Of the following compounds, the one with the lowest boiling point is (a) Pentyl chloride (b) Tertiary butyl chloride (c) Isobutyl chloride (d) \(\mathrm{n}\) -butyl chloride

When considering the boiling points of alkyl halides, it's vital to understand intermolecular forces, which are the attractive forces between molecules. In general, stronger intermolecular forces result in higher boiling points because greater energy is required to separate the molecules. For alkyl halides, the most relevant forces are van der Waals forces or London dispersion forces. These forces are dependent on the surface area of the molecule: the larger the surface area, the stronger the van der Waals forces.

The branching of a molecule can significantly reduce the surface area available for these forces to act upon. For example, in tertiary butyl chloride, the high degree of branching results in a compact structure, which has a smaller surface area compared to its less branched counterparts. This leads to weaker van der Waals forces and therefore a lower boiling point. On the other hand, pentyl chloride, having a straight chain, exhibits stronger intermolecular forces due to its larger surface area, and so has a higher boiling point.

While analyzing boiling points, molecular weight is often a critical factor to consider. Generally, as the molecular weight increases, the boiling point also rises because heavier molecules usually experience stronger van der Waals forces. However, when comparing isomers, such as the alkyl halides in the given exercise, the molecular weight is not a differentiating factor because isomers share the same molecular formula, hence have the same molecular weight.

All four compounds in the problem—pentyl chloride, tertiary butyl chloride, isobutyl chloride, and n-butyl chloride—have the same molecular weights, but distinctly different boiling points due to their differing structures. In this context, while molecular weight could be a useful indicator of boiling point trends across different molecules, it does not help in predicting the relative boiling points of these particular isomers.

The boiling point of a molecule also depends significantly on its chemical structure. For alkyl halides, the type of carbon atom to which the halogen is attached can influence how tightly the molecules pack together. Straight-chain alkyl halides like pentyl chloride and n-butyl chloride are able to pack closely, which maximizes intermolecular forces and, consequently, the energy needed to separate them (boiling point).

In contrast, branched molecules such as tertiary butyl chloride do not pack as efficiently due to their shape, which leads to decreased van der Waals forces. As we've identified in the exercise, tertiary butyl chloride is highly branched, which makes its structure unique among the given options and contributes to its notably lower boiling point.

Isomers are compounds with the same molecular formula but different arrangements of atoms. The properties of isomers, including their boiling points, can be markedly different due to these variations in structure. The impact of isomerism is illustrated in this exercise by comparing the boiling points of pentyl chloride, isobutyl chloride, n-butyl chloride, and tertiary butyl chloride.

Chain isomerism, where the carbon skeleton differs among isomers, is particularly influential on the boiling point. Linear isomers like n-butyl chloride tend to have higher boiling points than their branched counterparts such as isobutyl chloride. Even more branched structures like tertiary butyl chloride have the lowest boiling points among these types of isomers due to the reasons previously discussed. Understanding how the structural isomerism of compounds affects their boiling points is fundamental for predicting their physical properties.