Which of the following alkenes has the lowest boiling point? (A) Hexene, (B) pentene, (C) propene, (D) butene, or (E) ethene.
A boiling point is the temperature at which a liquid is converted to a gas. Some species will require a higher temperature to convert to a gaseous state than others. We are looking for the lowest boiling point. Let’s have a look at the molecules given in (A) to (E). All of them contain the suffix -ene. This is because they are alkenes, as stated in the question. The difference between them is not the functional group but the number of carbon atoms. The prefix hex- tells us that this molecule has six carbon atoms. Pentene contains five carbon atoms, propene contains three, butene contains four, and ethene contains two.
We need to know the relationship between the number of carbon atoms in alkenes and their boiling points. To understand the relationship between number of carbon atoms or chain length and boiling point, let’s look at two of the molecules given in options (A) to (E). I have drawn out propene and hexene. Due to the random motion of electrons in molecules, small instantaneous dipoles can form. These dipoles interact with the electron clouds of nearby molecules, inducing more dipoles. This causes an electrostatic attraction between the two molecules. This type of interaction is called the London dispersion force.
This interaction can occur between all atoms and molecules, so it will occur between alkene molecules. They are weak interactions, but they increase in strength when the number of electrons increases and there is more surface contact. So dispersion forces are stronger for longer molecules. So if there is an increase in chain length, there will be an increase in the strength of electrostatic attractions between molecules. Therefore, there will be an increase in the energy needed to break these attractions. Breaking these attractions will cause a substance to change from a liquid to a gas. So there will be an increase in the temperature of the boiling point.
We now know that as the chain length increases, the boiling point increases. The question is asking us which alkene has the lowest boiling point. So it is essentially asking us for the molecule with the shortest chain length. As ethene has only two carbon atoms and the other molecules have either three or above, we know that ethene has the shortest chain length. And therefore, it has the lowest boiling point.