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In a typical nucleophilic substitution reaction, a nucleophile Nu⁻ attacks a substrate R-LG.
Nu⁻ + R-LG → Nu-R + LG⁻
The leaving group LG departs, and the Nu replaces (substitutes) it in the substrate.
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Step 1: Loss of the leaving group, LG, to generate a carbocation intermediate.
R-LG → R⁺ + LG⁻
Step 2: Attack of the nucleophile on the electrophilic carbocation to form a new σ bond
Nu⁻ + R⁺ → Nu-R
Step 1 is the rate determining step. Only the substrate is involved in this step, so this is a unimolecular reaction.
The "1" in
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