The Melting Point Of An Alkane

The melting point of solid molecules also increases with the increase of carbon atoms, but it is not as regular as the change of boiling point, and the same series C1-C3 is not so regular, but the number of carbon atoms above C4 increases with the increase. This is because the force between crystal molecules depends not only on the relative molecular mass, but also on the arrangement of molecules in the lattice. The higher the molecular symmetry, the closer the arrangement, the greater the attraction between the molecules, the higher the melting point. In normal alkanes, the melting point of alkanes containing odd carbon atoms rises less than that of alkanes containing even carbon atoms. Even in the melting point curve of straight-chain alkanes, alkanes with odd and even carbon atoms each form a melting point curve, with even numbers on the top and odd numbers on the bottom.
By X-ray diffraction analysis, the solid n-alkane crystal is zigzag shape, with both methyl groups on one side in the odd-numbered carbon toothed chain, such as n-pentane. The methyl groups at both ends of the even carbon chain are not on the same side, such as n-hexane, the even carbon chain is closer to each other, and the interaction force is large, so the melting point rise value is larger than the rise value of the singular carbon chain.