Question

Which among following compounds has highest boiling point?

• A. $\mathrm{C_4H_9NH_2}$
• B. $\mathrm{C_2H_5COOH}$
• C. $\mathrm{CH_3(CH_2)_2CH_2OH}$
• D. $\mathrm{C_2H_5CH(CH_3)_2}$

Hint:

When comparing compounds of similar molecular mass, carboxylic acids always beat alcohols, amines, and hydrocarbons in boiling point rankings because they pair up into stable dimers through double hydrogen-bond interactions.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks to compare the boiling points of four different organic compounds (an amine, a carboxylic acid, an alcohol, and a branched alkane) of comparable molecular masses and find the one with the highest value.

Step 2: Key Formula or Approach:
The boiling point of a molecular compound is determined by the strength of its intermolecular forces. Intermolecular forces follow this order of decreasing strength:
Extensive Hydrogen Bonding>Standard Hydrogen Bonding>Dipole-Dipole>London Dispersion Forces

Step 3: Detailed Explanation:
Let's analyze the functional groups and associated intermolecular forces for each option:
Option (D) $\mathrm{C_2H_5CH(CH_3)_2}$ (isopentane) is a hydrocarbon featuring only weak London dispersion forces. It has the lowest boiling point.
Option (A) $\mathrm{C_4H_9NH_2}$ (butylamine) forms intermolecular hydrogen bonds via its $-\mathrm{NH_2}$ group, but nitrogen is less electronegative than oxygen, making these bonds moderately weak.
Option (C) $\mathrm{CH_3(CH_2)_2CH_2OH}$ (butan-1-ol) contains a highly polarized $-\mathrm{OH}$ group resulting in strong intermolecular hydrogen bonding.
Option (B) $\mathrm{C_2H_5COOH}$ (propanoic acid) contains a carboxylic acid functional group ($-\mathrm{COOH}$). Carboxylic acids are capable of forming stable, cyclic dimeric structures via two strong hydrogen bonds per pair of molecules. This extensive network persists even in the vapor state, resulting in a higher boiling point than equivalent alcohols.

Step 4: Final Answer:
Propanoic acid ($\mathrm{C_2H_5COOH}$) has the highest boiling point due to its exceptional hydrogen-bonding capability, which matches option (B).