Which of the following will have the highest boiling point?

KPSC Staff Nurse - 2015

Biochemistry and Nutrition

Medium

Appeared in: KPSC Staff Nurse - 2015

Explanation

n-heptane has the highest boiling point because it possesses the longest carbon chain (7 carbons) among the given options.
The boiling point of nonpolar compounds like alkanes increases with molecular weight and surface area.
Longer, straight-chain alkanes have a larger surface area, leading to stronger intermolecular London dispersion forces, which require more energy to break.
With seven carbon atoms and an unbranched structure, n-heptane has the strongest intermolecular attractions and thus the highest boiling point.

Option A: 2-methylbutane has only 5 carbon atoms, the lowest number among the options. This results in the weakest London dispersion forces and the lowest boiling point.
Option C: n-hexane has 6 carbon atoms, which is one less than n-heptane. A shorter carbon chain leads to weaker intermolecular forces and a lower boiling point compared to n-heptane.
Option D: 2,2-dimethylbutane has 6 carbon atoms, the same as n-hexane, but it is highly branched. Branching reduces the molecule's surface area, weakening the intermolecular forces and lowering the boiling point significantly compared to its straight-chain isomer (n-hexane) and the longer-chain n-heptane.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Boiling points of alkanes and structural isomers as background academic context rather than a clinical decision trigger.
Understanding how molecular structure affects physical properties like boiling point is a fundamental concept in organic chemistry, which is the basis for pharmacology.
The principle applies to many drug molecules, where structure dictates solubility, volatility (as in anesthetic gases), and ability to cross cell membranes.
For example, the volatility of inhalational anesthetics like desflurane (boiling point 24°C) versus sevoflurane (boiling point 59°C) affects their delivery and speed of action.

How to Approach the Question

First, identify that the question is asking to compare the boiling points of several organic compounds, which are all alkanes.
Recall the two primary rules governing the boiling points of alkanes: 1. Boiling point increases as the number of carbon atoms (chain length) increases. 2. For isomers (compounds with the same number of carbons), boiling point decreases as the degree of branching increases.
Count the number of carbon atoms in each molecule: 2-methylbutane (5), n-heptane (7), n-hexane (6), and 2,2-dimethylbutane (6).
Identify the molecule with the most carbon atoms. In this case, n-heptane has 7 carbons, more than any other option.
Based on the first rule, the compound with the longest carbon chain will have the highest boiling point. Therefore, n-heptane is the correct answer.
Confirm this by noting that the other compounds either have fewer carbons or are branched isomers with fewer carbons, both of which would lead to a lower boiling point.

Concept Tested & Keywords

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E6 Pharmacology KD Tripathi Essentials 9e Part 2 p. 52-54