Why do the boiling points of alcohols decrease with increase of branching in the carbon chain?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Due to the decrease in van der Waals forces with decrease in surface area.
Besides hydrogen bonding through the –OH group, alcohol molecules attract one another by van der Waals forces along their carbon chains. These forces grow with the area of contact between molecules. Branching makes a molecule more compact, so its surface area decreases, the van der Waals forces become weaker, and less energy is needed to boil the alcohol.
Why the other options are incorrect
Option (1) Due to the steric hindrance with decrease in surface area. — Incorrect. Steric hindrance is the crowding of bulky groups around an atom; it is not an attractive force between molecules, and boiling point depends on the attractive forces between molecules.
Option (2) Due to the decrease in dipole-dipole interactions with decrease in surface area. — Incorrect. The polar part of each isomer is the same –OH group; the force that falls with the smaller surface area is the van der Waals attraction between the carbon chains.
Option (3) Due to the formation of hydrogen bonds. — Incorrect. Hydrogen bonding raises the boiling points of alcohols compared with hydrocarbons and ethers of similar mass; it cannot explain a fall in boiling point with branching.
NCERT (2026-27 edition):
Boiling Points – “The boiling points of alcohols and phenols increase with increase in the number of carbon atoms (increase in van der Waals forces). In alcohols, the boiling points decrease with increase of branching in carbon chain (because of decrease in van der Waals forces with decrease in surface area).”