MCQ Detailed View

The boiling point of a substance depends mainly on the strength of its intermolecular forces. Ethanol (C₂H₅OH) and acetone (CH₃COCH₃) have similar molecular sizes, but ethanol boils at 78°C, while acetone boils at 56°C. The main reason behind this difference is the presence of hydrogen bonding in ethanol.

Ethanol contains a hydroxyl group (-OH) attached to its carbon chain. The highly electronegative oxygen atom pulls electron density away from hydrogen, creating a strong partial positive charge on the hydrogen atom. This allows ethanol molecules to form hydrogen bonds with each other. Hydrogen bonding is much stronger than dipole–dipole or London dispersion forces and requires significantly more energy to break. This extra energy translates into a higher boiling point.

Acetone, on the other hand, contains a carbonyl group (C=O). While the oxygen atom in the carbonyl can act as a hydrogen bond acceptor, acetone lacks the –OH group, so it cannot form strong hydrogen bonds with itself. Instead, acetone molecules interact primarily through dipole–dipole attractions, which are weaker than hydrogen bonds. As a result, acetone requires less energy to separate its molecules, and therefore it has a lower boiling point.

For comparison:

• 
  

  Methanol (CH₃OH): Boiling point ~65°C

  
  


• 
  

  Ethanol (C₂H₅OH): Boiling point ~78°C

  
  


• 
  

  Acetone (CH₃COCH₃): Boiling point ~56°C
  This data clearly shows the effect of hydrogen bonding in alcohols versus carbonyl compounds of similar size.

  
  

The concept is very important in physical chemistry and in practical fields:

• 
  

  Distillation processes rely on boiling point differences, where ethanol can be separated from acetone-containing mixtures.

  
  


• 
  

  Solvent selection in laboratories considers both volatility and intermolecular forces.

  
  


• 
  

  Biological systems also depend heavily on hydrogen bonding (e.g., water’s high boiling point compared to molecules of similar mass).

  
  

Thus, ethanol has a higher boiling point than acetone due to hydrogen bonding, not because of molecular weight or acidity.