The melting points of -dichlorobenzene, -dichlorobenzene, and -dichlorobenzene are , , and , respectively. What is the fundamental physical-chemical reason for the anomalously high melting point of th

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?

Prashant Kotian · Accepted Answer

Correct answer: B. 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 -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 - and -isomers have asymmetrical, "kinked" structures that frustrate close packing, sharply lowering their melting points. Note that -dichlorobenzene actually has a dipole moment of zero.

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