unimolecular nucleophilic substitution reaction

chemistry

Also known as: unimolecular substitution reaction

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July 19, 2026

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Unimolecular nucleophilic substitution reactions (often abbreviated as SN1) are a two-stage chemical process. In the first, rate-determining step, a molecule breaks apart, forming a positively charged organic ion (a carbonium ion) and another ion. This step is unimolecular because it involves only one molecule of the starting material. The second stage involves the carbonium ion reacting with a nucleophile (an electron-seeking reagent) to form the final products.1

These reactions are characterized by first-order kinetics, meaning the reaction rate depends solely on the concentration of the starting material, not the nucleophile. They are also influenced by electron-releasing groups attached to the reaction center and can result in a mix of stereochemical configurations, including inversion and racemization.1

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reaction mechanism
Unimolecular
Unimolecular nucleophilic substitution reactions proceed by a two-stage mechanism in which heterolysis precedes reaction with the nucleophile. The following equation is a typical example:in which the symbols are the same as in earlier equations, with the addition of delta plus (δ+) and delta minus…
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Unimolecular nucleophilic substitution reactions proceed by a two-stage mechanism in which heterolysis precedes reaction with the nucleophile. The following equation is a typical example:in which the symbols are the same as in earlier equations, with the addition of delta plus (δ+) and delta minus…
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organohalogen compound
Nucleophilic substitution
…the mechanism is described as unimolecular, and the term S_N1 (substitution-nucleophilic-unimolecular) is applied. The species formed in the slow step contains a positively charged, electron-deficient carbon and is called a carbocation. Carbocations are unstable and react rapidly with substances such as nucleophiles that have unshared electrons available for bond formation.
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…the mechanism is described as unimolecular, and the term S_N1 (substitution-nucleophilic-unimolecular) is applied. The species formed in the slow step contains a positively charged, electron-deficient carbon and is called a carbocation. Carbocations are unstable and react rapidly with substances such as nucleophiles that have unshared electrons available for bond formation.
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