Sn1 vs. Sn2 Mechanisms. Strong or Weak Nucleophiles?

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SUMMARY

The discussion focuses on the distinction between Sn1 and Sn2 mechanisms in nucleophilic substitution reactions, emphasizing the role of substrate structure and nucleophile strength. It highlights that methanol, when reacting with cyclohexyl bromide, leads to an Sn1 mechanism due to methoxide being a weak nucleophile despite being a strong base. The conversation also touches on the possibility of competing reactions, such as E2, indicating that methoxide does not facilitate substitution in this scenario.

PREREQUISITES
  • Understanding of Sn1 and Sn2 reaction mechanisms
  • Knowledge of nucleophile strength and its impact on reaction pathways
  • Familiarity with substrate classification (primary, secondary, tertiary carbon)
  • Basic principles of elimination reactions (E2)
NEXT STEPS
  • Study the characteristics of strong vs. weak nucleophiles in organic chemistry
  • Explore the factors influencing Sn1 and Sn2 mechanisms
  • Research the role of substrate structure in determining reaction pathways
  • Investigate the E2 elimination mechanism and its competition with substitution reactions
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Chemistry students, organic chemists, and anyone interested in understanding nucleophilic substitution mechanisms and reaction dynamics.

premed
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One can predict by which mechanism a reaction will occur, Sn2 or Sn1, by the substrate's order( i.e. primary, secondary, or tertiary carbon) and by the strength of the nucleophile. How can I tell if a nucleophile is strong or weak? For instance, one problem showed methanol combining with cyclohexyl bromide. I thought the methoxide would be a strong nucleophile but it was in fact weak and went through the Sn1 mechanism instead of Sn2. How could I tell if it is strong or weak. Thanks.
 
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It is a bit more complicated than "strong nucleophile" vs "weak nucleophile". Are you sure it didn't go through an E2 process in the presence of methoxide? You have two or more competing processes at work. One process occurs most readily with increasing carbon substitution and in the presence of strong bases. Methoxide is a strong base and will readily deprotonate things...

Methoxide will not make methoxycyclohexane from cyclohexyl bromide by any mechanism!
You don't get substitution in that case.
 

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