a
\((a)\) The steric demand of \(H^{\ominus}\)  is, however extremely small, and  when attack on \(C_6H_5Y\) is by any other electrophile, \(E^{\oplus}\) , which  will necessarily be larger, there will be increasing interaction between \(E\) and \(Y\) in the transition state for attack at the position \(o-\) to \(Y (57 \,b, R = E)\)  as attacking electrophile and substituent increase in size ; there can be no such interaction in the transition state for \(p-\) attack \((57a, R = E)\). This will  be reflected in an increasing \(\Delta \,G^+\) for \(o-\) attack, a consequently  slower reaction, and the relative proportion of \(o-\) product will thus fall as the  size of \(E\) and / or \(Y\) increase. This is illustrated by the falling \(f_{o^-} /f_{p^-}\) ratios which are  observed for the nitration of alkylbenzenes \((Y ---CH_3 \to CMe_3)\) under comparable conditions ; 

Increase in size of \(Y\) \(\begin{gathered}
   \downarrow  \hfill \\
   \downarrow  \hfill \\
   \downarrow  \hfill \\
   \downarrow  \hfill \\
   \downarrow  \hfill \\ 
\end{gathered} \) \(\begin{array}{*{20}{c}}
  Y&{\% \,\,o - }&{\% \,p\, - } \\ 
  {C{H_3}}&{58}&{37} \\ 
  {C{H_2}Me}&{45}&{49} \\ 
  {CHM{e_2}}&{30}&{62} \\ 
  {CM{e_3}}&{16}&{73} 
\end{array}\)