Organic Chemistry - "Change", Part 4: The SN1 Reaction - A Summary

 Greetings,

This is a short introduction to the SN1 reaction.  SN1 has the following characteristics:

• Monomolecular - 1st Order
• Rate only depends on the leaving-group concentration
• Leaving group on second or third degreee carbon atom
• Products are a mix of mirror-image optical isomers

Monomolecular 1st Order

The kinetics of SN1 is a simple one-to-one relationship between reaction rate and leaving group concentration. For example, doubling the concentration of the leaving group (in essence, doubling the concentration of the substrate-electrophilic reactant) doubles the reaction rate.  Changing the nucleophile concentration has no effect on reaction rate.

Rate change only depends on changes in the leaving-group concentration

The overall reaction happens in two steps: 1. Leaving group departs (fast) & 2. Nucleophile attacks the remaining carbocation (slow). Step 1 is the rate-determining part of the SN1 reaction and therefore the only step affecting reaction rate.

Leaving group on second or third degreee carbon atom

The SN1 reaction happens when a stable intermediate carbocation can form (in addition to a leaving group that can have a stable solvated form). Leaving groups on second and third degree carbon atoms create a more stable carbocation.

Products are a mix of mirror-image optical isomers

The carbocation intermediate can be attacked by the nucleophile from both sides.  This causes a pair of optical isomers (enantiomeric mirror-image) to form.

Thant's all for now.  As always, thank you for reading!