The correct order of increasing boiling points is: (a) Isobutane \(<\mathrm{n}\) -butane \(<\mathrm{n}\) -butyl chloride \(<\mathrm{n}\) -butanol (b) Isobutane \(<\mathrm{n}\) -butyl chloride \(<\mathrm{n}\) -butane \(<\mathrm{n}\) -butanol (c) \(\mathrm{N}\) -butane \(<\mathrm{n}\) -butanol \(<\mathrm{n}\) -butyl chloride \(<\) isobutane (d) N-butane \(<\) isobutane \(<\mathrm{n}\) -butyl chloride \(<\mathrm{n}\) -butanol

Understanding the factors affecting boiling points starts with a grasp of intermolecular forces—the attractions that exist between molecules. These forces are the glue that holds matter in a condensed state (either liquid or solid). There are several types of intermolecular forces, each varying in strength and impact on physical properties like boiling points.

London Dispersion Forces: These are the weakest intermolecular forces and exist between all molecules, whether polar or nonpolar. Caused by the temporary shifting of electron clouds, they grow stronger with increased molecular size or mass.
Dipole-Dipole Interactions: Occurring in polar molecules, these are medium-strength forces resulting from the attraction between the partial positive end of one molecule and the partial negative end of another.
Ion-Dipole Interactions: These forces are present in solutions of ions and polar molecules and are stronger than dipole-dipole interactions.

In summary, the force that is predominant in a substance determines how much energy is required to move molecules apart, turning a solid to a liquid or a liquid to a gas, which is associated with melting and boiling points.

Among all intermolecular forces, hydrogen bonding is peculiar because of its strong influence on boiling points. This specific interaction occurs in molecules containing a hydrogen atom bonded to electronegative elements such as oxygen, nitrogen, or fluorine. The electronegative atom draws electron density away from the hydrogen, creating a significantly polar bond.

When this hydrogen atom comes near a lone pair of electrons on another electronegative atom, a hydrogen bond forms. These bonds are strong enough to have a pronounced effect, significantly raising the boiling point of substances compared to those where hydrogen bonding is not possible. For instance, in the context of the original exercise, n-butanol exhibits hydrogen bonding due to its -OH group and experiences a much higher boiling point compared to compounds lacking this feature.

The boiling point trends in organic compounds are dictated largely by molecular mass and intermolecular forces. As the molecular mass increases, the boiling point typically rises since there is a greater area for London dispersion forces to act. Moreover, the structural complexity of molecules can affect this trend. For example, branching often decreases boiling points, which can be seen in the comparison of isobutane to n-butane. Isobutane, being more branched, has a lower boiling point.

Furthermore, the functional groups present within organic molecules play a pivotal role. Groups that can engage in hydrogen bonding, like hydroxyl (-OH) groups, lead to significantly higher boiling points. In the exercise, although n-butyl chloride has a polar bond, n-butanol's ability to hydrogen bond trumps that, placing it highest in the boiling point order.

The molecular structure of an organic compound profoundly influences its boiling point. Linear molecules, which have a larger area of contact between them, exhibit stronger London dispersion forces as compared to their branched counterparts, resulting in higher boiling points. On the other hand, branched molecules like isobutane have lower surface areas that interact with each other, leading to weaker dispersive forces and therefore lower boiling points.

Functional groups add another layer of complexity. The presence of electronegative atoms can introduce polarity to a molecule, enhancing dipole-dipole interactions and increasing the boiling point. However, if the molecule can create hydrogen bonds, this will overshadow the effects of dipole-dipole interactions, as these bonds are generally the strongest type of intermolecular force that occurs in organic molecules. Consequently, molecules such as alcohols, which can form these strong links, have notably higher boiling points compared to other organic compounds with similar molecular weights but without the capacity for hydrogen bonding.