SN1
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Definition:

  • SN1 stands for Nucleophilic Substitution Unimolecular.
  • This means that the rate of these reactions appears to depend only on the substrate concentration and not on the concentration of nucleophile.
  • The rate of the disappearance of substrate = d[ substrate ]/dt = -k[ substrate ], negative slope asymptotic to both axis.
  • The truth is that the rate of this reaction is really pseudo-first order in substrate concentration because the concentration of solvent and acid catalyst, which affect this reaction, don't change that much as the reaction proceeds.

Mechanism:

  • These substitutions occur by a removal (ionization) of the leaving group to make a carbocation intermediate.
  • They share the same first step as an E1 reaction.
  • The nucleophile attaches itself to the carbocation intermediate in an asynchronous fashion.
  • The electrophilic carbon must be sp3 hybridized.
  • The hybridization of the carbocation intermediate of this process is sp2.
  • This reaction leads towards racemization at carbons that are chiral centers:
    • sometimes not a perfect 50/50 mixture because of a cage effect or favoring one diastereomer,
    • When a neighboring group can stabilize the carbocation intermediate, inversion of configuration is observed.  This is called anchimeric assistance.
    • Epoxides substitute by this mechanism at the most sterically hindered position with inversion of configuration because the leaving oxygen is anchored as a neighbor, SN1 with "goal tending".

Factors:

  • The rate of this reaction is affected by:
    • The stability of the carbocation intermediate, rate greater for tertiary > secondary >> primary, neopentyl
      • Inductive effect
      • Hyper-conjugation
    • Solvent, works best with polar protic solvents like water, alcohols, etc.
      • A polar solvent is required for solubility.
      • A protic solvent hydrogen bonds to the leaving group helping to ionize it off.
      • When the solvent is also the nucleophile the reaction is called a solvolysis.
      • This means that SN1 substitutions are really pseudo-first order because the solvent has an effect on the rate but its concentration does not change that much!
    • Leaving group, works best with leaving groups that are weak bases because they have a weaker bonds with carbon and can stabilize negative charge best.
      • F > HO > H2N
      • I > Br > Cl > F
      • right-down, more right than down
      • cation > neutral
  • Though not affecting the rate of the reaction, less electronegative nucleophiles (but not bases) works best:
    • OH2 > HF
    • Se > S > O
    • I > Br > Cl > F
    • left-down, more left than down

 

 

 

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Thursday, April 06, 2017 13:18