a
\((a)\) That the steric factor is not the sole determinant is, however, seen in the  figures for the nitration of the halobenzenes, which are \(o-/p-\) directing but on which  overall attack is slightly slower than on benzene.

Despite the increase in size of the substituent \(Y\) from \(F \to I\), the  proportion of \(o-\) isomer increases. An increasing steric effect will, as with the  alkyl benzenes, be operating to inhibit \(o-\) attack, but this must here be  outweighed by the electron-withdrawing inductive/field effect exerted by the  halogen atom \((Y)\). This effect will tend to decrease with distance from \(Y\), being exerted somewhat less strongly on the distant \(p-\) position compared  with the adjacent \(o-\) position. Electronegative \(F,\) and relatively little \(o-\)  attack thus takes place on \(C_6 H_5F\), despite the small size of \(F.\) The electron-  withdrawing effect of the halogen \((Y)\) decreases considerably from  \(F\) to \(I\) (the biggest change being between \(F\) and \(Cl\)), resulting in  increasing attack at the \(o-\) position despite the increasing bulk of \(Y.\)

\(Increase\,\,in\,\,size\,\,of\,\,Y\)  \(\begin{gathered}
   \downarrow  \hfill \\
   \downarrow  \hfill \\
   \downarrow  \hfill \\
   \downarrow  \hfill \\ 
\end{gathered} \) \(\begin{array}{*{20}{c}}
  Y&{\% \,o\, - }&{\% \,p\, - } \\ 
  F&{12}&{88} \\ 
  {Cl}&{30}&{69} \\ 
  {Br}&{37}&{62} \\ 
  I&{38}&{60} 
\end{array}\)