Stoichiometry Calculator for Organic Synthesis

Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction. It tells you how much of each reagent to use and how much product to expect, based on the balanced chemical equation.

In practice, organic chemists describe reagent amounts using equivalents (equiv) relative to a limiting reagent — the reactant that determines the maximum amount of product. All other reagents are measured as multiples of the limiting reagent.

The core relationship is: mass (g) = mmol × MW (g/mol) / 1000

Where mmol = equiv × scale (mmol of limiting reagent), and MW is the molecular weight. For a 1.0 mmol scale reaction with a reagent at 1.2 equiv and MW 150.2 g/mol: mass = 1.2 × 1.0 × 150.2 / 1000 = 0.180 g (180 mg).

The theoretical yield is the maximum amount of product that could be formed from a given amount of limiting reagent, assuming 100% conversion and no side reactions.

Step-by-step calculation:

1. Identify the limiting reagent and its scale in mmol
2. Apply the stoichiometric coefficient: product mmol = limiting reagent mmol × (product coefficient / limiting reagent coefficient). For most reactions, both coefficients are 1.
3. Convert to mass: theoretical yield (g) = product mmol × product MW / 1000

Worked example: Suzuki coupling at 1.0 mmol scale, product MW = 182.22 g/mol. Theoretical yield = 1.0 × 182.22 / 1000 = 0.182 g (182 mg).

Percent yield = (actual yield / theoretical yield) × 100. A yield of 150 mg from the example above gives 150 / 182 × 100 = 82.4% yield.

Yields above 100% always indicate an error — impure product, incorrect molecular weight, or a weighing mistake. Never report a yield >100% in a publication.

A reagent table is the standard way organic chemists organize reaction information. It lists every component of a reaction with its equivalents, molecular weight, amount, and physical form.

Building a reagent table:

1. Choose the limiting reagent — typically the most valuable or most difficult-to-obtain starting material. Set its equivalents to 1.0.
2. Set the reaction scale — the amount of limiting reagent in mmol. This determines all other quantities.
3. Add other reagents with equivalents — follow the literature procedure or standard practice. Common excess: 1.1–1.5 equiv for coupling partners, 2–3 equiv for bases.
4. Specify the physical form — solid (weigh), neat liquid (measure by volume using density), or solution (measure by volume using molarity). This determines whether you see mass or volume in the output.
5. Account for catalysts — catalysts are described in mol% (e.g., 5 mol% Pd catalyst), not equivalents.

For salt forms and hydrates, use the molecular weight of the specific form you are weighing. For example, if using NaH as a 60% dispersion in mineral oil, adjust for the purity using the purity correction field.

Atom economy measures how efficiently a reaction converts reactant atoms into desired product atoms. It was introduced by Barry Trost in 1991 as a metric for evaluating reaction efficiency from a green chemistry perspective.

The formula is: AE = (MW of desired product / total MW of all products) × 100%

In practice, when you do not know all byproduct MWs, the formula simplifies to:AE ≈ (product MW / sum of all reactant MWs) × 100%

Benchmarks by reaction type:

• Addition reactions: approach 100% AE (all atoms incorporated into product)
• Substitutions: typically 60–80% AE (leaving group becomes waste)
• Eliminations: typically 50–70% AE (atoms lost as small molecule, e.g. water)

Key distinction: Atom economy is a theoretical metric based on the reaction equation — it does not account for excess reagents, solvent, or purification losses. Percent yield measures how much product you actually isolated. A reaction can have excellent atom economy but poor yield, or vice versa.