Which of the following is/are the reason for the relatively low boiling point of branched alkanes? 

CONCEPT:

Boiling Point and Intermolecular Forces

• The boiling point of a substance is influenced by the strength and type of intermolecular forces between its molecules.
• The primary types of intermolecular forces are:
  • Ionic Bonds: Strong electrostatic interactions between oppositely charged ions.
  • Hydrogen Bonds: Strong dipole-dipole interactions involving hydrogen bonded to highly electronegative atoms like O, N, or F.
  • Van der Waals Forces: Weak interactions due to temporary dipoles in nonpolar molecules, which are the dominant forces in alkanes.
• Branched alkanes have weaker Van der Waals forces compared to their straight-chain counterparts due to reduced surface area for interactions.

EXPLANATION:

• The boiling point of branched alkanes is relatively lower due to the following reasons:
  • Branched alkanes have a more compact molecular structure, reducing the surface area available for Van der Waals attractions.
  • Since Van der Waals forces are directly proportional to surface area, the reduced surface area in branched alkanes results in weaker intermolecular forces.
  • Weaker intermolecular forces require less energy to overcome, leading to a lower boiling point.
  •  (Van der Waals Attraction): This is the primary intermolecular force acting in alkanes, and its strength is reduced in branched alkanes due to a decrease in surface area.
• Incorrect Options:
  • Option 1 (Ionic Bonds): Alkanes are nonpolar molecules and do not exhibit ionic bonding.
  • Option 2 (Hydrogen Bonds): Alkanes do not contain highly electronegative atoms like O, N, or F, so they cannot form hydrogen bonds.
  • Option 4 (Ionic Bonds and Hydrogen Bonds): Both ionic and hydrogen bonding are irrelevant for alkanes.

Therefore, the relatively low boiling point of branched alkanes is due to weaker Van der Waals forces, making Option 3 the correct answer.