Factors affecting rate of SN1

The reactivity of a SN1 reaction is determined by 3 factors:

1. the substituent
2. leaving group
3. solvent effect
Note that the nucleophilicity is not considered here but a weak nucleophile is usually preferred.

Substituent
Substituent is the things that attached to the carbocation. As the carbocation formation is rate-determining step, a stable carbocation is necessary. A carbocation is positively charged and it is electron deficient. So it needs some groups/ substituents that are electron-donating. Methyl group (not hydrogen anymore) stabilizes the carbonation by
  1. the inductive effect (donating electrons through the sigma bond)
  2. hyperconjucation (donating electrons through the overlap of the sp3 orbital and the empty p orbital)
Therefore, a more substituted alkyl halide is more favored than a primary alkyl halide, which is the opposite of the SN2 reaction. Resonance (typically double bond or oxygen lone pair e-) also stabilized the carbocation.

Leaving Group
A good leaving group is necessary for the reaction to proceed because it is involved in the rate-determining step. It leaves the carbon during the slow ionization step. Therefore, it should be:
  • highly polarizing (to stabilize the transition state) (this also means electron-withdrawing)
  • a weak base (so it is stable after getting the electrons from its bonding to the carbon atom)
Note that the qualities of a good leaving group is the same as that of a SN2 reaction!

Solvent
Recall again that the rate-determining step is ionization (separation of two charges: the carbocation and the leaving group anion). Therefore, we need a highly polar solvent that favors and stabilizes these charges. Protic or aprotic then? Protic! Hydrogen bonding is formed between the anion and the hydrogen of the solvent (typical protic solvent: alcohol and water). The carbocation is stabilized by forming a complex with the non-bonding/ lone pair of electrons of the oxygen of the solvent.

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