$\underset{A}{\mathop{PhF}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\underset{B}{\mathop{PhCl}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\underset{C}{\mathop{PhBr}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\underset{D}{\mathop{PhI}}\,$
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} $ $\begin{array}{*{20}{c}}
Y&{\% \,o\, - }&{\% \,p\, - } \\
F&{12}&{88} \\
{Cl}&{30}&{69} \\
{Br}&{37}&{62} \\
I&{38}&{60}
\end{array}$
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$2 A ( g ) \rightleftharpoons A _{2}( g )$
at $400\, K$ has $\Delta G ^{\circ}=+25.2\, kJ mol ^{-1}$.
The equilibrium constant $K _{ C }$ for this reaction is $...... \times 10^{-2}$. (Round off to the Nearest integ $\left[\right.$ Use $: R=8.3\, J mol ^{-1} K ^{-1}, \ln 10=2.3$
$\left.\log _{10} 2=0.30,1\, atm =1\, bar \right]$
$[$ antilog $(-0.3)=0.501]$
