Question

Explain only reaction mechanism for alkaline hydrolysis of tert-butyl bromide

Answer 1

\[\ce{\underset{\text{(t-butyl bromide)}}{(CH3)3C}-Br + \underset{\text{(Nuclophile)}}{:OH-}->(CH3)3\underset{\text{(t - butylalcohol)}}{C-OH}+\underset{\text{(Bromide ion)}}{:Br-}}\]

The rate of the above reaction depends upon the concentrations of the substrate, methyl bromide and the nucleophilic, OH ion.

Mechanism of SN¹ reaction: The alkaline hydrolysis of tertiary butyl bromide (a tertiary alkyl halide) proceeds through SN¹ reaction mechanism. This reaction is a two-step process. The first step is a slow step while the second one is a fast step. The hydrolysis reaction can be written as follows.

Step-1 Formation of carbocation

\[\begin{array}{cc}
\phantom{....}\ce{CH3}\phantom{............}\ce{CH3}\phantom{..}\\
\phantom{.....}|\phantom{................}|\phantom{......}\\
\ce{H3C-C-Br⇌H3C-C^⊕ +Br^Θ}\\
\phantom{.....}|\phantom{................}|\phantom{......}\\
\phantom{....}\ce{CH3}\phantom{............}\ce{CH3}\phantom{..}
\end{array}\]

Step-2 Attack of the nucleophile on carbocation

\[\begin{array}{cc}
\phantom{.......}\ce{CH3}\phantom{...........}\ce{CH3}\phantom{...}\\
\phantom{.........}|\phantom{..............}|\phantom{........}\\
\ce{H3C-C^⊕->[OH^Θ]H3C-C-OH}\\
\phantom{.........}|\phantom{..............}|\phantom{........}\\
\phantom{.......}\ce{CH3}\phantom{...........}\ce{CH3}\phantom{...}\\
\end{array}\]

As carbocation has planar geometry, therefore, attack of nucleophile takes place from both the side to give retention inversion in configuration, therefore, the product formed is optically inactive racemic mixture.