Question

Among the given pairs of isomeric organic compounds, which pair is correctly arranged in order of INCREASING boiling point?

• A. $n$-Butyl alcohol < $t$-Butyl alcohol
• B. $t$-Butyl chloride & Lt; $n$-Butyl chloride
• C. $p$-Dichlorobenzene & Lt; $o$-Dichlorobenzene
• D. Isopentane & Lt; Neopentane

Hint:

As molecular branching increases $\Rightarrow$ Surface area decreases $\Rightarrow$ Van der Waals forces weaken $\Rightarrow$ Boiling point drops.

Answer 1

The Correct Option is B

Concept: For structural isomers sharing identical molecular formulas and molecular weights, variations in boiling point are governed by molecular shape and branching. Nonpolar or weakly polar covalent molecules rely on temporary London dispersion forces, which depend directly on the total accessible molecular surface area. The relationship between molecular structure and boiling point follows a reliable trend: \[ \text{Increased Branching} \Rightarrow \text{Spherical Shape} \Rightarrow \text{Smaller Surface Area} \Rightarrow \text{Weaker Dispersion Forces} \Rightarrow \text{Lower Boiling Point} \]

Step 1: Analyzing the structural differences between butyl chloride isomers.
We compare the straight-chain and branched isomers of butyl chloride:
• $t$-Butyl chloride ($\text{(CH}_3)_3\text{C-Cl}$): Features a highly branched, compact, and nearly spherical geometry. This spherical shape minimizes the outer surface area available for contact with neighboring molecules.
• $n$-Butyl chloride ($\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Cl}$): Possesses a fully extended, linear, straight-chain hydrocarbon conformation that provides a significantly larger molecular surface area.

Step 2: Correlating surface area to the final boiling point inequality.
The larger surface area of linear $n$-butyl chloride allows neighboring molecules to align closely, maximizing the strength of intermolecular London dispersion forces. Conversely, the compact shape of $t$-butyl chloride limits molecular contact, weakening these temporary attractions. Consequently, $t$-butyl chloride boils at a significantly lower temperature ($324\text{ K}$) than $n$-butyl chloride ($351\text{ K}$). This confirms that the arrangement $t$-butyl chloride & Lt; $n$-butyl chloride correctly demonstrates a trend of increasing boiling point.