Selectivity Ratio Tool Calculator

• Parallel kinetics selectivity (instantaneous):
  For parallel pathways with rate laws r₁ = k₁ C^m and r₂ = k₂ C^n, the instantaneous selectivity is:
  S_inst = r₁ / r₂ = (k₁ / k₂) · C^(m−n)
• Yield or distribution selectivity (measured):
  S_yield = Amount_desired / Amount_undesired
  Percent selectivity for the desired product is 100 · Amount_d / (Amount_d + Amount_u).

1. Choose a calculation mode based on what you know. Use kinetics for model-based comparisons, and yields for experimental outcomes.
2. For kinetics: enter k₁, k₂, reaction orders m and n. If orders are unknown, leave them blank to assume first order.
3. Enter the reactant concentration C. Use C = 1 to isolate the pure k₁/k₂ ratio.
4. For yields: enter measured desired and undesired amounts using the same basis. Common choices are moles, mass, or chromatogram area.
5. Click Calculate to view results below the header. Use the download buttons to export a CSV or PDF report.

Interpreting Instantaneous Selectivity

Instantaneous selectivity describes how strongly a desired pathway dominates at a given composition. When m and n are equal, selectivity collapses to k1/k2, so temperature or catalyst changes that shift rate constants are main levers. When orders differ, the same catalyst can behave differently across feed strengths because C^(m−n) amplifies or suppresses r1 relative to r2. Use this value to compare mechanisms, not final isolated product.

Concentration Effects and Control

If m>n, increasing reactant concentration increases Sinst and raises the desired fraction. This often occurs when the desired route is higher order due to bimolecular coupling or surface coverage effects. If m<n, dilution can improve selectivity, common in competitive adsorption or inhibition. In practice, run a short concentration series, compute Sinst at each point, and fit a log–log slope versus C to validate the order difference.

Measured Distribution and Yield

Yield selectivity uses product amounts after a reaction step, integrating kinetics, mixing, and side reactions. A high Sinst does not guarantee a high Syield if undesired products form later through consecutive reactions or if the desired product decomposes. Record sampling time, quench method, and analytical calibration, then compute Syield from consistent bases such as moles, mass, or chromatogram area corrected by response factors. Track desired percent to communicate performance.

Data Sources and Uncertainty

Rate constants may come from initial-rate experiments, model fits, or literature, while product amounts typically come from GC, HPLC, NMR, or gravimetry. Uncertainty matters because selectivity is a ratio; small errors can become large relative errors when the denominator is small. Report significant figures, replicate injections, and blank corrections. If the undesired amount approaches zero, treat the value as below detection and avoid overinterpreting extreme ratios.

Consistent Reporting Practices

For reliable comparisons, keep units and definitions stable across notebooks and teams. Document temperature, solvent, and catalyst loading. Note k units and concentration units beside every entry, and state whether selectivity is instantaneous or yield-based. Use the CSV export to archive raw inputs and outputs, and the PDF export for batch reports. When comparing conditions, rank by desired percent and absolute desired amount so selectivity gains do not hide throughput losses.

1) Which mode should I use for early screening?

Use parallel kinetics when you have rate constants and reaction orders. It highlights how conditions change the pathway competition before downstream effects alter the final mixture.

2) What if I do not know reaction orders?

Leave orders blank to assume first order for both pathways. Then run a concentration series later to estimate whether m and n differ and update the calculation.

3) Can I use chromatogram area as the amount basis?

Yes, but apply response-factor corrections if available. Uncorrected areas can bias Syield when detector responses differ between desired and undesired products.

4) Why is percent selectivity different between modes?

Kinetics percent selectivity is instantaneous from r1 and r2, while yield percent selectivity reflects accumulated products after time, workup, and possible consecutive reactions.

5) How should I treat an undesired amount near zero?

Very small denominators inflate ratios. If the undesired signal is below detection, report it as below detection and rely on desired percent and absolute desired amount.

6) Do the exports include my input assumptions?

Yes. The CSV and PDF include the selected mode, key inputs, and computed outputs so results can be reviewed and reproduced from the same dataset.