Why does toluene have a lower melting point than benzene despite having a higher molecular weight?

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Toluene has a lower melting point than benzene primarily because it does not pack as tightly in a crystal lattice. The crystal structure that solids adopt can significantly influence their physical properties, including melting points. Toluene's molecular structure, which features a methyl group attached to the aromatic ring, creates steric hindrance. This results in less efficient packing compared to benzene, which is a planar molecule and can pack more closely in the solid state.

As a result of these packing differences, the intermolecular forces holding the molecules together in the solid state are less effective in toluene than in benzene, leading to a lower melting point despite toluene’s higher molecular weight. Understanding crystal packing and the effects of molecular geometry on melting points is essential when analyzing properties of organic compounds.