Problem 9 - A Complete Resource Book in Chemistry for JEE Main

Question

Of the following compounds, the one with the lowest boiling point is? (a) pentyl chloride (b) tertiary butyl chloride (c) isobutyl chloride (d) n-butyl chloride

Step-by-Step Solution

Answer

The compound with the lowest boiling point is tertiary butyl chloride (b).

Step 1: Understand Boiling Point Factors

Boiling points are influenced by molecular structure, specifically factors like molecular weight, polarity, and branching. The more branched a compound is, the lower its boiling point tends to be due to decreased surface area and intermolecular forces.

Step 2: Analyze Molecular Structures

Examine the structures of the given compounds: - Pentyl chloride is a straight-chain compound. - Tertiary butyl chloride is highly branched. - Isobutyl chloride has a branched structure, but less so than tertiary butyl. - n-Butyl chloride is also a straight-chain compound. The high branching in tertiary butyl chloride suggests a lower boiling point.

Step 3: Compare Boiling Points

Remember, more branchings reduce intermolecular forces like Van der Waals forces. Among the given options, tertiary butyl chloride with the highest level of branching, will have the lowest boiling point due to its reduced surface area which limits intermolecular interactions.

Step 4: Conclusion

Based on structural analysis and understanding of molecular interactions, tertiary butyl chloride (option b) has the lowest boiling point among the given compounds.

Key Concepts

Molecular Structure

Molecular structure plays a key role in determining the physical properties of compounds, such as their boiling points. Specifically, the arrangement and bonding between atoms within a molecule can greatly influence how molecules interact with each other. These interactions are crucial because they determine how much energy is required to transition from the liquid to the gas phase, in other words, to boil the compound.
In the exercise, we are looking at chlorinated hydrocarbons where the arrangement of the carbon atoms affects the molecule’s shape and size. For example, pentyl chloride features a longer, straight-chain structure, while tertiary butyl chloride is more compact due to its branching. These differences in molecular structure lead to variations in the boiling points of the compounds.

Intermolecular Forces

Intermolecular forces are the attractions that occur between molecules due to various interactions. These forces include hydrogen bonds, dipole-dipole interactions, and Van der Waals forces. The strength of these forces impacts how easily molecules can move past each other, affecting boiling points.
In the case of the exercise compounds, Van der Waals forces are primarily at play as these molecules do not have the capability for hydrogen bonding. These forces are generally weaker than hydrogen bonds, which means that the molecular structure, which influences how these forces operate, becomes even more important. Less branching generally means stronger intermolecular forces and hence a higher boiling point, as seen with pentyl chloride compared to the highly branched tertiary butyl chloride.

Branching Effect

Branching within a molecule refers to how its carbon atoms are connected. When a molecule is highly branched, its shape is more compact and spherical. This impacts how the molecules pack together in the condensed phases. Because branched molecules have a smaller surface area compared to straight-chain molecules, they tend to have weaker intermolecular forces.
In the selection of compounds in the exercise, tertiary butyl chloride is the most branched. This extensive branching decreases the surface area available for intermolecular attractions, such as Van der Waals forces, thereby lowering its boiling point compared to less-branched or straight-chain isomers like pentyl chloride and n-butyl chloride.

Van der Waals Forces

Van der Waals forces are a type of weak intermolecular force that includes attractions like London dispersion forces and dipole-induced dipole interactions. These are significant in nonpolar or slightly polar molecules such as chlorinated hydrocarbons.
These forces depend highly on the surface area of a molecule; larger surface areas allow for more effective Van der Waals interactions. When the surface area is reduced through branching, as with tertiary butyl chloride, the Van der Waals forces are weakened, requiring less energy to break these interactions during boiling. Consequently, understanding these forces clarifies why highly branched structures boil at lower temperatures than their linear counterparts.