Question

What happens to the boiling points of organic compounds as they get bigger? Why?

Answer 1

As molecules get bigger, their boiling points should increase.......Of course there is a catch.

Explanation:

Look at the `"normal boiling points"` of the linear alkane series: `"methane; ethane; n-propane; n-butane; n-pentane"`. These are in order: `-161.5` `""^@C`; `-88` `""^@C`; `-42.3` `""^@C`; `-1.0` `""^@C`; `+36.0` `""^@C`.

As the chain gets longer, there is more opportunity for chain-chain interactions, and the boiling points increases because of this intermolecular van der Waals force of attraction.

Of course a molecule such a `"neopentane"`, i.e. `(H_3C)_4C`, has lesser opportunity for such van der Waal interaction (because of its shorter chain), and its boiling point, `9.5` `""^@C` is substantially lower than that of `"n-pentane"` because of the reduced opportunity for such chain-chain interaction.

Once we introduce polarity into the molecule, in one fashion or another, the boiling points should increase again. Introduce an hydroxyl group, and all bets are off again. Why so?