What are the key differences between SN1 and SN2 mechanisms of nucleophilic substitution?

Okay, here is a shorter and more direct comparison of SN1 and SN2 mechanisms, focusing on their key differences: ### SN1 vs. SN2 Mechanisms of Nucleophilic Substitution Nucleophilic substitution in halogenoalkanes occurs via two main pathways: SN1 and SN2. --- **Summary Table of Differences:** | Feature | SN2 Mechanism (Substitution Nucleophilic Bimolecular) | SN1 Mechanism (Substitution Nucleophilic Unimolecular) | | :------------------ | :---------------------------------------------------- | :------------------------------------------------------- | | **Steps** | One-step process | Two-step process | | **Intermediate** | No intermediate (forms a transition state) | Carbocation intermediate formed | | **Rate Law** | Rate = k [R-X] [Nu⁻] (Second-order) | Rate = k [R-X] (First-order) | | **Reactivity Order**| Methyl > Primary (1°) > Secondary (2°) > Tertiary (3°) | Tertiary (3°) > Secondary (2°) > Primary (1°) > Methyl | | **Stereochemistry** | Inversion of configuration (Walden Inversion) | Racemization (mixture of enantiomers) | | **Solvent** | Favoured by Aprotic polar solvents (e.g., acetone) | Favoured by Protic polar solvents (e.g., water, alcohol) | | **Attack** | Backside attack by nucleophile | Nucleophile attacks planar carbocation from either side |