Question

why boiling point of alkanes decreases with branching

Answer 1

With branching the London forces of attraction get weaker and hence the boiling point decreases.

Answer 2

This is due to the weaker Van der Waals dispersion forces between branched organic molecules compared to straight chain molecules. Van der Waals dispersion forces are proportional to the surface area of the molecule.

For the same number of carbon atoms, branched molecules are more compact, with fewer atoms at the surface of the molecule able to interact with the atoms on the surface of other molecules.

Weaker intermolecular forces means that less energy is required to separate the molecules of a compound from its liquid surface, thus lowering the boiling point.



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