An unknown, absolutely pure sample of a dichlorobenzene isomer is thoroughly characterized in the la — Haloalkanes and Haloarenes Chemistry Question
Question
An unknown, absolutely pure sample of a dichlorobenzene isomer is thoroughly characterized in the laboratory. It is found to be a solid at room temperature ($25^\circ C$) and exhibits the absolute lowest net dipole moment of all possible positional isomers. Which isomer is it, and what is its approximate relative boiling point compared to the other two isomers?
💡 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 ($\approx 181^\circ C$).