Question
Distinguish between $SN^1$ and $SN^2$ mechanisms.
| Factor | $SN^1$ | $SN^2$ |
| Kinetics | $1^{st}$ order | $2^{nd}$ order |
| Molecularity | Unimolecular | Bimolecular |
| Number of steps | Two steps | One step |
| Bond making and bond breaking | First the bond in the reactant breaks and then a new bond in the product is formed. | Simultaneous |
| Transition state | Two steps, two transition states | One step, one transition state |
| Direction of attack of nucleophile | Back side attack and front side attack | Only back side attack |
| Stereochemistry | Racemisation $($If the substrate is optically active$)$ | Inversion of configuration $($If the substrate is optically active$)$ |
| Type of substrate | Mainly $3^\circ$ substrates | Mainly $1^\circ$ substrate |
| Polarity of solvent | Polar protic solvent favorable | Aprotic $($non$-$polar$)$ or solvent with low polarity favourable |
| Intermediate | Intermediate involved | No intermediate |
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