Σ-constant

{
  "id": "MLC-T-CHM-010588",
  "term": "Σ-constant",
  "field": "Chemistry",
  "definition": "A quantitative measure in physical organic chemistry that describes the electronic effect of a substituent on the reactivity of a molecule. It is a parameter in the Hammett equation, reflecting the electron-donating or electron-withdrawing capability of a substituent relative to hydrogen. Positive σ-values indicate electron-withdrawing groups, while negative values indicate electron-donating groups, influencing reaction rates and equilibrium constants.",
  "subject": "Organic and Biomolecular Chemistry",
  "see_also": [
    "ρ-value",
    "Hammett equation",
    "Taft equation",
    "ionization",
    "power",
    "mesomeric effect"
  ],
  "references": [
    "PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1171 (https://doi.org/10.1351/pac199466051077)"
  ],
  "url": "https://mlchart.com/terminology/chemistry/s-constant/"
}