Hydrogen bonding in water.

The hydrogen bonding associated with the hydroxyl groups of alcohols cause them to be higher boiling than alkanes of similar structure and molecular weight. This statement is very much at the level of fundamental knowledge, so it is good to review the underpinnings. To boil a substance is to carry out the transformation of the substance from state A, the liquid phase, into state B, the gaseous phase. The liquid phase is a condensed state of matter. At the molecular level, down in the liquid, the molecules are largely trapped within each other's orbit, having fallen down into mutual wells of electrostatic potential energy along lines of intermolecular force. How easily can the molecules escape from each other? The stronger the intermolecular forces between them, the more difficult it will be and the higher the boiling point. Between alkanes there is only Van der Waals attraction, but between alcohol molecules, there is hydrogen bonding, which is much stronger.

If you have a sample of an alcohol in one sealed jar at a given temperature and an alkane of similar molecular weight in another sealed jar, within each jar there will be a vapor phase of the substance in the space above each liquid. Within each liquid, the molecules have a distribution of kinetic energies, a Boltzmann distribution. The molecules within the some fastest fraction within each liquid have enough kinetic energy to climb out of the potential energy well of intermolecular force and escape into the space above the liquid as vapor. At a given temperature, a greater fraction of the alkane molecules possess enough kinetic energy because it is easier for them to escape from each other. At a given temperature, there will be a greater concentration of alkane vapor in its head space versus the head space in the alcohol canister. The partial pressure of alkane vapor will be higher. The alkane has a higher vapor pressure. We say it is more volatile.

Boiling happens when you have increased the temperature of a liquid enough that its vapor pressure has come to equal atmospheric pressure. This will happen at a lower temperature with the alkane. The alcohol is higher boiling.