The $S_{N}1$ (Substitution Nucleophilic Unimolecular) mechanism proceeds through the formation of a carbocation intermediate.

1. Step 1 (RDS): Formation of carbocation by leaving group departure.
2. Step 2: Rapid attack of the nucleophile.

• The rate of reaction depends solely on the stability of the carbocation formed.
• Stability Order: $3^{\circ }$ (Tertiary) $>2^{\circ }$ (Secondary) $>1^{\circ }$ (Primary) $>C{H}_3^{+}$ (Methyl).
• This is due to inductive effect ($+I$) and hyperconjugation from alkyl groups which disperse the positive charge. Therefore, tertiary halides are most reactive in $S_{N}1$.