Question

Explain $S_{N}1$ mechanism with an example.

Answer 1

An example of a reaction taking place with an $S_{N^1}$ reaction mechanism is the hydrolysis of tert-butyl bromide with water forming tert-butanol-

$\underset{\text{tert-butyl bromide}}{{\left(C{H}_3\right)}_{3}C-Br}+2H_{2}O⟶\underset{\text{tert-butanol}}{{\left(C{H}_3\right)}_{3}C-OH}+{Br}^{-}+{H_{3}O}^{+}$

The above $S_{N^1}$ reaction takes place in three steps-

• Formation of a tert-butyl carbocation by separation of a leaving group $\left({Br}^{-}\right)$ from the carbon atom (this step is slow and reversible)-
  

${\left(C{H}_3\right)}_{3}C-Br⟶{{\left(C{H}_3\right)}_{3}C}^{+}+{Br}^{-}$

• The carbocation reacts with the nucleophile. The intermediate is an oxonium ion (this reaction step is fast).
  

${{\left(C{H}_3\right)}_{3}C}^{+}+H-O-H⟶{\left(C{H}_3\right)}_{3}C-H-\stackrel{\oplus }{O}-H$

• Deprotonation will take place and alcohol will form along with hydronium ion (this reaction step is fast).
  

${\left(C{H}_3\right)}_{3}C-H-\stackrel{\oplus }{O}-H+H-O-H⟶{\left(C{H}_3\right)}_{3}C-OH+{H_{3}O}^{+}$