How should pentane, propane, and hexane be ranked in order of increasing boiling point?

Introduction / Context:
Boiling point trends within homologous series such as alkanes provide insight into intermolecular forces and molecular size effects. As the carbon chain length increases, the surface area and polarizability typically increase, leading to stronger London dispersion forces and higher boiling points. This question asks how to rank three alkanes, propane, pentane and hexane, by boiling point, which requires understanding that longer chains usually boil at higher temperatures.

Given Data / Assumptions:
- Propane has three carbon atoms (C3H8).
- Pentane has five carbon atoms (C5H12).
- Hexane has six carbon atoms (C6H14).
- All three are straight chain alkanes in this context.
- Other factors like branching are not introduced in the question.

Concept / Approach:
In nonpolar molecules like alkanes, the dominant intermolecular forces are London dispersion forces. These forces increase with molecular size and surface area because larger molecules have more electrons and greater polarizability. As a result, alkanes with more carbon atoms and longer chains have higher boiling points. Therefore, among propane, pentane and hexane, hexane should have the highest boiling point, pentane an intermediate boiling point and propane the lowest. Ordering from highest to lowest helps identify the correct option.

Step-by-Step Solution:
Step 1: Identify the smallest molecule. Propane, with three carbon atoms, is the smallest of the three alkanes. Step 2: Identify the intermediate molecule. Pentane has five carbon atoms, making it larger than propane but smaller than hexane. Step 3: Identify the largest molecule. Hexane, with six carbon atoms, is the largest and has the greatest surface area. Step 4: Apply the rule that larger alkanes with more carbon atoms generally have higher boiling points due to stronger dispersion forces. Step 5: Conclude that the boiling point order from highest to lowest is hexane greater than pentane greater than propane, which corresponds to option C.

Verification / Alternative check:
Approximate boiling points from data tables reinforce this reasoning. Propane boils at a temperature well below zero degrees Celsius, pentane boils near thirty six degrees Celsius and hexane boils higher, near sixty eight degrees Celsius. These values clearly show that hexane has the highest boiling point, followed by pentane, with propane having the lowest. Even without memorising exact numbers, remembering that boiling points increase with chain length provides a reliable cross check for the ranking.

Why Other Options Are Wrong:
Option A, pentane greater than propane greater than hexane, incorrectly places hexane below pentane and propane, which contradicts the trend of increasing boiling point with increasing carbon chain length. Option B, propane greater than pentane greater than hexane, reverses the correct order entirely. Option D, propane greater than hexane greater than pentane, again misorders the compounds by placing propane above the larger alkanes. These options do not reflect the consistent relationship between chain length and boiling point in straight chain alkanes.

Common Pitfalls:
A common error is to overlook the relative sizes of molecules and instead guess based on the names or familiarity. Some learners may mistakenly think that more volatile fuels like propane must have higher boiling points, when in fact they have lower boiling points and are gaseous at room temperature. Confusing boiling point with melting point is another pitfall. Keeping in mind that greater molecular size in alkanes leads to stronger dispersion forces and therefore higher boiling points helps avoid these mistakes.

Final Answer:
The correct ranking of pentane, propane and hexane in order of increasing boiling point is Hexane > Pentane > Propane.