Editing Nucleophile (section)

==== Ritchie equation ====
The Ritchie equation, derived in 1972, is another free-energy relationship:<ref>{{cite book |doi=10.1351/goldbook.R05402 |doi-access=free |chapter=Ritchie equation |title=The IUPAC Compendium of Chemical Terminology |year=2014 }}</ref><ref>{{Cite journal |last=Ritchie |first=Calvin D. |date=1972-10-01 |title=Nucleophilic reactivities toward cations |url=https://pubs.acs.org/doi/abs/10.1021/ar50058a005 |journal=Accounts of Chemical Research |language=en |volume=5 |issue=10 |pages=348–354 |doi=10.1021/ar50058a005 |issn=0001-4842|url-access=subscription }}</ref><ref>{{Cite journal |last=Ritchie |first=Calvin D. |date=March 1975 |title=Cation-anion combination reactions. XIII. Correlation of the reactions of nucleophiles with esters |url=https://pubs.acs.org/doi/abs/10.1021/ja00838a035 |journal=Journal of the American Chemical Society |language=en |volume=97 |issue=5 |pages=1170–1179 |doi=10.1021/ja00838a035 |issn=0002-7863|url-access=subscription }}</ref>
:<math>\log_{10}\left(\frac{k}{k_0}\right) = N^+</math>

where ''N''<sup>+</sup> is the nucleophile dependent parameter and ''k''<sub>0</sub> the [[reaction rate constant]] for water. In this equation, a substrate-dependent parameter like ''s'' in the Swain–Scott equation is absent. The equation states that two nucleophiles react with the same relative reactivity regardless of the nature of the electrophile, which is in violation of the [[reactivity–selectivity principle]]. For this reason, this equation is also called the ''constant selectivity relationship''.

In the original publication the data were obtained by reactions of selected nucleophiles with selected electrophilic [[carbocation]]s such as [[tropylium]] or [[diazonium]] cations:
:[[File:RichieEquationDiazonium.png|400px|Ritchie equation diazonium ion reactions]]

or (not displayed) ions based on [[malachite green]]. Many other reaction types have since been described.

Typical Ritchie '''N<sup>+</sup>''' values (in [[methanol]]) are: 0.5 for [[methanol]], 5.9 for the [[cyanide]] anion, 7.5 for the [[methoxide]] anion, 8.5 for the [[azide]] anion, and 10.7 for the [[thiophenol]] anion. The values for the relative cation reactivities are −0.4 for the malachite green cation, +2.6 for the [[benzenediazonium cation]], and +4.5 for the [[tropylium cation]].