An unknown, absolutely pure sample of a dichlorobenzene isomer is thoroughly characterized in the la — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

The only dichlorobenzene isomer that is a solid at room temperature is the highly symmetric $p$ -dichlorobenzene ($m.p. \approx +54^\circ C$). The other two are liquids ($o$ - m.p. $\approx -17^\circ C$, $m$ - m.p. $\approx -25^\circ C$). Furthermore, due to its diametrically opposed $C-Cl$ bonds ($180^\circ$), the $p$ -isomer has a net dipole moment of exactly zero. Because it lacks permanent dipole-dipole attractions in the liquid phase, its boiling point ($\approx 170^\circ C$) is slightly but noticeably lower than the polar $m$ -isomer ($\approx 173^\circ C$) and the highly polar $o$ -isomer ($\approx 181^\circ C$).