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Alkylhalides: Substitution reactions 4 (Sn1)

Alkylhalides: Substitution

Nucleophile substitution reaction (Sn1 reaction)

To kick off this new chapter from the alkylhalides, some experimental background theory concerning Sn1 reactions:
  • the reaction rate of the reaction depends ONLY on the concentration of the alkyl halide (unlike Sn2 reactions where the nucleophile concentration was important aswell !!!).
  • the reaction rate of the reaction INCREASES if the alkylhalide is more branched at its reaction center (again unlike Sn2 reactions where more branched reaction centers are LESS active !!!). This means a tertiary alkyl bromide f.e. will have a huge relative reaction rate, methylbromide on the other hand will have no Sn1 reaction.
  • when an alkyl halide reacts with its enantiomere (cfr. stereochemistry), the reaction product will be a racemic mixture (this is a mixture of two enantiomeres of the same compound) or partial racemic product.
Conclusion: this indicates at the presence of a CARBOCATION intermediate

The Sn1 reaction is a two step reaction. The leaving group has already left the alkylhalide before the nucleophile comes in to play. See the example for more details:


Reaction rate Sn1 reaction

The reaction rate of the Sn1 reaction is dependent on the following basic rules:
  • the better the leaving group the faster the reaction will take place and be finished.
  • if the carbocation is very very stable, the reaction rate will be high. 
  • the more polar the solvent is, the higher the reaction rate. 
as stated before, the concentration of the attacking nucleophile has effect at all on the Sn1 reaction. Why? It is not implemented in the rate-determing step.

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