Question: Rank the following isomers in order of increasing boiling point.

Alkenes generally have low boiling points.

The boiling point of an alkene increases with the number of carbons, owing to its larger surface area.

The boiling point of cis alkenes is higher than that of trans alkenes as they have a small net dipole present.

In the following compound, the dipole moments of all alkyl groups present around C=C cancel out each other. Thus, this compound has no net dipole and has the smallest surface area out of all the three compounds.

Four dipoles cancel out each other

Hence, the above compound has the lowest boiling point.

In the following compound, two ethyl groups cancel out the dipoles. Thus, there is no net dipole. And the compound has an intermediate boiling point.

Two dipoles cancel out each other

In the following compound, the two alkyl groups are cis to each other. Thus, their dipoles reinforce. As a result, this compound has the highest boiling point.

Two dipoles reinforce each other

Thus, the increasing order of the boiling point of the given compounds is as shown:

Increasing order of boiling point