[CSG]

Hi,

I do not understand what the following phrase out of my text book means, with regard to boiling point: "when branching occurs the molecules between them become more spherical in shape, which reduces the contact surface area between them and lowers the boiling point."

Could someone please explain what this means please?

Especially what contact surface area means. Also, I thought that the boiling point generally increases as the carbon chain gets longer - isn't that branching?

[Smrt guy]

The sentence sounds very jumbled and may be some sort of typo.  What it is trying to say is that branching makes the molecules resemble spheres more than their straight-chain counterparts (spheres have minimum surface area relative to volume).  This in turn means that the intermolecular interactions are decreased because the surface contact is less (imagine placing two balls next to each other and then two blocks:  compare the surface contact).  Boiling point is a measure of intermolecular forces, so the molecules with less surface contact have smaller intermolecular forces and thus lower boiling points.  Finally, no branching is not related to number of carbons.  Instead it is comparing the straight chain to constitutionally isomeric molecules (e.g. pentane to 2-methylbutane or 2,2-dimethylpropane).