The melting points of -dichlorobenzene, -dichlorobenzene, and -dichlorobenzene are , , and , respect — Haloalkanes and Haloarenes Chemistry Question
Question
The melting points of $o$ -dichlorobenzene, $m$ -dichlorobenzene, and $p$ -dichlorobenzene are $-17^\circ C$, $-25^\circ C$, and $+54^\circ C$, respectively. What is the fundamental physical-chemical reason for the anomalously high melting point of the para-isomer?
💡 Solution & Explanation
Melting point is highly dependent on the crystal lattice enthalpy, which is governed by how closely and efficiently molecules can pack together in the solid state. The $p$ -isomer is perfectly symmetrical and linear, allowing it to fit very tightly into the lattice, requiring much more thermal energy to break apart. In contrast, the $o$ - and $m$ -isomers have asymmetrical, "kinked" structures that frustrate close packing, sharply lowering their melting points. Note that $p$ -dichlorobenzene actually has a dipole moment of zero.