Definition:

• S_N1 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, S_N1 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 S_N1 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