In alcohols, the boiling points decrease with increase of branching in carbon chain. Why?

Due to the decrease in van der Waals forces with decrease in surface area.

The correct answer is option 4. Due to the decrease in van der Waals forces with decrease in surface area.

In alcohols, when the carbon chain branches, the overall surface area of the molecule decreases. This decrease in surface area impacts the strength of the intermolecular forces that hold the alcohol molecules together.

• Van der Waals forces: These are weak attractive forces between uncharged molecules. In alcohols, the main type of van der Waals forces are London dispersion forces, which arise from temporary fluctuations in electron distribution within the molecules.

•Impact of surface area: When the surface area of a molecule decreases (due to branching), there are fewer opportunities for London dispersion forces to form between neighboring molecules. This results in weaker intermolecular forces overall.

Lower boiling point:

Weaker intermolecular forces require less energy to overcome during the boiling process. Therefore, alcohols with branched carbon chains (and lower surface areas) have lower boiling points compared to straight-chain alcohols with the same number of carbon atoms.

Other factors to consider:

•Hydrogen bonding: While present in alcohols, the impact of hydrogen bonding on boiling point changes with branching is relatively minor compared to the change in van der Waals forces.

So, the decrease in surface area due to branching leads to a decrease in van der Waals forces, ultimately resulting in lower boiling points for branched-chain alcohols.