Question

Which of the following would you expect to have the highest boiling point? C8H18, CH4, C4H10, C2H6, C3H8

Answer 1

Generally speaking, boiling points increase as carbon chain length increases. This is because there are more atoms present in the hydrocarbon molecule and therefore more intermolecular forces (eg Van der Waals') that must be overcome. In addition the now heavier molecule requires greater energy (heat) to make the phase change to a gas. However, branching decreases boiling point; branched hydrocarbons cannot pack together as closely as unbranched ones, and so intermolecular forces are less strong.

Basing on this, the C₈H₁₈ should have the highest boiling point, followed by C₄H₁₀, C₃H₈, C₂H₆  and CH₄.

Hope this helps