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Why do butane and isobutane have different boiling points and freezing points?

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Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — isomerism Tetrahedral geometry of methane: (A) stick-and-ball model and (B) diagram showing bond angles and distances. (Plain bonds represent bonds in the plane of the image; wedge and dashed bonds represent those directed toward and away from the viewer, respectively.)

Butane and isobutane are constitutional isomers, meaning they share the same chemical formula ( $C_{4} H_{10}$ ) but possess different structural arrangements. Butane features a straight-chain structure, while isobutane is a branched-chain isomer.123

These structural differences lead to variations in their physical properties, such as boiling and freezing points. The boiling point of an alkane is influenced by London forces, which are weak intermolecular attractive forces. Because the straight-chain structure of butane is more extended, it allows for a greater number of these intermolecular interactions compared to the more compact, branched shape of isobutane. Consequently, more energy is required to overcome these forces in butane, resulting in a higher boiling point (-0.5 °C) compared to isobutane (-11.7 °C). Similarly, their distinct molecular shapes affect how they pack into a crystal lattice, which accounts for the difference in their freezing points (butane at -138.3 °C and isobutane at -159.6 °C).342