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Sn1 vs Sn2 reactions

  1. May 10, 2005 #1
    Hello.

    How do we know which reaction is favored?
    I know that:
    -a primary alkyl halide favors the Sn2
    -weak bases promote sn2 because they can attack sp3 carbon more easily

    Let's say we are given an electrophile and a nucleophile. Is there a foolproof trick which determines which reaction occurs?
     
  2. jcsd
  3. May 10, 2005 #2

    GCT

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    No foolproof tricks, you'll need to study this in detail because there are quite a few factors which influence Sn2 and E2 reactions. You've got a carbocation formation in the sn1 usually with the aid of a catalyst and sn2 requires a stronger nucleophile; in most cases, sn2 reactions occur at the terminal motif of the molecule, while sn1 reactions are prefered at the secondary, tertiary carbons due to the stabilization of the carbocation.
     
  4. May 10, 2005 #3
    The rate-limiting step of the Sn1 reactions is the formation of the carbocation/carbanion - it is limited by the degree to which the molecule disassosciates in a solution. Sn1 is essentially a Lewis acid-base reaction. As GCT indicated, this can be influenced by a variety of factors, including solvents. The classic example is that is that of a polar aprotic solvent can increase the rate of a Sn1 reaction by a million fold simply because it solvates the anions well but does not solvate cations to the same degree.
     
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