25 g/mol.
Grignard stoichiometry: frequently asked questions
What is a Grignard reaction?
A Grignard reaction uses an organomagnesium halide (RMgX, the Grignard reagent) as a nucleophile. The reagent adds to electrophiles such as carbonyl compounds (ketones, aldehydes, esters) to form carbon-carbon bonds. Most additions are 1:1 (one mole of Grignard per mole of electrophile), but addition to esters requires 2 moles.
How many equivalents of Grignard reagent does an ester need?
Esters require 2 equivalents of Grignard reagent to give a tertiary alcohol. The first equivalent adds to give a ketone intermediate, and the second equivalent adds to the ketone before work-up. Aldehydes and ketones require 1 equivalent.
What is the limiting reagent?
The limiting reagent is the reactant that runs out first, determining the maximum theoretical yield. This calculator compares moles of the Grignard reagent to moles of the electrophile (adjusted for stoichiometry) and identifies which is limiting.
Why must Grignard reactions be anhydrous?
Grignard reagents react violently with water (RMgX + H2O gives RH + Mg(OH)X), destroying the reagent before it can react with the electrophile. All glassware, solvents, and substrates must be rigorously dried. Diethyl ether and THF are the typical solvents.
How do I calculate the mass of Grignard reagent needed?
Mass (g) = moles of electrophile x stoichiometric equivalents x molar mass of Grignard reagent. For example, to add methylmagnesium bromide (MW 119.24 g/mol) to 1 mmol of benzaldehyde (1:1 ratio): mass = 0.001 mol x 1 x 119.24 g/mol = 0.119 g. In practice, use 1.2 to 1.5 equivalents to ensure complete reaction.
Official sources
- NIST Chemistry WebBook: Organometallic compound data.
- ACS: ACS Publications (Journal of the American Chemical Society).
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.