Which Melts Higher?
Melting point is set by two things at once: the strength of the intermolecular forces and how neatly the molecules pack into a crystal. A symmetric molecule can melt far above a floppier isomer that attracts identically. All melting points here are measured values.
Pick the compound with the higher melting point. When the pair has a clean packing story, a follow-up asks you why.
Compound A
Compound B
Which melts higher?
Common Questions
Why isn't melting point a pure measure of intermolecular forces?
Melting also destroys the crystal lattice, so how neatly molecules pack matters as much as how strongly they attract. Anthracene and phenanthrene are isomers with nearly identical boiling points (340 vs 339 °C), yet anthracene melts at 216 °C and phenanthrene at 99 °C — the linear isomer packs, the bent one doesn't.
Why do symmetric molecules melt higher?
A symmetric molecule fits into an ordered lattice in more equivalent orientations, so it loses less entropy on crystallizing. Neopentane boils 27 °C below pentane (weaker dispersion contact) yet melts 113 °C above it (near-spherical packing).
Why does a trans isomer usually melt higher than its cis isomer?
The trans double bond keeps the chain straight, and straight molecules stack tightly into a crystal. The cis kink disrupts the stack. Elaidic acid (trans) melts at 43 °C while oleic acid (cis) melts at 14 °C — the same contrast that separates saturated fats from oils.
Why do melting points zigzag in homologous series?
Boiling points climb smoothly with chain length, but melting points alternate: chains with an even number of carbons pack end-to-end more efficiently than odd ones, so the even members melt anomalously high. This zigzag is one more sign that melting is about packing, not just attraction.