Introduction
The SN1 reaction, a type of unimolecular nucleophilic substitution in organic chemistry, is characterized by two key features. “SN” stands for “nucleophilic substitution”, and the “1” indicates that the rate-determining step involves a single molecule. As such, the rate equation typically shows a first-order relationship with the substrate and no dependence on the nucleophile. In the initial phase of the SN1 mechanism, a planar carbocation is formed. This allows the nucleophile to attack from either side in the second step, potentially leading to a racemic mixture. However, complete racemization is not achieved because the nucleophile attacks the carbocation before the leaving halide ion fully detaches. The halide ion’s negative charge prevents frontal attacks on the carbocation, making a backside attack more likely, resulting in a predominance of inverted configuration enantiomers over a complete racemic mixture. The carbocation intermediate, pivotal in the rate-determining step, exhibits sp2 hybridization with a trigonal planar shape, allowing the nucleophilic attack to occur from both sides of the molecule. In scenarios where neither path is preferred, the outcome is a racemic mix of enantiomers, especially if the reaction occurs at a chiral center.
Side reactions such as elimination and carbocation rearrangement often accompany the SN1 process. Under elevated temperatures that favor entropy, E1 elimination becomes the dominant pathway, leading to alkene formation. Conversely, at cooler temperatures, SN1 and E1 reactions compete, making it challenging to predict the predominant reaction. Even under cold conditions, some alkene byproduct may still be produced.
Reaction

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2-step reaction
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The intermediate is a carbocation
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The reaction rate depends only on the substrate, not the nucleophile
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The rate depends on how good the leaving group is and how stable the resulting carbocation is
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Mechanism

Electrophile Trends

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SN1 reactions are faster for substrates in which the leaving group is bonded to a more substituted carbon because the resulting carbocation is be more stable.
Keywords
unimolecular substitution | SN1 | carbocation | electrophile | nucleophile |