As steric hindrance (crowding) increases, the rate of \({S_{{N^2}}}\) reaction decreases.
The order of reactivity towards \({S_{{N^2}}}\) reaction for alkyl halides is
Primary halides \(>\) Secondary halides \(>\) Tertiary halides
$\left( C _6 H _5\right)_3 C - Cl \frac{ OH ^{-}}{\text {Pyridine }}\left( C _6 H _5\right)_3 C - OH$
$\begin{array}{*{20}{c}}
{Ph - CH - CH - CHO\xrightarrow[\Delta ]{{{K_2}C{O_3}}}} \\
{\,\,\,\,\,\,\,\,\,\,\,|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\
{\,\,\,\,\,\,Br\,\,\,\,\,\,\,\,\,\,Br\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}
\end{array}$