ESn1

ESn1 is an abbreviation sometimes used in organic chemistry education to denote the first elementary step of the SN1 reaction mechanism. In this usage, ESn1 refers specifically to the ionization step in which the leaving group departs from a substrate to generate a carbocation (or resonance-stabilized cation). This step is unimolecular and typically rate-determining, with a rate law of rate = k [substrate]. The nucleophile does not influence the rate at this stage.

Mechanism involves two stages. Step 1: ionization to form a carbocation, often facilitated by polar protic solvents and good leaving groups. Step 2: nucleophilic attack on the carbocation by the solvent or other nucleophile, followed by any necessary proton transfers or rearrangements. The second step is usually rapid relative to the first.

Key characteristics include a preference for substrates that can stabilize carbocations, such as tertiary, benzylic, or allylic centers, and the use of polar protic solvents that stabilize charged intermediates. The rate-determining ionization explains the observed first-order kinetics with respect to substrate and a lack of dependence on nucleophile concentration.

Stereochemistry is notable: if the substrate is chiral, formation of a planar carbocation intermediate leads to racemization at the reactive center, though neighboring-group effects can influence outcome in some cases.

ESn1 is not universally used as a standalone term; more commonly the SN1 mechanism and its rate-determining ionization step are discussed without the acronym. See also SN1, SN2, E1, carbocation rearrangements.