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Sn1 vs. Sn2 Mechanisms. Strong or Weak Nucleophiles?

  1. Jun 16, 2008 #1
    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.
     
  2. jcsd
  3. Jun 16, 2008 #2

    chemisttree

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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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