Arrange the following from highest to lowest boiling point? Paraffin, Methane, Octane, Isooctane, Octadecane.?

Paraffin, Octadecane, Octane, Isooctane, and methane.

That's in order from highest to lowest boiling point.

The larger molecules tend to have higher boiling points, because they are more massive and require greater energy to enter the gaseous phase. In the case of octane and isooctane, while they are the same size, the long chain-like structure of octane lets the separate molecules of octane almost weave together so it boils at a higher point than the isooctane.

Paraffin [if by that you mean the wax] is highest, then octadecane, the octanes [not sure which is higher], then methane is the lowest.