SN1 mechanism with stereochemistry; solvent

Unimolecular nucleophilic substitution (SN1)

Example: hydrolysis of an optically active tertiary halide, R₁R₂R₃C–Br + OH⁻.

Step 1 (slow):  R₃C—Br  →  R₃C⁺ + Br⁻           planar carbocation
Step 2 (fast):  R₃C⁺ + OH⁻  →  R₃C—OH

• Rate = k[RX], first order.
• Tertiary halides react because the carbocation is stabilised by hyperconjugation and the +I effect of alkyl groups.

Stereochemistry

In SN1 the carbocation formed is planar (sp²), so the nucleophile can attack from either face: from the face opposite to the leaving group gives inversion, from the same face gives retention. This gives a mixture (racemisation). Inversion slightly predominates because the departing leaving group stays near the front face as an ion pair and partly blocks it, so attack from the rear is easier.

              Nu attacks from rear   → inversion  (slightly more)
 planar R₃C⁺ 
              Nu attacks from front  → retention
Overall: partial racemisation of an optically active halide

Solvent

Solvent: polar protic solvents (water, ethanol, methanol, formic acid). They stabilise the carbocation and the halide ion by solvation (ion–dipole interaction and hydrogen bonding with the leaving group), which lowers the energy of the transition state of the slow step.