why do branched chains have lower boiling points?

hi, the text book says that alkanes with branched chains have lower boiling points thn the straight carbon chain with the same number of carbon atoms. but it doesn't explain. does anyone know why?

Reply 1

3.14159265

branched alkanes have weaker van der waals forces because of the smaller area of contact between the molecules.

so less energy is needed to overcome the intermolecular force.

Linear alkanes can line up more closer and therefore have stronger van der waals forces than unbranched alkanes.

so just remember that van der waals forces are stronger when there is more area of contact between molecules, and obviously area of contact is smaller in branched molecules compared to linear molecules (when they both have the same molecular formula).

Reply 2

charco

Study Forum Helper

Just to add that a sphere is the shape with the smallest surface area to volume ratio. The more branching there is the more spherical is the molecule and hence the smaller surface area for a given volume (relative molecular mass).

Reply 3

Magnamind

Alkanes have intermolecular forces, i.e. Van Der Waal's forces that control their boiling points. ... A straight chain alkane will have a boiling point higher than a branched chain alkane because of the greater surface area in contact with other molecules.

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