Chemistry · Hydrocarbons · NEET
For isomers (same molecular formula, same molar mass), branching DECREASES the boiling point. A straight chain like n-pentane boils highest; the most branched one like neopentane boils lowest. Remember: this rule is only for comparing isomers of the same formula, not for comparing alkanes of different sizes.
A straight chain is long and can lie close to other chains along its whole length, so the two molecules touch over a large surface. A branched molecule is squeezed into a rounder, more ball-like shape, so two of them only touch at small points. Less contact surface means weaker van der Waals (London dispersion) forces, so the molecules separate easily and boil at a lower temperature.
Van der Waals (London dispersion) forces are weak attractions between temporary charges in non-polar molecules. They act only over the surface where two molecules are close together. A bigger touching surface = more of these weak forces adding up = stronger total attraction = higher boiling point. Branching shrinks the touching surface, so the total attraction drops.
n-pentane (309 K) > isopentane / 2-methylbutane (301 K) > neopentane / 2,2-dimethylpropane (282.5 K). n-pentane is a straight chain (most surface), neopentane is the most branched and nearly spherical (least surface), so it boils lowest. This exact order was asked in NEET 2024.
No. Isomers have the SAME molecular formula, so the SAME molar mass. Since mass is equal, mass cannot explain the difference. The only thing that changes is the SHAPE, which changes the surface contact and therefore the strength of van der Waals forces. That shape difference is the whole reason for the boiling point difference.
Neopentane has a central carbon with four methyl groups around it, so it is compact and almost ball-shaped (spherical). A sphere has the smallest surface area for its size. Small surface area = smallest contact between molecules = weakest forces = lowest boiling point among the pentane isomers.
Given below are two statements: Statement I: The boiling point of three isomeric pentanes follows the order n-pentane > isopentane > neopentane. Statement II: When branching increases, the molecule attains a shape of sphere. This results in smaller surface area for contact, due to which the intermolecular forces between the spherical molecules are weak, thereby lowering the boiling point. In the light of the above statements, choose the most appropriate answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It lowers the boiling point only when you compare ISOMERS (same molecular formula). If you compare alkanes of different sizes, the bigger molecule usually boils higher because boiling point rises steadily with molar mass. The branching rule is strictly for same-formula isomers.
n-pentane, the straight-chain isomer, has the highest boiling point (about 309 K) because its long shape gives the largest surface contact and the strongest van der Waals forces.
Neopentane (2,2-dimethylpropane), the most branched and nearly spherical isomer, has the lowest boiling point (about 282.5 K) because its small surface contact gives the weakest van der Waals forces.
Yes. NCERT states the boiling point of isomeric haloalkanes and alcohols also decreases with branching, for the same reason — more branching means smaller surface area and weaker van der Waals forces. So the idea carries over to other chapters, which is useful for NEET.
Alkanes are non-polar, so they are held together only by weak van der Waals (London dispersion) forces. These forces get stronger with more surface contact and with higher molar mass, and weaker with more branching.
