Normal butane and isobutane have boiling temperatures of \(-0.5^{\circ} \mathrm{C}\) and \(-12.3^{\circ} \mathrm{C}\) (31.1 \(\mathrm{F}\) and \(9.9^{\circ} \mathrm{F}\) ), respectively. Briefly explain this behavior. on the basis of their molecular structures, as presented in Section \(14.2\).

Understanding the molecular structure of substances is crucial in predicting their physical properties, like boiling points. In the case of normal butane and isobutane, both share the molecular formula C₄H₁₀ but differ drastically in their molecular architecture. The structural differences have profound effects on the strength of interactions known as London dispersion forces.

Molecular structure refers to the three-dimensional arrangement of atoms within a molecule. Normal butane has a linear, extended shape which allows it to have more points of contact with neighboring molecules compared to isobutane's more compact, branched shape. This difference in touch points is fundamentally connected to the London dispersion forces, which are weak, temporary attractive forces caused by the movement of electrons that induce temporary dipoles in molecules.

Molecular Structure and Strength of London Dispersion Forces

A straight-chain molecule like normal butane has a larger surface area exposed to adjacent molecules which increases the opportunity for these temporary dipoles to occur. Thus, normal butane experiences stronger intermolecular attractions. Conversely, isobutane with its branched configuration has less surface area available to interact with other molecules resulting in weaker London dispersion forces.

Boiling temperature is an important physical property determined by the strength of the intermolecular forces within a substance. The need to understand boiling temperature may seem remote until one considers processes like distillation or everyday tasks like cooking.

For molecules with only London dispersion forces at play, the more substantial these forces are, the higher energy is required for molecules to break free from the liquid and enter the gas phase, which is the essence of boiling. This fundamental relationship allows scientists and students alike to infer molecular interactions based on boiling points.

Connecting Boiling Temperature to Molecular Interactions

When comparing substances with similar molecular weights but different boiling temperatures, one can deduce that those with a higher boiling point have stronger intermolecular forces. In the example of normal butane and isobutane, since normal butane has a higher boiling temperature (-0.5°C) compared to isobutane (-12.3°C), it implies that the intermolecular forces in normal butane are stronger, demanding more energy to convert from liquid to gas.

The term intermolecular forces refer to the forces of attraction or repulsion which act between neighboring particles: atoms, ions, or molecules. They are the glue that holds substances together and dictate state changes from solid to liquid to gas.

London dispersion forces are a type of intermolecular force present in all molecules, especially significant in nonpolar compounds such as butane isomers. They are usually the weakest of the intermolecular forces, but their cumulative effect, especially in larger or elongated molecules, can be substantial enough to affect physical properties such as boiling temperature.

Role of London Dispersion Forces in Physical Properties

In summary, the London dispersion forces, arising from fluctuations in electron distributions within molecules, result in temporary dipoles inducing others in close proximity. The strength and therefore the impact of these forces can be altered by the molecular structure of the substance, as evidenced by the straight-chain form of normal butane having stronger forces and consequently a higher boiling temperature than the branched isobutane.