The predominant intermolecular forces in hydrocarbons are dispersion forces. There are two factors that increase the dispersion forces between molecules:
Molecule size: The larger the electron cloud around the molecule, the greater the dispersion forces. The length of the carbon chain among similarly branched hydrocarbons can be used to assess this factor.
Molecule shape: Molecules that can align easily will have stronger dispersion forces. The linearity of the molecule among similarly branched hydrocarbons can be used to assess this factor.
Note that the size of the molecule has a much greater affect on the dispersion forces than the shape of a molecule.
Arrange the following hydrocarbons in order of decreasing melting point.
ethane
butane
trans-2-butene
2-butyne
cis-2-butene