Question

Place these hydrocarbons in order of decreasing boiling point?

London dispersion forces are weak intermolecular forces that work to attract both polar and nonpolar molecules to one another via instantaneous dipole moments. These are extremely short-lived fluctuations in the distribution of electrons around an atom. London forces are extremely sensitive to distance, being inversely proportional to the distance raised to the seventh power. They are also sensitive to the size of the molecules, because greater surface area allows for more molecular interaction.

Place these hydrocarbons in order of decreasing boiling point:

paraffin : C36H74

methane: CH4

octane: C8H18

isooctane: C8H18 (also known as 2,2,4 trimethylpentane)

octadecane: C18H36

Rank from highest to lowest boiling point.

Answer

basically what you do first, is looking at the number of carbon atoms in the molecule. the bigger a molecule, the higher its boiling point. if you have molecules with the same number of Cs, you look at the structure. linear molecules have a higher bp than shorter ones with .. (i'm literally translating, not sure about the correct terminology) side chains. so in your example we have the highest number of Cs in paraffin, then octadecane, then the linear C8 octane (also called n-octane to show that its linear), isooctane and then with the lowest bp we have methane, which is a gas.

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